Leatherback Sea Turtle Interactions in Western and Central Pacific Deep-Set Longline Fisheries

Pacific leatherback sea turtle (Dermochelys coriacea) populations continue to decline at alarming rates and fishery bycatch is considered a primary threat. Incidental capture, or bycatch, in deep-set fisheries targeting tuna are especially concerning because ~ 79% of longline fisheries operating in the Western and Central Pacific Ocean (WCPO) are deep-set fisheries that do not have sea-turtle specific required conservation and management measures (CMMs). This report compiles leatherback turtle bycatch and mortality estimates using data from the Western and Central Pacific Fisheries Commission (WCPFC), and also focuses on a subset of this larger data set by analyzing leatherback turtle interactions and estimated mortality from the Hawaii longline fisheries. Leatherback interaction rates are 0.0055 and 0.00026 per 1,000 hooks in Hawaii’s shallow-set (15 hooks between floats) and deep-set (=> 15 hooks between floats) fisheries, which have 100% and ~20% fisheries observer coverage, respectively. Despite a lower rate of interaction in the deep-set fishery, its effort represents ~ 97% of total longline fishing effort (per number of hooks set), and thus results in a much higher number of estimated interactions and mortalities. Therefore, we report that in Hawaii’s fisheries, there are more interactions and mortalities of leatherback turtles in deep-set fisheries as compared to shallow-set. Previous estimates of fisheries interactions and impacts to sea turtle populations in the WCPFC Convention Area indicate a trend of both increased effort in deep-set longline fisheries as well as an associated increased interaction rate and potential impacts to Pacific leatherback turtles (Peatman et al. 2019). However, fisheries interaction data reported by the WCPFC are derived from fleets with low observer coverage (1 to 3%) and spatially limited coverage (Peatman et al. 2019), thereby limiting abilities to understand the impacts of these trends on leatherback populations. Data from Hawaii’s longline fleets, which have ~20% (deep-set) and 100% (shallow-set) observer coverage, could be useful in providing a more accurate understanding of the impacts of deep-set longline fishing on leatherback populations in the Pacific Ocean.

In this paper, we apply mortality rates based on data obtained from Hawaii’s deep-set longline fishery to better understand the potential impacts to leatherback turtles from fisheries interactions in the WCPFC Convention Area. Previous work (Peatman et al. 2019) estimates an annual interaction of ~ 257 leatherbacks per year, which we estimate to result in 90 mortalities using the mortality rate from Hawaii’s deep-set longline fishery. We also report on an alternative approach to estimate leatherback interactions in the WCPFC using interaction rates derived from Hawaii’s longline fishery, for which there is relatively high (~20%) observer coverage. Given concerns regarding the population decline of Pacific leatherback turtles, our data indicate a high conservation value if the current WCPFC Sea Turtle CMM (2018-04) were expanded to include measures to address deep-set longline fisheries. Additionally, our findings highlight the importance of increasing observer coverage levels in order to improve accuracy of estimates of bycatch and mortality, thereby improving the Commission’s ability to sustainably manage fisheries and resources within the WCPO.