Post-release Mortality of Longline-caught Loggerhead Turtles Estimated from Satellite Tracking Data and Anatomical Hooking Location

The interaction of sea turtles with pelagic fishing gear is a problem shared by many nations. Accordingly, there is considerable international collaborative research on the nature of such interactions and ways to mitigate them. Recently, scientists from PIFSC and the University of Hawaii Joint Institute for Marine and Atmospheric Research have been working to understand the fates of loggerhead sea turtles after interactions with longline gear. The study is being conducted in collaboration with researchers from the non-governmental organization SUBMON in Barcelona, Spain and the consulting firm Biolintech on the island of Oahu, in Hawaii. In a forthcoming journal article the team of scientists will report estimates of the likelihood of loggerhead mortality resulting from interactions with longline gear. The estimates refer to mortality of the turtles after they are freed from the fishing gear and released back into the sea and were based on satellite telemetry data from longline-caught loggerheads. The estimates took into account information on days at liberty (DAL, the number of days a turtle was successfully tracked after release), the turtle's dive depth, and anatomical location of the hooking.

Pop-up satellite archival tags were deployed by NOAA Fisheries observers on 29 loggerhead sea turtles (Caretta caretta) caught between 2002 and 2006 by the U.S.-based pelagic longline fishery operating from California and Hawaii in the North Pacific. The observers classified each turtle as shallow-hooked if it was entangled in the line or the hook was in the flipper, jaw or mouth and could be removed (55% of turtles were shallow-hooked); or deep-hooked if the hook was ingested and could not be removed (45% were deep-hooked). The vertical movements of turtles post-release were used to infer potential mortalities.

Of the 25 tags that reported data, the DAL ranged from 3 to 243 days (overall mean =68 days). The observed mean DAL was shorter by nearly 50% for shallow-hooked turtles (mean=48 days, range: 3 to 127) than for deep-hooked turtles (mean=94 days, range: 5 to 243), but the difference was not statistically significant (P =0.0658). Although aspects of these analyses may be considered speculative, the data provide empirical evidence to indicate that deep-hooking is not linked to shorter DAL.

Information on DAL, anatomical hooking location, and gear removal were evaluated along with inferences about the extent of injuries and rates of infection to estimate an overall post-release mortality rate of 28% (bootstrap 95% confidence interval for mortality: 16–52%). These results are consistent with the range of estimates used to shape some U.S. fisheries management plans, suggesting that the expected mortality rates for released turtles postulated for those management measures are credible.