Stock units: Identification of distinct biological units (stock units) for different fish and shellfish species and among different GFCM-GSA

STOCKMED has originally tackled the identification of the stock units and their boundaries for 19 species of commercial interest in the Mediterranean. The study is based on available data concerning all those domains that are recognized in the literature as determinants in multidimensional identification of stocks. During the project, since these data derive from studies carried out for purposes other than the identification of units of stock, it was necessary to develop an appropriate methodology that allowed dealing in a rigorous way with the problem. The developed methodology has allowed devising a framework that combines spatial analysis (Constrained Clustering) and Multi Criteria Decision Analysis (MCDA), incorporating geo-referred information derived from both biological (e.g. abundance, growth, maturity and parasite) and environmental (e.g. habitats and currents) domain. Considering that detailed knowledge about the spatial distribution of populations are essential to identify stock boundaries, the hypothesis to be tested with the MCDA were constructed using the information of scientific surveys carried out within the Data Collection Framework funded by the European Union . The results of the analysis depend on both the number and the quality of the indicators used. Despite the difficulties of integrating in a rigorous analytical procedure diverse information types (based on different spatial scales and time intervals), the methodology developed has been able to identify possible distribution patterns of stock, including the assessment of their uncertainty. These represent the first example of the distribution of units of stock in the Mediterranean according to a holistic approach. For management purposes, the distribution of the stock of each species was compared with available information on the distribution of fishing fleets. This was used to provide a guideline on areas to be considered homogeneous for the assessment and management of resources. Depending on the quality of the available data, both in terms of discriminatory ability and spatial/temporal coverage, the distribution patterns obtained do not all have the same degree of plausibility. For example while some species (e.g. Merluccius merluccius) show a high degree of acceptability others present a high degree of uncertainty (e.g. Eledone moschata). However, the distribution patterns obtained represent a starting point to improve the knowledge of the distribution of the stocks in the Mediterranean. These proposed patterns will be subject to re-evaluation according to the improvement of knowledge on stock properties. One of the main difficulties in this study was that in the majority of the cases the information used, were not derived from studies done with the specific aim of identifying stock boundaries. Therefore the units of stock with relative boundaries provided by STOCKMED should be reassessed in the future in order to provide a specific collection of data similarly to methodologies proposed in other areas (ICES). In order to achieve this the knowledge gaps need to be filled. In particular modelling the retention and/or dispersion of eggs and larvae from the spawning ground in relation to physical oceanographic processes could represent a valid approach.. For some selected species (for which the ecology is well documented) eggs and larvae, , will be represented as Lagrangian drifters released in the main spawning ground. Successively the simulated transport of the pelagic stages will be considered to identify connectivity mechanism among stock subunits and units. Another gap to be overcame is to improve the use of 'last generation' markers (SNPs) (e.g. Engraulis encrasicolus), coupled with an extensive sampling scheme realized within the framework of EU funded projects (e.g. Merluccius merluccius and Solea solea), which markedly increase the power of genetic data for stock identification. Although the growing importance in the international literature, studies on micro-chemical composition and shape of otoliths are almost absent in the Mediterranean. Furthermore investigation on adults migration and movements, which is a main topic in stock identification, is very poor and needs to be improved at Mediterranean scale. Finally a major availability of standardized information on the spatial distribution could improve the accuracy in selecting data for stock assessment and the consequent adoption of appropriate management measures.