Comparing demersal megafaunal species diversity along the depth gradient within the South Aegean and Cretan Seas (Eastern Mediterranean)

Abstract

Knowledge on biodiversity patterns of demersal megafaunal species in the Mediterranean and particularly in its eastern basin is still very scarce. In the present study, fine-scale diversity patterns in relation to depth were analyzed for three major megafaunal groups (fish, cephalopods and crustaceans) in three subareas of the eastern Mediterranean (Crete, Cyclades and Dodecanese islands). The analysis was based on data from the Mediterranean International Trawl Survey conducted during 2005–2014 and the relationship between depth and two different diversity measures (species richness and Shannon-Weaver) was examined using Generalized Additive Modeling (GAM) techniques. Species richness of fish decreased with depth in two of the three subareas (Cyclades, Dodecanese), while the opposite was true for crustaceans in all subareas. Cephalopods had higher species richness at intermediate depths, near the shelf break. Significant differences among subareas were found, with Crete showing a distinct species richness-depth pattern, which was more obvious for fish and cephalopods. The differences among subareas were also highlighted based on the occurrence of alien species of Indo-Pacific origin, which were more frequent in Crete. Our results suggested that the importance of depth-related factors in structuring communities was higher for cephalopods and less important for fish, and that Crete showed a distinct diversity-depth relationship, a fact that can be attributed to its specific geographical and oceanographic characteristics. These results support the current GFCM/FAO’s characterization of Crete as a unique geographic subarea. The findings of the study contribute to understanding the causes of underlying diversity patterns and would assist various environmental management actions, particularly those related to the establishment of marine-protected areas.

Distribution and spatio-temporal biomass trends of red mullets across the Mediterranean

“…In the Aegean Sea for instance, increasing abundance trends for several demersal species have been partially attributed to recruitment increases owing to increased productivity (Tserpes and Peristeraki 2002). Certainly, the intensity of fishing activities always plays an important role, and this may explain why areas of low fishing pressure, such as GSA 23 (Crete), show relatively high abundances (Table 1) although they are characterized by low primary production (Peristeraki et al 2017). Levi et al (2003), suggested that positive sea surface temperature (SST) anomalies (warmer waters than average) could enhance recruitment of M. barbatus, probably through a reduced upwelling regime, which-although it may involve lower productivitywould result in lower offshore transport of early fish stages, a key factor for the survival of juvenile stages.…”

Spatio-temporal trends in diversity of demersal fish assemblages in the Mediterranean

“…data concerning only one guild of fishes or limited to specific depths, gear or habitat), some of regional inventories are useless for comparative studies (Psomadakis et al 2012). Recent studies based mainly on standardized time series data also question the previously considered west-east decreasing diversity trend in the Mediterranean (Gaertner et al 2013, Granger et al 2015, Peristeraki et al 2017.…”

Long-term spatiotemporal dynamics of cephalopod assemblages in the Mediterranean Sea

“…As most Lessepsian species inhabit depths shallower than those prospected during the MEDITS and/or non-trawlable grounds, they in principle are not prone to being captured by trawling gears. A few Lessepsian fish species have been recorded in the MEDITS surveys in the south Aegean and Cretan Seas (Peristeraki et al 2017), but no Lessepsian cephalopod species have been recorded so far, although some of them have well-established populations in the eastern Mediterranean (Zenetos et al 2011;Lefkaditou, unpublished data). It is therefore expected that these Lessepsian migrants will spread westwards, as some fish species have already done (Bianchi 2007, Calvo et al 2011, probably leading to spatiotemporal differences in cephalopod assemblages through the Mediterranean, at least during the westward spreading phase.…”