engleski

Relative impact assessment for loggerhead bycatch in northern Adriatic fisheries

Fisheries bycatch often affects particular size classes of turtles. Because of the late age at maturity for loggerhead turtles, the reproductive value of individual small juveniles, large juveniles and adults may vary by orders of magnitude. The reproductive value, which is based on age, provides a relative measure of value for individuals, thereby allowing us to estimate the effects of different stressors based on the number of animals killed. Using reproductive value as a comparative index, we assessed the relative impact of trawl and gillnet fisheries operating in the northern Adriatic Sea (Croatia and Slovenia) on loggerhead populations. The northern Adriatic host one of the most important developmental habitats for the species in the Mediterranean, and, at the same time, is one of the heaviest fished areas, resulting in high bycatch rates. We tested several scenarios for bycatch and mortality in both fisheries. Despite high bycatch rates, bottom trawl fisheries result in direct death of fewer loggerheads (267-355 vs. 361-2217 in gill nets), but affect individuals that are older than average and hence have higher reproductive values. Because of data limitations for vital rates and bycatch estimates, we looked at a distribution of potential impact for each fishery. Lessons learned include the need for relative, rather than absolute, impact assessment because of the sensitivity of adult reproductive value to parameters such as clutch frequency, sex ratio and remigration interval. We anticipate that our analysis will help managers prioritize needs for regulation and mitigation among different fisheries.

sea turtles; bycatch; trawls; gillnets; population modeling; conservation; fishery management

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