Thomas Schmitt

Philippe Caillou

Michele Sebag

Matching Jobs and Resumes: a Deep Collaborative Filtering Task

This paper tackles the automatic matching of job seekers and recruiters, based on the logs of a recruitment agency (CVs, job announcements and application clicks). Preliminary experiments reveal that good recommendation performances in collaborative filtering mode (emitting recommendations for a known recruiter using the click history) co-exist with poor performances in cold start mode (emitting recommendations based on the job announcement only). A tentative interpretation for these results is proposed, claiming that job seekers and recruiterswhose mother tongue is Frenchyet do not speak the same language. As first contribution, this paper shows that the information inferred from their interactions differs from the information contained in the CVs and job announcements.

The second contribution is the hybrid system Majore (MAtching JObs and REsumes), where a deep neural net is trained to match the collaborative filtering representation properties. The experimental validation demonstrates Majore merits, with good matching performances in cold start mode.

Introduction

Among the key problems of the 21 st century is unemployment, a multi-faceted societal and industrial phenomenon. This paper considers a single facet thereof, referred to as informational market labor frictions : the fact that high unemployment rates can co-exist with high numbers of job unfilled. Machine learning and Knowledge Discovery have been investigating human resource-related applications for over a decade (more in section 2). Focusing more specifically on e-recruitment, the state of the art essentially considers the matching issues related to high paying jobs [START_REF] Shaha | A survey of job recommender systems[END_REF]. A majority of approaches support the recruiter or headhunter task, using natural language processing, statistical or hybrid approaches [START_REF] Kessler | A hybrid approach to managing job offers and candidates[END_REF][START_REF] Bollinger | Using social data for resume job matching[END_REF][START_REF] Senthil Kumaran | Towards an automated system for intelligent screening of candidates for recruitment using ontology mapping expert[END_REF][START_REF] Faliagka | Application of machine learning algorithms to an online recruitment system[END_REF] to identify and rank top management and executive-level candidates with targeted professional skills. Other approaches aim at supporting the seeker's search and retrieving job announcements best fitting her professional skills and goals [START_REF] Ioannis Paparrizos | Machine learned job recommendation[END_REF][START_REF] Yi | Matching resumes and jobs based on relevance models[END_REF]. Yet other approaches, inspired by social matching and dating [START_REF] Diaz | Relevance and ranking in online dating systems[END_REF], handle the seekers and recruiters matching as a reciprocal search [START_REF] Mine | Reciprocal recommendation for job matching with bidirectional feedback[END_REF].

This paper instead considers the matching issues related to temporary and low-wage jobs, tackling the matching of CVs and job announcements and providing support to recruiters. The choice of low-wage jobs is guided by both scientific and applicative motivations. On the applicative side, the needs are huge: the vast majority of job announcements concerns low-wage jobs (according to the French Ministry of Labor, 2016, over 90% staff recruited every month are on temporary, fixed-term contracts). On the scientific side, the challenge is that recruiters and job seekerswhose mother tongue is Frenchyet do not seem to speak the same language (more in section 3). More precisely, CVs and job announcements tend to use different vocabularies, and same words might be used with different meanings. In other words, the task of accurately matching jobs and CVs might be more a machine translation problem, than an information retrieval one. These remarks echo recent advances in information retrieval, also suggesting that that people tend to use different vocabularies depending on whether they write or search for information [START_REF] Furnas | The vocabulary problem in human-system communication[END_REF][START_REF] Ruotsalo | Interactive intent modeling: Information discovery beyond search[END_REF].

The presented study exploits the logs of a recruitment agency1 over a 2-month period, including 143,000 job announcements, 65,000 CVs and the usage traces recording which job seeker applied on a job announcement. A first question concerns the quality of the dataset w.r.t. the heterogeneity of the low-wage job labor market, and whether the data enables accurate collaborative filtering, making accurate recommendations for known seekers or recruiters [START_REF] Färber | An automated recommendation approach to selection in personnel recruitment[END_REF][START_REF] Malinowski | Matching people and jobs: A bilateral recommendation approach[END_REF]. Job and CV matching however significantly differs from collaborative filtering, as popularized by the Netflix challenge [START_REF] Bennett | The netflix prize[END_REF]: once a seeker (respectively a recruiter) has found a job (resp. an employee), the search is over, in general. In other words, job and CV matching mostly deals with the cold-start problem of making recommendations for new recruiters or seekers. The second question thus is whether the available textual information, that is, the CV and job announcement documents, enables efficient cold-start recommendation.

The contribution of the proposed paper is twofold. The representativity of the data is established as excellent matching performances can be obtained using collaborative filtering. Nevertheless, poor performances are obtained in cold-start mode. A first contribution regards the interpretation of this counter-performance, blamed on the fact that job seekers and recruiters do not speak the same language. Additional experiments are conducted to back up this claim. The second contribution is the hybrid system Majore, using the representation learned by collaborative matching and its properties [START_REF] Sam | Nonlinear dimensionality reduction by locally linear embedding[END_REF], to train a deep neural net [START_REF] Bengio | Representation learning: A review and new perspectives[END_REF]. The merits of Majore are empirically shown, with significantly improved cold-start performances compared to baseline methods.

The paper is organized as follows. Section 2 briefly reviews related work concerned with e-recruitment, focusing on the job and CV matching task. The basics of collaborative filtering and singular value decomposition are introduced for the sake of self-containedness. Section 3 presents the data used in the paper. Section 4 gives an overview of the Majore system. Experimental settings are reported in section 5 and the empirical validation in section 6. The paper concludes with a discussion and some perspectives for further research.

Background knowledge

This section first briefly reviews some machine learning-based approaches related to e-recruitment. Recommendation systems are thereafter introduced for the sake of self-containedness.

E-recruitment

The state of the art mostly focuses on high paying jobs, exploring several approaches [START_REF] Shaha | A survey of job recommender systems[END_REF]. A first approach is that of Content-and Knowledge Based Recommendation (CKBR) [START_REF] Kessler | A hybrid approach to managing job offers and candidates[END_REF][START_REF] Bollinger | Using social data for resume job matching[END_REF][START_REF] Senthil Kumaran | Towards an automated system for intelligent screening of candidates for recruitment using ontology mapping expert[END_REF]. With roots in Natural Language Processing, CKBR involves the extraction of grammatical, lexical and semantic features from the CVs, application letters and job announcements. These features are possibly augmented using external databases or domain ontologies (e.g. augmenting a job announcement with the skills required for this job). Additionally, similarities are built from statistical models [START_REF] Kessler | A hybrid approach to managing job offers and candidates[END_REF] or expert knowledge to compute the degree of matching between jobs and resumes. [START_REF] Senthil Kumaran | Towards an automated system for intelligent screening of candidates for recruitment using ontology mapping expert[END_REF] builds an ontology from the candidate documents and matches it with the job ontology to compute the degree of matching.

Another approach is concerned with the exploitation of new sources of information, e.g. social networks. Firms can examine the applicant profiles (including the restricted parts, e.g. asking the applicant's password) [START_REF] Bachrach | Human judgments in hiring decisions based on online social network profiles[END_REF][START_REF] Faliagka | Application of machine learning algorithms to an online recruitment system[END_REF][START_REF] Doherty | Getting social with recruitment[END_REF]. The human perception of job-suitability is correlated with many profile components (e.g., posts, photos, likes). Beyond the education and skill factors, screening based on social networks retains personality-related and other demographic criteria.

Machine learning is a key enabling technology to build scalable systems. In [START_REF] Ioannis Paparrizos | Machine learned job recommendation[END_REF], job recommendation is addressed as a supervised machine learning problem, exploiting past job transitions, employee CVs and institution data. In [START_REF] Yi | Matching resumes and jobs based on relevance models[END_REF], semi-structured supervised ML is used to exploit the structure of the CV and job announcement documents. In [START_REF] Faliagka | Application of machine learning algorithms to an online recruitment system[END_REF], the online ranking of candidates is based on features extracted from LinkedIn profiles (including personality features from the linguistic analysis of posts), and yields similar performances as human recruiters. A source of inspiration is that of social recommendation and dating [START_REF] Diaz | Relevance and ranking in online dating systems[END_REF], accounting for the two-sided relevance aspects of CVs and job announcements. [START_REF] Mine | Reciprocal recommendation for job matching with bidirectional feedback[END_REF] propose a reciprocal recommendation method, where recruiters and seekers exchange messages, iteratively assessing the popularity of their offer.

Recommendation approaches

A main trend of e-recruitment is based on adapting and extending recommendation systems [START_REF] Färber | An automated recommendation approach to selection in personnel recruitment[END_REF][START_REF] Malinowski | Matching people and jobs: A bilateral recommendation approach[END_REF]. Recommendation systems [START_REF] Su | A survey of collaborative filtering techniques[END_REF][START_REF] Agarwal | Content recommendation on web portals[END_REF], popularized by the Netflix challenge [START_REF] Bennett | The netflix prize[END_REF], often exploit the (n, m) matrix M reporting the feedback of the user community about the products (e.g., items for Amazon or movies for Netflix). Letting n (respectively m) denote the number of users (resp. items), M i,j describes the ranking of the j-th item by i-th user. Optionally, some description and metrics upon the user and item spaces are available.

Memory-based approaches use the metrics on the user (respectively item) spaces, with convolution from matrix M, to recommend items liked by users similar to the target user (resp. items similar to the items liked by the target user):

Recomm(i, j) = i ′ Sim(i, i ′ ) • M i ′ ,j i ′ M i ′ ,j (1)
Model-based approaches rely on a low-rank decomposition of the CF matrix M, using Singular Value Decomposition (SVD) [START_REF] Strang | Linear Algebra and its Applications[END_REF] and variants thereof [START_REF] Koren | Matrix factorization techniques for recommender systems[END_REF]:

M ≈ U • V t (2)
where U and V respectively are (n, ℓ) and (m, ℓ) matrices and the rank ℓ of the decomposition is determined by cross-validation. U and V are referred to as the latent representation of users and items respectively. The decomposition enables to fill the full matrix (with M i,j ≈ ⟨U i • V j ⟩), thus achieving collaborative filtering (recommending items to known users). The limitation is that the latent representation is unknown for new items or users, preventing its usage to achieve cold start recommendation. The e-recruitment approach proposed by Malinowski et al [START_REF] Malinowski | Matching people and jobs: A bilateral recommendation approach[END_REF], extending the pioneering approach of [START_REF] Färber | An automated recommendation approach to selection in personnel recruitment[END_REF], builds a probabilistic model based on a set of m boolean, manually defined features used to describe the documents. The collaborative filtering matrix is modified, with M i,j reporting the probability for the i-th user (job seeker or recruiter) to select a document, conditionally to the j-th feature being satisfied by this document.

The corpus

The presented study is based on the corpus provided by the Qapa company, which includes 4 million job announcements, 5 million CVs, and 8 billions clicks of job seekers applying on Figure 1: Distribution of recruiters neighbors, where two recruiters are neighbor if at least one job seekers applied to both propositions. The bump after the 800 mark is explained as a job seeker applied to 767 jobs; each of these 767 recruiters has at least 766 neighbors. job announcements over the 2012-2016 period. A 2-month extract thereof has been used in the experiments, including n=143,000 job announcements and m=65,000 CVs, with M i,j = 1 iff the j-th seeker clicked on the i-th job announcement. The Qapa company specifically targets temporary contracts and low-wage jobs. The available textual information is accessible using web browsers and smartphones, often resulting in very short CVs (2 sentences, possibly a-grammatical).

A first filter, removing all CVs and all job announcements but those with at least 5 clicks, retains n = 11, 000 recruiters and m = 7, 500 CV, where matrix M reports 84,000 clicks.

The structure of the collaborative filtering matrix M is analyzed together with the contents of the CV and job announcement documents.

Exploratory analysis

Matrix M is first characterized from the median number of clicks, that is 7 for the job seekers and 6 for recruiters. Let two recruiters be defined as neighbors if they have been clicked by (at least) one same job seeker. The neighbors heavy-tailed distribution is displayed in Fig 1 . The median (resp. average) number of neighbors is 139 (resp. 210), which is explained as a few job seekers click on a huge number of announcements.

The sparsity of M is 0.1% (compared to 1% in e.g., Netflix). Complementary experiments suggest that 0 corresponds to a "don't like" much more often than in standard recommendation setting, where 0 more often corresponds to "don't know".

After tokenization and stop words removal, job announcements and CVs contain 9000 unique words. Terms made of 2 and 3 consecutive words have also been considered, resulting in a 30,000-sized vocabulary; only the 7,000 most frequent ones are used in the experiments.

Fig. 2 reports the distribution of the lengths of CVs (left) and job announcements (right). Note that the support of the job announcement distribution is more compact than for CVs. As said, the CVs acquired through Web or mobile phone are broadly diversified and often informal, e.g., "physical reception; management of communication lines; appointment scheduling; maintenance of medical supplies"; "The mission that I had in the boat that was storing the car and cleaning the deck"; "team leader, management, HACCP norm, Chef de cuisine". . Left: the textual information is processed using LSA (dimension 1,200, projected in 2D using t-SNE [START_REF] Van Der Maaten | Visualizing data using t-sne[END_REF]); for some categories such as graphic designers and waiters, the job announcements and CVs form distinct clusters, suggesting that job seekers and recruited do not use the same vocabulary. Right: the interaction information is processed using SVD (dimension 500, projected in 2D using t-SNE); job seekers and recruiters are localised in same clusters; the fact that job seekers might click on different job categories (e.g. waiters and dishwashers) is also visible. Figure better seen in color.

Job seekers and recruiters do not speak the same language

A most interesting finding is that, although job seekers and recruiters undoubtedly understand each other, CVs and job announcements are not expressed in the same language, at least for some job categories. This claim is supported by conducting the following experiment on the textual description (CVs and job announcements using the 7,000 vocabulary) on the one hand and the interaction matrix M on the other hand. The singular value decomposition (in dimension 1,200 for the textual description and in dimension 500 for M) yields a vectorial representation of the job seekers and recruiters. This representation, together with the Euclidean metrics, is used to map the job seekers and the recruiters on the 2D plane with t-SNE [START_REF] Van Der Maaten | Visualizing data using t-sne[END_REF]. Fig. 3 represents four job categories: photographers, dish washers, waiters and graphic designers, in textual space (left) and in interaction space (right). Photographers form a dense cluster in both textual and interaction spaces. Dishwashers form a single cluster in the textual space, and several clusters in the interaction space. For waiters, and even more so for graphic designers, two very distinct clusters are observed in the textual space: one for job seekers (legend star), and one for recruiters (legend circle).

The separation of the job seeker/recruiter clusters in the textual space is explained as people are indeed using different languages, e.g. "You take in charge the physical reception of the patients, the management of the planning and the patient records" (job announcement) versus "Secretary accountant; Capture and storage of documents" (CV).

The separation of the job seeker/recruiter clusters disappears when considering the interaction space (Fig. 3, right): each job category involves several clusters, but each cluster mixes job seekers and recruiters. The fragmentation of e.g., the graphic designer clusters in the interaction space is explained as these clusters capture some information such as the geographic localization, which is hardly captured in the textual information.

Discussion

The difference of language between the two populations might give some additional empirical support to e.g. the frictional unemployment theory, emphasizing the cost of acquiring and processing the relevant information on the job market. It might also explain the difficulties of NLP-based matching of job announcements and CVs3 , and the mandatory use of external resources, such as ontologies. Another approach will be proposed in this paper, using the interaction-based representation to achieve job-CVs matching.

Overview of Majore

Aimed at recommending job seekers to recruiters, Majore involves two modes respectively referred to as collaborative filtering (Cf) and cold-start (Cs) modes. Its input includes the collaborative matrix M recording the clicks of known job seekers applying on known job announcements, and the textual description of CVs and job announcements. For simplicity, it will be assumed in the remainder of the paper that a recruiter is interested in a single job announcement; the two terms will be used interchangeably.

In Cs mode, the recruiter is only known from the job announcement document. In Cf mode, the recruiter description also includes the job seekers having clicked on the job announcement, known from matrix M.

Majore Architecture

Majore involves three modules. The first module computes standard vectorial representations, mapping each recruiter/seeker onto IR d . The second module builds a metrics on IR d , either directly, or using matrix M. The third module, implemented as a neural net, trains a specific representation for the job matching task. This representation, hybridizing Collaborative Filtering and Locally Linear Embedding [START_REF] Sam | Nonlinear dimensionality reduction by locally linear embedding[END_REF] and called Cle for Collaborative Local Embedding, is the core of the Majore contribution.

In the following, a recruiter (respectively a job seeker) is denoted x (resp. y).

Standard representations and metrics

Four standard representations are defined. The former two are defined from the textual information; the latter two are defined from matrix M and hence only available for known job seekers and recruiters:

• Primary representation. The primary representation of a recruiter (resp. a job seeker) is the tf-idf representation of the job announcement (resp. CV) with dimension 7,000 (vocabulary size).

• LSA representation. The LSA representation is obtained through singular value decomposition of the primary representation, with dimension (SVD rank) k = 1, 200 in the experiments.

• Collaborative primary representation. The interaction matrix M is normalized by column and then by row, forming the matrix M. The collaborative primary representation of the i-th recruiter (respectively, j-th seeker) is given by the i-th row (resp., j-th column) of matrix M.

• Collaborative SVD representation. This representation is obtained through singular value decomposition of M, with dimension ℓ set to 500 in the experiments.

Two similarities are defined on the top of the above representations. The former one, noted sim is defined by the scalar product of two vectors. The latter similarity, defined on (recruiter, job seeker) pairs and noted sim c , is obtained by convolution of the former similarity with matrix M. Thus sim c gives a matching score between a CV and a job.

sim c (x, y) ∝ x ′ sim(x, x ′ )M x ′ ,y (3)

The Cle representation

As will be seen in section 6, the job matching performances are excellent in collaborative filtering mode while the baseline results in cold-start mode are poor. It thus comes naturally to view the collaborative representations extracted from matrix M (either the collaborative primary or the collaborative SVD representations) as an oracle representation, defining a relevant metrics on the job seeker and recruiter spaces. The goal thus becomes to learn a new representation from the primary representationthe only representation available in cold-start modesuch that it preserves the good topology of the oracle representation.

The proposed approach takes inspiration from the Locally Linear Embedding (LLE) approach [START_REF] Sam | Nonlinear dimensionality reduction by locally linear embedding[END_REF]. LLE, aimed at non-linear dimensionality reduction, expresses any point x i in the initial representation as the weighted sum of its p nearest neighbors, where p is a hyper-parameter of the approach, such that the weights sum to 1: with I e the indicator function of event e (thus ensuring that the numbers of neighbors is equal to p). Dimensionality reduction is thereafter achieved by looking for points z i preserving the same relations as the x i s, i.e. such that ∀i, z i ≈ j w i,j z j Interestingly, the LLE mapping (mapping each x i onto ϕ(x i )) is invariant by translation, homothety and rotation of the x i s.

∀i, x i ≈ j w i,j x j s.t. w i,i = 0, j w i,j = 1 and j I (wi,j ̸ =0) = p
The LLE principle is adapted to find a mapping ϕ from the initial representation IR D onto IR d such that

ϕ * = arg min      i   ϕ(x i ) - j w i,j ϕ(x j )   2 -αR(ϕ)      (4)
where regularizing term R(ϕ), set to the variance of the ϕ(x)s, is meant to avoid the trivial solution ϕ(x) = 0.

Algorithmically, the Cle representation is learned by training a neural net to minimize Eq. 4. The mapping from the input to the first hidden layer is initialized using the LSA decomposition; the following layers are initialized to random matrix (Fig. 4).

The Majore options

Finally, Majore involves 8 options, summarized in Table 1.

Metrics Representation tf-idf LSA M SVD Cle scalar product tf-idf m LSA m SVD m convolution with M (Eq. 3) tf-idf u LSA u M u SVD u Cle u Table 1: The Majore options
Subscript m refers to model-based options, only using the textual information (tf-idf m computes the scalar product of job announcement and CV words; LSA m achieves the dimensionality reduction of the documents) or the interaction matrix (SVD m is the dimensionality reduction of interaction matrix M). All model-based options yield a vector representation for jobs and CVs, enabling to reconstruct M (Eq. 2). Subscript u refers to user-based options, indicating that the considered representation (textual, latent or neural) is used in convolution with matrix M (more precisely, M) to define a finer-grained metrics (Eq. 3). Note that the user-based option only requires the textual description of the recruiter.

Experimental Setting and Goals

The goal of the experiments is threefold. The first one consists of assessing the quality of the data, already discussed in section 3. The second one concerns the performance of the Majore algorithm in collaborative filtering mode.The third goal concerns the Majore performance in cold-start mode, which is the most relevant one from the application viewpoint. In Cf mode, the experimental setting is as follows:

• For each job announcement, one click is randomly removed, defining a lesioned matrix M ′ .

• Majore.Cf is applied on the representation made of all documents and M ′ ;

• For each job announcement, the CVs are ranked by decreasing matching score;

• The rank r(j) of the removed CV is computed;

• The first performance indicator noted Recall @200 (Cf) reports the percentage of job announcements (averaged over 20 independent drawings of M ′ ) where r(j) is less than 200.

• The second performance indicator noted AU C 200 (Cf) reports the area under the recall curve truncated at 200, likewise averaged over 20 independent drawings of M ′ .

In Cs mode, the experimental setting is as follows:

• Recruiters are equi-partitioned in 20 folds;

• Matrix M -i , involving all recruiters except those in the i-th subset, is considered;

• Majore.Cs is applied on the representation made of all documents and M -i ;

• For each job announcement in the i-th subset, the CVs are ranked by decreasing matching score;

• The percentage of true CVs with rank less than 200 is computed, averaged over the 20 folds and over 20 different partitions of the folds; it yields the indicator noted Recall @200 (Cs).

The Majore hyper-parameters are summarized in Table 2. The computation times are measured on IntelCore i7-4800MQ 2.70GHz × 8 with Ubuntu 14.04. LSA with k = 1, 200 requires less than 2 minute computational time.

The Cle neural net (with 7000-1200-1200-1200 architecture and tanh neurons), is trained and converges after a couple of epochs (with 11,000 examples per epoch) with computational time 30 mn. On-going research is investigating the impact of the number of layers and the type of neuron in the neural net.

Empirical Validation of Majore

This section presents and discusses the results obtained for the empirical validation of the Majore algorithm in collaborative filtering (section 6.1) and cold start (section 6.2) modes. The dimension of the LSA (resp. SVD) decomposition is 1200 (resp. 500).

Collaborative Filtering Performances

The very poor performance of the word-only based approach tf-idf m has been omitted (see section 6.2). The counter-performance of the baseline metrics LSA m on the top of the LSA representation is blamed on the poor overlap of the job announcement and CV vocabularies, discussed in section 3. The limitations of the LSA representation can be alleviated using the convolutional metrics (legend LSA u ), with an improvement of circa 25%.

Two main lessons are learned. On the one hand, the relevant information is captured by matrix M, reflecting the interaction history: SVD m , using M singular value decomposition with a baseline metrics yields more than twice better results than using the textual information with LSA m . Still, the convolutional metrics SVD u improves by more than 10% on the baseline metrics SVD m ; a tentative interpretation for this improvement is that the convolutional metrics compensates for the sparsity of M. Finally, the best results are obtained by M u , combining the convolutional metrics with M (though the difference between M u and SVD u vanishes as the rank of the singular value decomposition increases).

The complementarity of the textual and collaborative information is shown in Fig. 5, displaying a few hundred CVs in the 2D plane defined from i) its rank according to LSA u (x axis) and ii) its rank according to M u (y axis), the ideal result being located at (1,1). The vast majority of points have a very low rank according to M u . A few others, with high rank after M u , have a low rank according to LSA u ; however the detailed inspection of these cases did not reveal any general pattern up to now.

The fact that 80% of the relevant resumes are found in the top-200 recommended resumes is very satisfactory from the application viewpoint, as each recruiter (resp. job seeker) commonly examines several hundred CVs (resp. job announcements). This good result confirms the quality and representativity of the data. As already said however, the collaborative filtering setting does not correspond to the general setting of the job matching application, that mostly considers The shortcomings of the textual representation are confirmed by the poor performance of the baseline metrics in tf-idf and LSA representation. The use of the convolutional metrics significantly improves the result, for the raw tf-idf representation and even more so for the LSA representation. The best performances are observed for the Cle representation, and Fig. 6 confirms that Cle consistently outperforms all other options whatever the threshold value. The slight differences observed for LSA m and LSA u between the Cf and Cs modes are explained as i) the LSA representation is not extracted from the same data (all job announcements are considered in Cf); ii) the performance indicator does not measure the same information: in Cf mode, one examines whether one missing click is ranked in the top 200 on average; in Cs mode, one examines the fraction of missing clicks (over all job announcements) ranked in the top 200; the last indicator is biased toward job seekers with more clicks, who are harder to rank due to their diversity. People easily overcome this difference by "reading between the lines" to see whether a given document is relevant to their search. A different story is whether an automatic system can be educated to read between the lines in the same way.

Cold Start Performances

Answering this question leads to further questions. Firstly, how much data is required in order to do so? Secondly, how fast is the e-recruitment domain evolving? Typically, the availability of the e-recruitment platform on mobile phones already had a very significant impact on the information content of the CVs.

A promising approach is presented in this paper, using a deep neural network to emulate the metric properties of the oracle, collaborative filtering-based, representation.

More complex architectures (e.g., handling the geographic information; considering domain adaptation among the job seeker and the recruiter spaces [START_REF] Ganin | Domain-adversarial training of neural networks[END_REF]) will be considered in further work. A main challenge will be to adapt to the evolution of the actual user behaviors, responding to the evolution of the job matching platform.

2

 2

Figure 2 :Figure 3 :

 23 Figure 2: Distribution of the document lengths.

Figure 4 :

 4 Figure 4: The Cle architecture: the number of input neurons is the vocabulary size (7,000); the number of neurons in layers 2 and 3 is the dimension k of the LSA representation (k = 1, 200).

Figure 5 :

 5 Figure 5: Respective performance of LSA u (x axis) and M u (y axis), reporting the rank respectively attributed to 2,000 CVs (the lower the better). While M u outperforms LSA u on average, a small but non negligible minority of CVs is well ranked by LSA u and badly ranked by M u .

Figure 6 :

 6 Figure 6: Majore Cold Start Recall curves: x → percentage of relevant recommendations with rank < x

Table 2 :

 2 The Majore hyper parameters .01 .79 ± .01 .68 ± .01 .57 ± .02 .31 ± .02 Recall @200 (Cf) .93 ± .01 .93 ± .01 .82 ± .01 .74 ± .02 .45 ± .02

	Hyper parameters		Range	Model
	Vocabulary size		7,000 tf-idf, LSA, Cle
	Latent dimension ℓ		500	SVD
	Latent dimension k		1,200	LSA
	Hidden layer size		1,200	Cle
	Number of hidden layers	2	Cle
	Nonlinearity function	tanh	Cle
	α			1	Cle
	Learning rate		20	Cle
	Batch size		100	Cle
	Score	M u	SVD u	SVD m	LSA u	LSA m
	AU C 200 (Cf)	.80 ±

Table 3 :

 3 Majore Collaborative Filtering Performance: percentage of cases where the relevant recommendation is in the top-200, and Area Under the Recall curve (averaged over 20 runs).

Table 4 :

 4 ). While M u outperforms LSA u on average, a small but non negligible minority of CVs is well ranked by LSA u and badly ranked by M u . Majore Cold Start Performances: percentage of relevant recommendations in the top-200, averaged over 20 folds and 20 runs.

	Score Cle new job announcements and CVs. tf-idf m tf-idf u LSA m LSA u

u AU C 200 (Cs) .32 ± .01 .35 ± .01 .29 ± .01 .51 ± .01 .55 ± .01 Recall @200 (Cs) .48 ± .01 .51 ± .01 .42 ± .02 .68 ± .01 .74 ± .01

Table 4

 4 describes the cold-start performances averaged on 20 runs; representative recall curves are depicted in Fig 6.

With kind permission from the Qapa company and its CEO S. Delestre.

In contrast, a typical recruiter post would read: "Administrative assistant : Company of industrial cleaning, sweeping and sanitation. In connection with the accounting department, you will mainly billing. You have a good computer skills (internal software on which you will be trained). You have accounting concepts.".

The fact that merely matching the key words in job announcements and CVs yields poor results will be evidenced in section 6 (methods tf-idfm and LSAm).

Acknowledgments

The first author's PhD is funded by a grant of the Lidex Institut de la Société Numérique from the Paris-Saclay University. All authors warmly thank S. Delestre who made this study possible.


Catherine Dupont
email: catherine.dupont@univ-rennes1.fr

Could occupation duration be related to the diversity of faunal remains in Mesolithic shell middens along the European Atlantic seaboard?

Keywords: Mesolithic, Shell midden, occupation, maritime hunter-gatherers

    Could occupation duration be related to the diversity of faunal remains in Mesolithic shell middens along the European Atlantic seaboard?

Introduction

One of most fundamental criteria defining Mesolithic populations is the way of life of hunter-gatherers. In archaeology, this has classically given rise to a rather negative image of Mesolithic populations, as well as the hypothesis of seasonal (nomadic) settlement. This hypothesis was challenged from the outset by the notion that early farmers stabilized their settlement mode (sedentary versus permanent dwellings) in order to cultivate part of their food [START_REF] Zvelebil | Farmers, Our Ancestors and the Identity of Europe[END_REF]. However, a review of the archaeological literature shows little evidence of this stabilization for Mesolithic people for more than one season. Indeed, while there are some "snapshot" indications of the presence of Mesolithic groups on the coastal belt, such evidence remains almost non-existent for Neolithic populations. However, we could infer that the presence of certain Neolithic populations close to the coast was justified by the presence of certain marine resources that were easier to exploit on a seasonal basis. Such is the case for salt, algae in island environments or certain migratory food resources in estuaries [START_REF] Barber | The Wheeler's site: A Specialized Shellfish Processing Station on The Merrimack River[END_REF]. Moreover, ethnographic examples have profoundly modified the image of an egalitarian nomadic community of hunter-gatherers [START_REF] Testart | Les chasseurs-cueilleurs ou l'origine des inégalités[END_REF]. But where is the archaeological evidence for the presumed presence of populations for more than one season? We wish to address this question by focussing on Mesolithic marine shellfish-consuming populations on the European Atlantic coast. First of all, we present a review of the literature, in the aim of summarizing the published data on issues relating to occupation for the whole set of listed Mesolithic coastal sites. The data used are extracted from an unpublished database which lists all the published archaeological components of Mesolithic shell middens. Based on these data, we address several questions. Do the published studies of Mesolithic shell middens take settlement modes into consideration? If so, what criteria are these based on? When (in which season) were sites occupied? Does the diversity of animal taxa reflect the duration of occupation (more than one season, more than one year…) at a given site? In the same way, could the number of consumed mollusc species be related to a certain degree of stabilization of these coastal populations? We use the database presented here to address these questions in general terms, and then consider the data from recently excavated archaeological sites. The current trend in archaeology aims at intensively excavating or even re-excavating sites in marine environments, namely Mesolithic shell middens, thus leading to new interpretations of these deposits. The diversity of the exploited resources can also be discussed in greater depth here since the detailed nature of the excavations and the specialization of archaeological disciplines allow us to "detect" archaeological remains that were not brought to light by previous excavations.

Materials and methods

We make use of two sources to discuss the settlement mode of Mesolithic populations. The first source is the bibliography listed in the database succinctly presented below. Based on this first approach, we draw up an assessment of our knowledge of the settlement modes of Mesolithic populations along the European Atlantic coast. The second source of information is based on two archaeological sites: Beg-er-Vil (France) and Cabeço da Amoreira (Portugal). These two recently excavated Mesolithic sites enable us to take stock of the archaeological data currently used to discuss the settlement modes of these hunter-gatherer populations. In 2004, a database was created in order to summarize information on Neolithization. This database was intended to be expandable and is amended regularly in the light of new publications. More than 400 bibliographic references responded positively, which allowed us to add data to the existing information.

Database

Unfortunately, it is impossible to mention all the references used in this paper. Here we will describe in detail the contents of the database. It lists the archaeological components of all the Mesolithic and Neolithic sites along the European Atlantic arc where molluscs were consumed (extending from Norway to the south of Portugal). This database is composed of several elements. The period (Mesolithic or Neolithic) attributed to a given site is that mentioned in the most recent publication concerning the site. The evidence supporting these attributions proves to be very varied: absence of domestic animals or cereals, lithic industry, malacofaunal composition and radiocarbon dating. We deliberately used the latest published data for the attribution of the period as criteria change regularly and vary from country to country. The raw dates (shell, wood, animal or human bone… with detailed species when they are specified) used for radiocarbon dates are also mentioned along with references to facilitate recalibration if necessary. The dimensions of shell middens are also listed, along with the proportion excavated, as well as the excavation techniques (with or without sieving), specifying the mesh-size used for sieving when published information is available. The published data relating to the mode of occupation or residence (seasonal, permanent, recurrent or limited in time; based on the size of the site, on its stratigraphy, on the quantity of food waste, on the diversity of lithic tools, on the presence of cemeteries, on sclerochronological analyses on shells or otoliths…), subsistence modes (hunting, fishing, gathering, breeding, farming) and specific characteristics of the site are also reported, while taking care to note the archaeological data on which researchers' interpretations are based. In addition, all the artefacts making up these accumulations are listed: plants (seeds, charcoal, phytoliths, etc.), animals (terrestrial and marine mammals, avifauna, amphibians, reptiles, fish, marine and terrestrial molluscs, crustaceans, sea urchins), as well as lithic and ceramic materials.

For the fauna and flora, the species present as well as their relative abundance are noted when this information is available. As regards the lithic industry, the types of tools and their relative abundance are recorded. For ceramics, the types of decoration and quantities of items are also noted. In this way, 400 shell deposits are described including 248 attributed to the Mesolithic (Fig. 1), 75 to the Mesolithic and Neolithic combined and 77 to the Neolithic. When several phases of occupation can be identified with some being assigned to the Mesolithic and others to the Neolithic, each chronological entity is noted as a distinct site. The sites listed as belonging to both the Mesolithic and the Neolithic correspond to sites where the archaeological composition of the various phases of occupation are not differentiated. The database is cross-examined as part of this study to determine the proportion of Mesolithic shelly deposits where published information on residence is available, and, if so, on what criteria. Many of these shell middens were excavated in the past, others are only known from prospecting.

Through the example of two recently excavated sites, we aim to reconsider the criteria used to determine whether it is possible to assess occupation continuity at certain sites.

Shell middens of Beg-er-Vil and Cabeço da Amoreira

Two shell middens were selected in a second approach to address the occupation mode of maritime populations of hunter-gatherers during the Mesolithic. This choice is based on several common denominators: both sites are currently being re-excavated using similar techniques. Both are dated to the late Mesolithic where the question of a possible continuity of occupation has become even more crucial.

Beg-er-Vil was discovered in the 1970s by G. Bernier (Bernier, 1970;cited in Kayser, 1987). This discovery is related to the erosion of the rocky coast on which the site is located (Marchand et al. in press). The site was then excavated between 1985 and 1988 by O. Kayser over an area of about 20 m² [START_REF] Kayser | Nouveaux objets décorés du Mésolithique armoricain[END_REF][START_REF] Kayser | Les industries lithiques de la fin du Mésolithique en Armorique[END_REF]. Following a rapid resumption of erosion, new excavations have been in progress since 2012 under the scientific supervision of G. [START_REF] Marchand | Maritime hunter-gatherers of the Atlantic Mesolithic: current archaeological excavations in the shell levels of Beg-er-Vil (Quiberon, Morbihan, France)[END_REF]. It would appear that 100 m² of this shell accumulation have now been observed (Kayser, 1990). However, it remains very difficult to evaluate the original area covered by the deposit due to the intense erosion of the coastline in this region, as well as the dissolution of part of the initial shell material and the presence of the dune masking the site, which ranges in thickness from 0.30 to 1.60 m [START_REF] Marchand | Maritime hunter-gatherers of the Atlantic Mesolithic: current archaeological excavations in the shell levels of Beg-er-Vil (Quiberon, Morbihan, France)[END_REF]. The chronological attribution of the site is corroborated by radiocarbon dating results. "Eight new dates have been obtained from short-lived samples (burnt fruit, roe deer bone and twigs or brushwood) which have enabled us to place the formation of this archaeological layer of human origin at the beginning of the 7 th millennium" [START_REF] Marchand | Maritime hunter-gatherers of the Atlantic Mesolithic: current archaeological excavations in the shell levels of Beg-er-Vil (Quiberon, Morbihan, France)[END_REF]. This site is associated with the Mesolithic on account of several factors: the absence of domesticated mammals, the absence of cereals, the dating results and the presence of a lithic industry compatible with the Mesolithic of the Brittany region.

The shell level is approximately 0.50 m thick. The various excavation campaigns have made it possible to observe several anthropic structures: a layer of burnt stone associated with oysters in the unit containing the shell midden, a hearth and pits dug from the shell level down into the substratum [START_REF] Marchand | Maritime hunter-gatherers of the Atlantic Mesolithic: current archaeological excavations in the shell levels of Beg-er-Vil (Quiberon, Morbihan, France)[END_REF].

Unlike the famous sites of Téviec and Hoedic discovered in the same area, and in the current state of research, burials are absent from this human occupation. This refuse zone is made up of abundant and varied food remains (molluscs, fish, crabs, birds, mammals, etc.) and flint knapping waste. Except for charcoal and some rare seeds, plants seem to have been poorly preserved within this accumulation.

During excavations in the 1980s, the sediments of the refuse heap were sieved without being analysed. Their study was resumed following wet sieving with mesh sizes of 1 and 5 mm. Since the renewed excavations in 2012, the sediments of this shell level have been sieved with water (using sea water and rinsing with fresh water) using mesh sizes of 2 and 4 mm [START_REF] Dupont | Marine invertebrates and models of economic organization of the coastal zone during the Mesolithic: French and Portuguese examples[END_REF]. The entirety of the sieve residue is dry sieved to recover the maximum amount of information from these zones occupied by human groups. A prolonged period of occupation has been proposed for this site, but without any proof of its continuity [START_REF] Dupont | Harvesting the seashores in the Late Mesolithic of north-western Europe. A view from Brittany[END_REF].

According to the model of economic organization along the French coastal fringe during the Mesolithic, Beg-er-Vil can be classified as a base camp with varied activities, as opposed to outlying stations with less varied activities and undoubtedly short durations of occupation [START_REF] Dupont | Coastal exploitation in the Mesolithic of western France: la Pointe Saint-Gildas (Préfailles)[END_REF].

The geographical position of the Cabeço da Amoreira shell midden is different from that of Beg-er-Vil with respect to marine influence. It is located along the River Muge inland from the coast. In contrast to Beg-er-Vil, which is still subject to the action of the sea, Cabeço da Amoreira is currently cut off from marine influences.

During the Mesolithic, however, the latter site was subject to marine influence, probably in the vicinity of vast [START_REF] Zbyszewski | Découverte d'un instrument préparé sur bois de Cervus elaphus au Cabeço da Amoreira (Muge)[END_REF][START_REF] Zbyszewski | Découverte d'un instrument préparé sur bois de Cervus elaphus au Cabeço da Amoreira (Muge)[END_REF] and José Rolão from 1998 to 2001 [START_REF] Rolão | O complexo Mesolítico de Muge: novos resultados sobre a ocupação do Cabeço da Amoreira[END_REF]. Since 2008, excavations have been resumed by Nuno Bicho [START_REF] Bicho | The 2008-2010 excavations of Cabeço da Amoreira, Muge, Portugal[END_REF]. The surface area of the shell heap is evaluated at 14,000 m² [START_REF] Cunha | The osteological series from Cabeço da Amoreira (Muge, Portugal)[END_REF]. The chronological attribution of the site is corroborated by radiocarbon dating. Although there is evidence for several occupations, the shell midden layers were deposited between ca. 7700 and 7600 yrs cal BP (2D) [START_REF] Bicho | Chronology of the Mesolithic occupation of the Muge valley, central Portugal: The case of Cabeço da Amoreira[END_REF]). In addition to dating, several other factors associate this shell midden with the Mesolithic: the absence of domesticated mammals, the absence of cereals and the presence of a lithic industry compatible with the Mesolithic typology of the Muge complex.

The currently visible part of the deposit is ca. 60 m in diameter and nearly 3.5 m high at the centre [START_REF] Bicho | The 2008-2010 excavations of Cabeço da Amoreira, Muge, Portugal[END_REF]). Habitat structures have been observed inside the shell midden, including hearths, postholes and pits. Unlike at Beg-er-Vil, burials are associated with the Cabeço da Amoreira shell midden. The refuse zone is made up of abundant and varied food remains (molluscs, fish, crabs, birds, mammals, etc.) and charcoal.

As with the Beg-er-Vil site, in spite of sieving during the successive excavations, the sieve residues have not been completely sorted. Often, only the most striking items were extracted from the sieve residue (ornaments, lithic materials, bones), and this operation was carried out directly in the field. The sieving protocol undertaken since 2011 for Cabeço da Amoreira is similar to that applied in Brittany. All the sediments are dry sieved using two screens, with mesh sizes of 6 mm and 2 mm [START_REF] Bicho | The 2008-2010 excavations of Cabeço da Amoreira, Muge, Portugal[END_REF]. The largest elements are extracted during the excavation and all the remaining sieve residues are then sorted in the laboratory. The duration of occupation for this shell midden is unknown, but is likely to have been long [START_REF] Dupont | Marine invertebrates and models of economic organization of the coastal zone during the Mesolithic: French and Portuguese examples[END_REF].

Results and discussion

First of all, a review of the mode of residence of Mesolithic maritime populations along the European Atlantic seaboard was carried out using the database. The database highlights a major bias: the geographical distribution of the sites is heterogeneous (Fig. 1). Site distribution is largely controlled by a whole range of phenomena accompanying the latest marine transgression (Dupont 2006, pp.7-8;[START_REF] Gutiérrez-Zugasti | Shell midden research in Atlantic Europe: state of art, research problems and perspectives for the future[END_REF]. In this way, the rise in sea level led to the erosion of the most exposed coasts, the burial of other zones beneath several metres of mud or sand, as well as the immersion of certain Mesolithic coasts. For example, the coasts of the South of France and the North of Portugal are covered by sand dunes which mask the old shoreline. This bias should be taken into account considering that we can only observe part of coastal Mesolithic occupations, and clearly limits the interpretation of data. A total of 248 Mesolithic shell middens are listed (Fig. 1). Issues relating to occupation residence are only mentioned in 19% of these cases (Fig. 2). While this proportion appears to be very low, it can be partly explained by the fact that some of the sites are only known from prospection and the long-established investigation of certain excavations. In addition, the underlying assumption that hunter-gatherers were mainly nomads could also have led certain researchers to elude the question in the past. Moreover, the results of this review are well contrasted when we consider the permanence of the occupation. For 22% of the 46 Mesolithic sites (Fig. 2), occupation duration is unknown, while temporary camps are evoked for 50% of the sites. However, the possible permanence of these settlements is proposed for 28% of the 46 Mesolithic sites. This last statistic remains high for populations entirely dependent on resources provided by nature. How should we interpret such a high proportion? It could be related to a bias in the representativeness of archaeological sites. For example, sites with a longer duration of occupation could correspond to sites where anthropic activities produced greater amounts of waste, thus making them more visible in the coastal landscape. If we only consider the criteria used to diagnose the occupation mode of populations, we observe a shortcoming in the published data. The criteria used to identify the suggested hypotheses are absent for 16% of the sites where an occupation mode is proposed (no permanence/possible permanence). This proportion is high considering the implications of the mode of occupation on the organization of hunter-gatherer populations. Unfortunately, the diverse archaeological disciplines are rarely applied to a same site and it is not so easy to recapitulate the occupation mode of these Mesolithic populations for the whole Atlantic façade. For sites where the original data sources for models of economic organization are published, 12 principal criteria can be identified (Fig. 2: source of occupation). Some of them are used individually, whereas others are combined in publications.

For example, some authors consider the shell midden more in terms of a single habitat. Thus, if the overall surface area of a midden is extensive, this is used to back up the hypothesis of a permanent occupation [START_REF] Rolão | Del Wurm final al Holocénico en el Bajo Valle del Tajo (Complejo Arqueológico Mesolítico de Muge)[END_REF]. This type of result remains contentious in the absence of meticulous excavations, since multiple excavations carried out on these shell levels sometimes reveal complex stratigraphies which are far from compatible with constant occupation [START_REF] Arnaud | Os concheiros mesoliticos dos vales do Tejo e Sado : semelhanças e differenças[END_REF][START_REF] Bicho | Chronology of the Mesolithic occupation of the Muge valley, central Portugal: The case of Cabeço da Amoreira[END_REF][START_REF] Roksandic | The Cultural Dynamics of Shell-Matrix Sites[END_REF]. In some cases, this same stratigraphy is studied in great detail to determine if the shell midden was abandoned during its formation (Merchant et al. in press). Dating results are also used to define the periods of occupation of human groups at certain sites (O' Sullivan and Breen, 2011). Other authors take into account the diversity of lithic components [START_REF] Valente | Recent Developments in Early Holocene Hunter-Gatherer Susbsistence and Settlement : a view from southwestern Iberia[END_REF] or the presence of structures such as hearths or burials [START_REF] Lubell | The Mesolithic-Neolithic transition in Portugal, Part1 : isotopic and Dental evidence of Diet[END_REF][START_REF] Cunha | The osteological series from Cabeço da Amoreira (Muge, Portugal)[END_REF][START_REF] Valente | Recent Developments in Early Holocene Hunter-Gatherer Susbsistence and Settlement : a view from southwestern Iberia[END_REF]. The accessibility of animal resources is also used, based on current animal ethology, as well as the definition of the season of capture of certain species, with methods such as sclerochronology [START_REF] Indrelid | Mesolithic economy and settlement patterns in Norway[END_REF]Mannino andThomas, 2007, Schaller Ahrberg, 2007;[START_REF] Gutiérrez Zugati | An examination of Mesolithic shellfishing activities in the lower Asón river basin (Cantabira, Spain)[END_REF]Carter, 2009;[START_REF] Dupont | Harvesting the seashores in the Late Mesolithic of north-western Europe. A view from Brittany[END_REF][START_REF] Mannino | The strategy of the shoreline? Social ecology of Mesolithic coastal subsistence, with reference to the site of Culverwell, Isle of Portland[END_REF]. Biases may arise on account of the fact that animal behaviour could have evolved since the Mesolithic in response to climatic variations, among other factors. In addition, periods of accessibility to food resources do not necessarily coincide with the presence of humans at the site. Some of the resources may have been stored or prepared. The storage of dried fruit, such as hazelnuts, is widespread but cannot be envisaged for the consumption of fruit with a short storage life, such as the wild pear. The transformation of fresh products into processed products (by drying, for example) could delay their consumption, which would therefore offset the information concerning human occupation. Nonetheless, most of these animal taxa are restricted to the marine environment (birds, shellfish, crabs and fish) and were generally consumed immediately by the coastal populations. Clearly, each of the criteria presented here has its drawbacks. This explains the cautious approach of archaeologists who almost unanimously evoke the "possible" permanence of habitats. The higher the diversity of the observed faunas, the more we are able to link up periods of resource accessibility to extend the snapshots of the presence of these coastal populations on the coast. Unfortunately, the whole range of archaeological disciplines is not systematically applied. After this rather negative evaluation of the archaeological resources used to define Mesolithic population occupations, we needed to examine the archaeozoological data in order to assess whether this diversity could reflect the more or less extended presence of humans near the coast during the Mesolithic. One of questions addressed here is also to determine whether the diversity of the exploited marine resources is related to the duration of human occupation on a site. For example, we could imagine that a short duration of occupation on the coast might enable the populations to exploit only the most visible and accessible resources, whereas a human population established for a more long term occupation would have time to explore all the environmental niches around the site. In Fig. 3, we compare the diversity of animal taxa (marine molluscs, fish, birds, crabs, mammals, terrestrial mammals, sea urchins, crabs, etc.) between Mesolithic and Neolithic sites along the French Atlantic seaboard. The x-axis coordinates represent the number of taxonomic classes identified in the shell middens (for example, 1 indicates the presence of shellfish alone, while 2 indicates the presence of shellfish associated with another animal class such as birds or mammals, and so on). This diagram shows that there is no overall difference between the diversity of animal classes exploited during the Mesolithic and the Neolithic along the European Atlantic seaboard (Fig. 3). For each economic system, whether it is based on hunting, gathering, fishing or breeding, the diversity of the animal classes used is very heterogeneous and varies from 1 to 6. Thus, there does not appear to be any direct link between the diversity of the exploited animal classes and the duration of occupation of the coastal sites. This variability can be masked by the diversity of human behaviours and circumstances. In the following discussion, we revisit the case studies described above.

Fig. 3 -Number of animal classes described in Mesolithic and Neolithic European shell middens

In the presentation of these case studies, we mentioned that the Beg-er-Vil site can be incorporated into an established model of economic organization in France [START_REF] Dupont | Coastal exploitation in the Mesolithic of western France: la Pointe Saint-Gildas (Préfailles)[END_REF], as it is classified as a base camp with prolonged occupation. The question of diversity is a crucial factor in this classification. Indeed, the broad range of animal classes exploited at Beg-er-Vil (6 animal classes represented) implies a wider exploitation of the environment. In the same way, when considering mollusc species, it is necessary to include 5 additional species to obtain 90% of the total MNI of seashells. Nine species represent more than 1%: Mytilus edulis, Cerastoderma edule, Patella sp., Littorina littorea, Phorcus lineatus, Ruditapes decussatus, Nucella lapillus, Ostrea edulis and Scrobicularia plana; Fig. 4). According to the sites incorporated in the database, 4.4 mollusc species represent on average more than 1% of the total MNI in Mesolithic shell middens. Thus, Beg-er-Vil is highly diversified in terms of the exploited species. The percentage of the total MNI and the abundance of these species demonstrate their use as food resources and their deliberate harvesting (Fig. 4). The gathering of these species in the vicinity of the site requires an unquestionable knowledge of the territory surrounding the habitat. These species also reflect the exploitation of three types of substratum from environments accessible less than 5 km from the shell midden, with a strong contribution from the rocky environment. In the model of economic organization for the French Atlantic coast, Beg-er-Vil clearly contrasts with outlying sites with very short durations of occupation. The site of Saint-Gildas 1b belongs to this latter occupation category, with only one animal class making up the diet: molluscs. No other animal or plant remains have been found there. Two mollusc species are represented at Saint-Gildas 1b, accounting for more than 1% of the total MNI (with Scrobicularia plana making up 95% and Patella sp. 2% out of a total MNI of 217). The muddy environment at this site was largely exploited, along with very small proportions of molluscs from the rocky substratum. Thus, for these two examples in France, as well as other archaeological criteria [START_REF] Dupont | Coastal exploitation in the Mesolithic of western France: la Pointe Saint-Gildas (Préfailles)[END_REF], the diversity as well as the number of intentionally exploited animal classes and mollusc species seems to be an index of the duration of occupation of the populations using these sites. For Beg-er-Vil, this hypothesis is supported by the combined accessibility of all the discovered food resources all year round [START_REF] Dupont | Harvesting the seashores in the Late Mesolithic of north-western Europe. A view from Brittany[END_REF]). But with the example of Cabeço da Amoreira, we will see that this schema is not as simple as it seems. Having recently studied the marine invertebrates of Cabeço da Amoreira using similar techniques to those used at Beg-er-Vil, it seems appropriate to compare these sites from the point of view of taxonomic class diversity and malacofaunal distribution. This comparison is also justified by the fact that archaeologists propose a long occupation duration for Cabeço da Amoreira [START_REF] Dupont | Marine invertebrates and models of economic organization of the coastal zone during the Mesolithic: French and Portuguese examples[END_REF]. Moreover, in contrast to Beg-er-Vil, the Portuguese site was located close to an estuary during its exploitation [START_REF] Van Der Schriek | Holocene Paleoecology and floodplain evolution of the Muge tributary, Lower Tagus basin, Portugal[END_REF]). In the case of Beg-er-Vil, the number of described animal classes (6) indicates very high diversity. On the other hand, at Cabeço da Amoreira, only two mollusc species account for more than 1% of the total MNI (Scrobicularia plana and Cerastoderma edule; Fig. 4). This low number of species is comparable to the results obtained for the site of Saint-Gildas. However, this poor diversity represents the environments accessible within a 5 km radius around the site. Thus, even if the site of Cabeço da Amoreira displays evidence of a prolonged occupation, the nearby environmental constraints limited the possibility of exploiting a varied number of molluscs.

Conclusions

It is difficult to investigate the occupation mode (nomadic or permanent/sedentary) and duration (all year round, seasonal…) of the Mesolithic shellfish-consuming populations who settled along the European Atlantic coast. Many factors interfere with the overall vision of the archaeologist. Even the distribution of archaeological sites on the coast is largely shaped by the effects of tidal fluctuations. Moreover, a review of the literature shows that the question of occupation mode has not been systematically addressed for all the sites. In cases where this issue is discussed, certain authors acknowledge their inability to identify any occupation traces of these huntergatherer populations, due to the lack of evidence or preservation of the artefacts. Even if new lines of research are opened up by addressing the occupation mode of Mesolithic hunter-gatherers along the European Atlantic seaboard, such an approach would tend to "smooth out" the data. Various authors have made use of at least 12 different criteria to define the season of occupation of Mesolithic coastal habitats. Sometimes these criteria are combined, sometimes not. Therefore, the heterogeneity of the published data makes a synopsis of the topic difficult. The results obtained from this approach show various occupation modes at Mesolithic sites during all seasons in a more or less occasional manner. On account of the difficulties involved in describing the permanence of a settlement, archaeologists are cautious in publications concerning the topic and many of them merely allude to the "possible" permanence of the presence of Mesolithic people near the sea. By studying the diversity of exploited faunas, we can obtain an index of the adaptation of human populations with respect to their nearby environments. Such an index is provided here by the number of animal classes represented at the scale of the European Atlantic seaboard. This approach shows that scenarios are very variable and that the representation of sites tends to decrease as the number of exploited animal classes increases. Two major variables should be taken in account to explain the diversity of scenarios and the fact that the range of exploited faunas cannot be used as an indication of occupation duration: the accessible environments and the resources they procure and the behaviour of coastal populations. If maritime people only have access to one kind of environment, they seem to be able to adapt to this unique environment and to the low faunal diversity. It appears that in spite of this low diversity they can establish long occupations (spanning more than one season).

This is what we seem to clearly observe in Cabeço da Amoreira. As for the second variable, i.e., the importance of the diversity of human behaviour in the same environment, we clearly observe that Mesolithic people on French coasts with access to a large diversity of marine environments explored and exploited the diverse resources in the nearby environment [START_REF] Dupont | La malacofaune de sites mésolithiques et néolithiques de la façade atlantique de la France: Contribution à l'économie et à l'identité culturelle des groupes concernés[END_REF]. During the Neolithic, the appearance of predicable resources like cereals and cattle completely alters the behaviour of coastal populations. Unlike Mesolithic people collecting very diverse shell types, Neolithic populations mainly select two mollusc species among the more accessible species at low tide, for example [START_REF] Dupont | La malacofaune de sites mésolithiques et néolithiques de la façade atlantique de la France: Contribution à l'économie et à l'identité culturelle des groupes concernés[END_REF].

It is always fundamental to return to the scale of an archaeological site to improve our understanding of the behaviours of Mesolithic coastal populations. Thus, for sites in the course of excavation at Beg-er-Vil and Cabeço da Amoreira, prolonged occupation by Mesolithic populations seems to be corroborated by the diversity of the animal classes exploited, as well as other previously exposed criteria (stratigraphy, presence of structures, burials, diversity of lithic tools...). Both of these sites show a taxonomic diversity of 6 at a class level. In the same way, the number of exploited shellfish species could very well increase with the duration of occupation. Prolonged occupation would allow human groups to explore all the niches along the foreshore. This latter scenario could well be applied to Beg-er-Vil, where 9 marine mollusc species were harvested in environments close to the site. Among these mollusc species, we note the presence of the dog whelk Nucella lapillus. Along with the sites of Oronsay (Scotland), this occurrence represents one of the rare traces of the consumption of this gastropod [START_REF] Mellars | Oronsay: prehistoric human ecology on a small island[END_REF]. At Beg-er-Vil, shellfish were harvested from rocks, sands and mudflats. Species diversity falls to 2 for the Mesolithic shell midden at Cabeço da Amoreira in Portugal. Should we conclude from this that occupation duration was short in spite of other archaeological evidence from the site indicating a wellestablished long-term coastal population? We consider that such a conclusion is unfounded. The relatively low number of exploited molluscs seems to be constrained by the rare occurrence of environments with diverse species that could be used as food resources. Thus, although the Cabeço da Amoreira example shows that environmental pressure is high, this did not prevent the stabilization of this Mesolithic population throughout the course of time.

The permanent nature of occupation can be raised but is difficult to prove for the European Atlantic seaboard as a whole as well as for the analysis of individual sites. The concept of occupation duration is a problematic issue.

Occupations can be intermittent over a relatively long period of time and continuity is hard to demonstrate. The diversity of the exploited resources is not directly related to the duration of occupation: it is often necessary to address this issue case by case. Two major parameters can interfere with the combination of diversity and occupation duration: the accessibility of resources in the vicinity of the site and human behaviour (such as cultural preferences) linked to the high diversity of ways of life of coastal human populations.

Fig. 1 -

 1 Fig. 1 -Map of the Mesolithic shell middens listed in the database and location of the sites mentioned in the text

Fig. 2 -

 2 Fig. 2 -Summary of literature concerning the mode of occupation of coastal shellfish-consuming Mesolithic populations with sources of criteria used to define the model of economic organization

Figure 4 -

 4 Figure 4 -Malacofaunal distribution of the sites of Beg-er-Vil and Cabeço da Amoreira and their original substrata

  mudflats[START_REF] Van Der Schriek | Holocene Paleoecology and floodplain evolution of the Muge tributary, Lower Tagus basin, Portugal[END_REF]. Discovered by Carlos Ribeiro and Pereira da Costa in 1863[START_REF] Ribeiro | Les kjoekkenmoedings de la vallée du Tage[END_REF], the site has been excavated several times by different teams: F. Paula de Oliveira between 1884 and 1885, MendeCorrêa, Serpa Pinto and Santos Junior from 1930 to 1933, Jean Roche and Veiga Ferreira from 1952 to 1965

Acknowledgements

The author would like to thank the archaeologists who contribute to the excavation of shell middens with adapted methods. The first version of the database was supported by the project "Action Concertée Incitative jeunes chercheurs 'La néolithisation de la façade atlantique de l'Europe : interactions culturelles, transferts techniques, implications des milieux naturels' (Supervisor: G. Marchand / UMR 6566) (2002)(2003)(2004)(2005)". We also thank the two anonymous reviewers who helped to improve this paper.