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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.

On the basis of trawl survey data collected during the MEDITS project (1994-1999), the distribution of Mediterranean hake populations was described in coastal areas corresponding to about three quarters of both the latitudinal and longitudinal extensions of the Mediterranean Sea, and in the 0-800 m range. Abundance and biomass indices (in terms of number and kg per km2) are presented in 15 geographical sectors and 40 subareas from the Alborán Sea to the Aegean Sea. A statistical analysis by generalized linear modelling performed on such indices per main national blocks (Greece, France, Italy and Spain), and considering the effects of the six years of sampling and the depth, showed that biomass increased from west to east, while for all areas the most significant depth effect appeared between 100 and 200 m, corresponding to the depth range that hosts most of the summer nursery areas. The overall size-frequency distributions in shelf and slope waters, average sizes and total mortality coefficient Z per sector showed that the bulk of the MEDITS samples consisted of young individuals and, with only a few exceptions (western Sardinia, central Aegean), a generalized condition of growth overfishing was apparent. However, no negative trend was found, either in biomass or in average size of fishes, during the six years of the present study. Recruitment patterns were studied by distinguishing the youngest fish as Gaussian groups (modal length from 6 to 9 cm total length) in length-frequency distributions of each subarea and main stratum (shelf and slope). Considering the timing of MEDITS surveys (May-July), this approach emphasised the nurseries in the northern part of the western Mediterranean (Gulf of Lions, Ligurian Sea, northern Tyrrhenian Sea) and in the northern central Mediterranean, where recruitment occurs mainly during spring. On the other hand, few recruits were found in part of the central and particularly in the eastern Mediterranean, where recruitment occurs mainly in summer. MEDITS recruitment patterns therefore gave support to distinctions among Mediterranean hake populations, previously suggested on the basis of vertebral counts and other morpho-physiological, albeit fragmentary, data. In particular the central Mediterranean hake populations do not seem to be homogeneous, with an Adriatic unit very close to the northwestern one. The MEDITS experience therefore suggests that future management measures could take into account both general and regional patterns, the latter concerning mainly the younger fraction of the populations.

The species richness pattern of groundfish species in the entire northern Mediterranean Sea was examined at 3 spatial scales: region, large biogeographical zone and basin. We analysed 1914 trawl hauls collected using a single sampling design in the trawlable areas of the continental shelves between the Strait of Gibraltar and the Strait of Dardanelles (from 36.3 to 45.7°N and 5.3°W to 28°E). Spatial pattern in species diversity was assessed using complementary methods (Chao2 estimates of total species richness, mean species richness and beta diversity). No matter which scale was used, the expected longitudinally decreasing trend in species richness, which has been widely described in previous studies, did not appear when comparing estimates of total species richness per unit of area. Only the mean species richness pattern showed a moderate eastwards decrease at the largest spatial scale, but the trend progressively disappeared as the scale of analysis was reduced. In contrast to what is usually expected, our results suggest that Atlantic inflow does not play a key role in the present spatial pattern of fish species richness within the northern Mediterranean Sea. Furthermore, we show that the Aegean Sea can no longer be considered the least species-diverse zone in the northern Mediterranean Sea. Our results provide the first description of a quantitative 'reference state', with which the temporal changes in species richness patterns throughout the entire northern Mediterranean Sea can be compared in the future. KEY WORDS: Species richness · Beta diversity · Groundfishes · Mediterranean Sea · Large scale · LessepsianResale or republication not permitted without written consent of the publisher

Recent demographic structure and distribution of the fish species Zeus faber in the Adriatic Sea were investigated in this study and compared with a 1948-1949 study. The presence of John Dory in the Adriatic Sea was recorded at a depth range from 21 to 239 m, with the highest biomass indices at depths between 50 and 100 m. The length frequency distribution, length-weight relationships, sex ratio and length at first sexual maturity were determined. Results based on data from the 1948-1949 ÔHvarÕ expedition and the MEDITS surveys of 1996-1998 showed in the 50-year interval that the index of biomass and area of distribution of this species in the Adriatic Sea have been reduced and that the demographic structure of the population has changed. Negative changes described in the paper indicate an overfishing of the Z. faber species in the Adriatic Sea.