Assessing the impact of small-scale fisheries on sea turtle populations in the Eastern Mediterranean
Palmer, J
Date: 10 June 2024
Thesis or dissertation
Publisher
University of Exeter
Degree Title
PhD in Biological Sciences
Abstract
Bycatch is a core issue recognised across industrial, semi-industrial and small scale fisheries (SSFs), with several million tonnes of bycatch landed globally each
year. It is a key driver of population declines, including those experienced by
marine turtle populations with complex life history strategies that expose them to
a ...
Bycatch is a core issue recognised across industrial, semi-industrial and small scale fisheries (SSFs), with several million tonnes of bycatch landed globally each
year. It is a key driver of population declines, including those experienced by
marine turtle populations with complex life history strategies that expose them to
a wide variety of fishing practices across all species and life stages. The
Mediterranean is one of the most overexploited seas worldwide and recognised
as a marine turtle bycatch hotspot with some of the highest mortality rates in the
world. Despite this, research focusing on bycatch in Mediterranean SSFs has
been limited, and regions such as the eastern basin are particularly lacking in
data. The main aims of this thesis were therefore to: (1) provide a comprehensive
characterisation of the operating behaviours and spatial extent of the Northern
Cyprus SSF situated in the eastern Mediterranean basin; (2) quantify marine
turtle bycatch and fisheries-related mortality rates and evaluate key drivers of
marine turtle bycatch within the SSF; (3) assess gut digesta of stranded and
bycaught marine turtles to determine interspecific dietary preferences and
ontogenetic shifts; and (4) establish location of foraging grounds, diel movement
patterns and depth utilisation of bycaught marine turtle populations resident in
Northern Cyprus. The main findings of the research conducted were that: (1) a
rapidly increasing active fleet size ranked 14th of 23 Mediterranean fleets
assessed where many have had reductions in size and activity, predominantly
coastal fishing practices are employed by the SSF that overlap considerably with
established Special Environment Protection Areas, with clear crepuscular peaks
in vessel activity and gear deployment; (2) an estimated 5,500 marine turtles are
caught in this fishery annually, with set nets being major contributors to marine
turtle bycatch, and bycatch probability largely driven by depth of fishing operation
and mesh size; (3) distinct dietary niche separation between green and
loggerhead turtles (Caretta caretta) but reliance of both species on seagrass
habitats, opportunistic foraging in loggerhead turtles, and a slow ontogenetic
dietary shift to herbivory in green turtles; and (4) distinct and overlapping foraging
sites between species, diel patterns in surfacing behaviours and core use areas,
and interspecific differences in depth utilisation. In conclusion, this thesis
establishes key pathways and drivers of marine turtle bycatch in a data deficient
region through examining the interactions between foraging and movement
ecology of marine turtle populations with operating behaviours of the SSF in
4
Northern Cyprus. The findings provide a comprehensive assessment of a main
threat to marine turtle population recoveries and considers relevant opportunities
for adaptation of fishery practices using a holistic approach to ensure
management strategies are reflective of the socioeconomic importance and
complexity of the fishery, and to minimise conflict with other ecologically important
species.
Doctoral Theses
Doctoral College
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