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Fishery and Sea Resourcesnetic heterogeneity in the Adriatic stock,so that the morphometric and meristic differencesfound by previous studies seemto be mainly due to environmental factorsrather than to the existence of two geneticallydistinct stocks. Anchovy spawningactivity takes place in the coastal watersof the western Adriatic between theGulf of Trieste and the Gargano Promontory[8, 10] and the largest number of eggsoccurs in the Gulf of Trieste and off theriver Po Delta [11]. Growth rates, morphometricand allozyme frequency analysessuggested the existence of two distinctstocks: one in the northern Adriatic,between Chioggia and Ancona, andthe other one in the area between SanBenedetto and Vieste. However, this featureis still unclear and needs further studies[5]. These issues are of great importancefrom a management point of view:in fact, in a closed basin like the AdriaticSea, agreement between countries exploitingthe same stocks is crucial. Since1975, CNR-ISMAR-Ancona has been conductingresearch on the biology and assessmentof the stocks of anchovy and sardinein the central and northern Adriatic Sea.Stock assessment has been carried out bymeans of population dynamics and acousticmethods. In the present work, emphasisis given to the former ones. These methodsare based on mathematical models andalways require catch data, along with informationon other features like, for example,the component of mortality not due to fishingactivity. One of the methods mainlyused for the two stocks [3, 4, 12, 13] isrepresented from Virtual Population Analysisor VPA, which requires catch data distributedinto age classes [14]. For both anchovyand sardine, total catch (i.e. fromfleet of both western and eastern side ofAdriatic) and stock biomass estimated byVPA are compared over years in Figure 1[15]. For anchovy, split year data wereused, assuming the first of June as the birthdate, consistently with the biology of Adriaticanchovy (e.g. split 1976 is from calendarfirst of June 1975 to the 31st May1976). The annual biomass at sea of thesestocks estimated by VPA display strongfluctuations along the last three decades.Anchovy progressively decreases from theend of the 1970s to 1987, the year of lowestabundance and fishery crisis, followedfrom partial recovery and a peak in recentyears; sardine abundance, on the otherhand, shows a continuous decrease of thepopulation since the middle of the 1980s[3, 4, 16, 17, 18, 13]. These patterns ofabundance over years are not very differentform those obtained by means of theecho-surveys carried out by the same CNR-ISMAR-Ancona [18]. Likely the decreasingtrends are imputable not only to thefisheries. In fact, the stock dynamics canbe influenced by physical oceanographicfactors [19, 14, 20]. From this point ofview the Adriatic Sea is emblematic becauseof the great number of environmentalvariables that affect daily and seasonallythe main characteristics of this basin(e.g. temperature, salinity, food availability,circulation). The general circulationis cyclonic with important influence, especiallyin the northern sub-basin, exerted byPo River runoff and dense water formationprocesses. Winds are able to induce relevantchanges in current patterns at severalspatial and time scales, with relevant differencesbetween winter and summer windregimes. Besides that, the great surface relatedto the small volume makes this basinstrongly variable regards several parameters:e.g. temperature in winter it is characterizedby a vertical homogeneity and bya pronounced horizontal thermal gradient1898