Endangered Species Research: sea turtle bycatch

S. Hoyt Peckham, David Maldonado-Diaz, Volker Koch, Agnese Mancini, Alexander Gaos, M. Tim Tinker, Wallace J. Nichols. 2008. High mortality of loggerhead turtles due to bycatch, human consumption and strandings at Baja California Sur, Mexico, 2003-2007, Endangered Species Research, preprint, October 2008.

Abstract

Assessing mortality of long-lived organisms is fundamental for understanding population trends and for implementing conservation strategies, but doing so for marine megafauna is challenging. Here we assessed anthropogenic mortality of endangered North Pacific loggerhead turtles in the coastal waters of Baja California Sur, Mexico (BCS), through the synthesis of 3 sources: (1) intensive surveys of an index shoreline from 2003 to 2007; (2) bimonthly surveys of additional shorelines and towns for stranded and consumed carcasses from 2006 to 2007; and (3) observations of bycatch by 2 small-scale fishing fleets. Using Monte Carlo simulations we estimate that 1500 to 2950 loggerhead turtles died per year at BCS from 2005 to 2006 due to bycatch in the 2 observed fleets. Actual mortality may be considerably higher due to bycatch in other fisheries, directed hunting for black market trade, and natural factors including predation and disease. From 2003 to 2007 we encountered 2719 loggerhead carcasses on shorelines and in and around towns of BCS. Along the 43 km Playa San Lázaro, 0.25 loggerheads km–1 d –1 were stranded during summer fishing months over 5 yr, which is among the highest reported stranding rates worldwide. This stranding rate corroborates similarly high observed bycatch rates for local small-scale longline (29 loggerheads 1000 hooks –1) and gillnet (1.0 loggerhead km–1 of net) fisheries. A significant increase in mean length of 2636 carcasses measured at BCS occurred from 1995 to 2007. Given the endangered status of the North Pacific loggerhead population, conservation action to reduce bycatch and poaching at BCS is urgently needed.