SUMMARY:The main characteristics concerning distribution, size structure and total mortality of two of the most important decapod crustaceans of commercial interest in the Mediterranean Sea, Nephrops norvegicus and Parapenaeus longirostris, are studied along the European Mediterranean coasts. The study is based on data collected during a series of six trawl surveys performed in spring from 1994 to 1999 from the Gibraltar Straits to the Aegean Sea. The population size structure identified in the different geographical sectors is analysed taking into account two bathymetric sectors: continental shelf and upper slope. Differences in N. norvegicus population demographic structure among geographical sectors, as well as in total mortality, appear to be highly related to different exploitation levels. Size structure in P. longirostris also shows a great heterogeneity throughout the different geographical sectors. Considering that both species are heavily exploited all along the studied area, the observed differences can be interpreted as different responses to exploitation related to the widely differing life history characteristics of the mentioned species. In fact, N. norvegicus is a long-lived, benthic, burrowing species with low growth and mortality rates, and P. longirostris an epibenthic short-lived species characterised by higher rates of growth and mortality.

Species diversity assessments are an important step to evaluate the conservation status of a community, both in marine and terrestrial ecosystems. These assessments are pivotal if related to both, the constant increase of human pressure on ecosystems and the anthropogenic climate change occurring nowadays. Sharks and rays are globally threatened, and the situation is particularly alarming in the Mediterranean Sea where more than 50% of species are listed at risk of extinction by the International Union for Conservation of Nature (IUCN). In this paper, we revise and discuss the chondrichthyan species richness of the Mediterranean and the Black Sea. Through an accurate review of published taxonomic studies, historical data on species occurrence, analyses of scientific survey data and biodiversity databases and other scientific papers, we produced a revised list of species whose presence in the Mediterranean Sea is confirmed or highly probable and discussed on current taxonomic and occurrence disputes on the species that are instead rarer or claimed to be locally extinct. We listed a total of 88 species, representing 30 families and 48 genera that are currently present in the Mediterranean and the Black Sea. This number includes 48 shark species, 38 batoids, and 2 chimaeras. The review represents a reference for future conservation assessments of cartilaginous fish in the region and a guide for decision-makers when promoting the sustainable exploitation of fisheries resource within an ecosystem-based framework. This paper can help to set a baseline of the Mediterranean species and thus resolve some uncertainties regarding their conservation status, explaining the reasons for their prolonged absence in the reports. Indeed, failure to record over time may not be due to grubbing up, but because after careful review this species was not really part of the Mediterranean fauna.

No abstract

Thc paper explores the apparcnt contradiction between a high trawling pressure on juveniles and sustained production of hakc that has occurred over the last decade in many Mediterranean fisheries. The practical consequenccs are followed of assurning rapid declines in natural mortality rate M in the first few years of life to a low, constant adult natural mortality, as well as the observation, for small-mesh trawl cod ends, of dcclining availability with agc. Several approaches arc proposed for fitting declining M-with-age with a rcciprocal function for hakc, using criteria based on rncan life-time fecundity, mcan age at egg production, existing estirnates of adult M, and vectors bascd on stock productivity assumptions. All vectors of M-at-age were similar to MSVPA estimates of North Sca stocks.The implications of the changes in mortality with age for stocks harvested by fine-mesh trawls were explored in yield per recruit calculations under 2 different hypotheses: 1) using current cstimates of growth and adult rnortality, 2) M-at-age vcctors for juveniles, dropping rapidly from age O+, and declining availability to trawling for oldcr fish. These hypotheses were compared within yicld per recruit analyses. Under the new assurnptions, given current F>>M (adults), yield isopleths predict no significant increases in Y/R with stretchcd mesh > 40 mm, but a substantial decline in fecundity per recruit with small incrcases in effort by gill nets or longlines, aimed at mature fish. These results are linked to the refugium concept for older fish, and it is speculated that this may be in part responsible for the continued productivity of other sustained fisheries for juvenile resources elsewhere.