| Abstract |
The present work aims to contribute to the knowledge of patterns and trends of marine demersal
communities in the Eastern Mediterranean, with emphasis in the Greek Seas. Three main subjects were
investigated in the present thesis: A) the fine-scale diversity patterns in relation to depth of three major
megafaunal groups (fish, cephalopods and crustaceans); B) the spatio-temporal abundance patterns of
demersal chondrichthyes in the Greek Seas in relation to anthropogenic and environmental factors and C)
the spatio-temporal patterns and trends of community structure indicators, such as the community mean
trophic level and mean inferred temperature, in order to detect any shifts in the demersal communities due
to anthropogenic impacts. This analysis was made on a large Mediterranean scale, including relevant data
from thirteen geographic areas of the North Mediterranean Sea. Summary information on each topic is
presented below.
A. 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 examined 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 (MEDITS) conducted during 2005-2014
and the relationship between depth and two different diversity measures (species richness and ShannonWeaver) 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 richnessdepth 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 marineprotected areas.
B. Chondrichthyes constitute one of the oldest and most ecologically diverse vertebrate taxonomic
groups. Their life characteristics make them highly susceptible to fisheries and other anthropogenic
activities and they are considered as “indicator species” for overfishing and ecosystem stability. In
European waters, chondrichthyes have become a focus of conservation actions due to the abundance
declines of several species. The present study aims to investigate trends and patterns in the abundance of
demersal chondrichthyes in the E. Ionian and the Aegean seas, in relation to anthropogenic and
environmental factors. The investigation of the common trends (CTs) in the abundance indices of demersal
chondrichthyan species and their relation to known features, in two major areas of the eastern
Mediterranean Sea, based on a long fishery-independent data series from 1994 to 2016, is also presented.
Data on biomass by haul, for each chondricthyan species, were used to estimate the annual standardized
biomass index by species, expressed in terms of kg per square km of swept area. CTs were extracted using
dynamic factor analysis. Factor loadings were plotted against maximum length (Lmax), median inferred
temperature (MT) and trophic level (TL) of the species, in order to identify potential relationships among
major trends and those parameters.
A total of 41 demersal chondrichthyan species were found in the Aegean Sea and 28 in the E. Ionian. In the
E. Ionian Sea one CT was detected, showing a generally increasing abundance trend until 2008 and a slight
decrease afterwards for all considered species, apart from R. clavata, which was negatively related to the
CT. Two almost opposite CTs were detected in the Aegean Sea: the first presented an increase of the
abundance index until 1999-2000 decreasing afterwards, while the second presented a decreasing trend
until 1999, increasing afterwards until 2012 and decreasing again during the last years. A significant inverse
relationship was found in the case of factor loading values of the second CT and the Lmax of the species,
while the relationship between factor loading values of the first CT and the MT of the species was
marginally significant.
The present results generally suggest that species either targeted by fisheries or psychrophilic ones were
represented by a CT characterized by declining biomass since 2001. It is likely that such species are affected
by fishing and/or sea warming owing to climatic changes. Small sized species with lower TLs, and rare
species, less vulnerable to fisheries, undergo a generally increasing trend since 2000, while thermophilic
species with relatively high TLs showed either increasing or no specific trends.
In addition, fishery independent data obtained from the MEDITS Surveys conducted in the Aegean and E.
Ionian Seas (eastern Mediterranean) were coupled with bottom trawl fishing effort data obtained from
vessel monitoring systems (VMS) in order to evaluate the effect of fishing on biomass variations of
demersal chondrichthyes. The study focused on commonly captured species such as Raja clavata, Galeus
melastomus, Scyliorhinus canicula and Squalus spp, as well as to the total chondrichthyan biomass. The
effect of fishing effort on biomass indices, expressed in terms of kg per square km of swept area, was
examined by means of Generalized Additive Model techniques, using also the sampling position, year and
depth as co-variates.
Overall findings suggested a biomass decline of chondrichthyan species with increasing fishing effort.
Nevertheless, it was also found that certain species (e.g G. melastomus in both areas and S. canicula in the
Aegean Sea) can withstand fishing pressure and it is likely that discard survival rates, depth preferences
and fisheries exploitation patterns, are the main drivers explaining variations regarding the impact of fishing
on the different stocks.
C. Mean inferred temperature (MTC) and mean trophic level (MTL) spatiotemporal patterns of
MEDITS survey catches were examined in thirteen geographic sub-areas (GSAs) in the Mediterranean,
between 1994-2016. The study aimed to detect changes in the demersal community structure related to
anthropogenic impacts. A Generalized Additive Modelling (GAM) approach was used to examine the
effects of year and GSA on MTC and MTL indexes and on bottom temperature by haul. Regarding MTC
index, the year was significant only in four GSAs, while for MTL it was significant in five out of thirteen.
Higher MTC values were observed in the central and eastern areas. Bottom temperature was increasing
after 2010, and also from west to east and from north to south. Our results indicated that the observed recent
increase in bottom sea temperature has not resulted in an immediate response to demersal marine
communities; however, areas with higher warming rates or shallow depths were found to be more
susceptible to sea warming. Regarding MTL, only in two GSAs decreasing trends were observed, while the
observed temporal trends in five GSAs possibly reflected changes in fishing activity patterns. However,
higher MTL values were observed in GSAs with generally higher exploitation rates, indicating that other
factors, apart from fishing, play an important structuring role on marine communities.
The present results indicate existing differences among Mediterranean subareas in regard to the community
structure changes attributed to different environmental conditions and exploitation patterns with
implications in the ecology and dynamics of the stocks.
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