Assessing the impact of small-scale fisheries on sea turtle populations in the Eastern Mediterranean

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 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.