Modelling post-release mortality of loggerhead sea turtles exposed to the Hawaii-based pelagic longline fishery

Abstract

Loggerhead sea turtles Caretta caretta are an endangered species exposed to anthropogenic hazards such as pelagic longline fisheries. Many loggerheads caught in these fisheries are alive when released from the gear, but many probably die soon after because of hook injuries or line entanglement. Robust estimates of post-release mortality are essential for stock assessment and evaluating the benefit of releasing turtles caught alive in the gear, yet none are available for any sea turtle species. Here, the post-release mortality of 40 loggerheads caught in the Hawaii-based pelagic longline fishery was investigated using satellite telemetry deployed by a National Marine Fisheries Service (NMFS) observer program. We modelled time-to-failure of all transmitters using nonparametric statistical modelling (Kaplan-Meier-Turnbull, local regression) to derive survival and hazard functions for light and deep hooked loggerheads. There was a significant difference between the survival functions for light and deep hooked loggerheads within 90 d of release, but no difference between survival functions after this time. But satellite transmitters fail for many reasons (defects, battery failure, transmitter detachment, turtle death), which results in a hazard function that confounds these competing risks. Hence we propose that it might not be possible to infer true postrelease mortality based on satellite telemetry unless the cause of each transmitter failure is known, which is rarely the case. We discuss other survey design and statistical modelling challenges involved in the evaluation of post-release mortality based on satellite telemetry.

Differential loggerhead by-catch and direct mortality due to surface longlines according to boat strata and gear type

“…Cheng and Cheng, 1997) and surface longline (i.e. Wetheral, 1977;Witzell, 1999;Chaloupka et al, 2004;Lewison et al, 2004). For example, several studies have focused on fishery-related loggerhead mortality in the Spanish surface drifting longline fisheries (Aguilar et al, 1992;Camiñas and de la Serna, 1995;Camiñas, 1997;Camiñas and Valeiras, 2001); however, these studies did not analyse the differences linked to the type of boat and gear.…”

Post-capture movements of loggerhead turtles in the southeastern Pacific Ocean assessed by satellite tracking

“…As such, while based upon a relatively small sample, it provides a valuable point of comparison with similar studies of conspecifics elsewhere and can help inform regional and global efforts to better understand fishery impacts on sea turtles (Lewison et al 2004, Blumenthal et al 2006, Wallace et al 2008, McClellan et al 2009, Alessandro & Antonello 2010 and their post-release mortality (Bjorndal et al 2003, Hays et al 2003, Chaloupka et al 2004, Swimmer et al 2006, Sasso & Epperly 2007.…”

“…Or they might indicate that our understanding of what entails a minor or severe injury to a sea turtle is incomplete. One transmitter failed after only 8 d but could not be attributed to death of the turtle, nor could death be attributed to any of the remaining turtles, which transmitted from 48 to 297 d. Use of PTTs or pop-up satellite archival tags (PSAT) with dive data or depth sensors could help investigators make more informed determinations, but still do not provide clarity in revealing a turtle's fate (Chaloupka et al 2004, Swimmer et al 2006. Future studies in this fishery should determine the prevalence of entanglements as well as minor and severe hookings of turtles in order to help evaluate their likelihood of survival and the relative risk posed by the fishery.…”

“…All loggerhead turtles used in the study were active at time of capture (not moribund or comatose) and fishers were given detailed instructions on how to safely handle and maintain the turtles aboard. All visible fishing hooks and entangling line were removed from the turtles before release.We assigned an injury score to each turtle based upon the location and severity of any visible injuries sustained during the capture process following criteria described by Chaloupka et al (2004). We used a 3 point injury scale in which level 1 referred to turtles with external injuries only (including those that were only entangled in longline branchlines), level 2 indicated minor injuries to the mouth cavity or lower mandible and level 3 indicated more severe injuries including turtles deeply hooked in the esophagus or soft palate.…”

Performance of pop-up satellite archival tags

“…The fail-safe option allows researchers to identify postrelease mortality (Swimmer et al 2002, Domeier et al 2003, Chaloupka et al 2004, Moyes et al 2006, Hays et al 2007). Ambiguity arises, however, when PSATs fail to report.…”