346 



Fishery Bulletin 105(3) 



Previous researchers have linked leatherback turtle 

 occurrence at high latitudes to the 15-16°C isotherm 

 (Stinson, 1984; McMahon and Hays, 2006). Off central 

 California, this reported pattern may reflect the pres- 

 ence of >15°C waters during summer and fall relax- 

 ation events and in upwelling shadows where jellyfish 

 aggregations are found (Graham, 1994; Graham and 

 Largier, 1997), rather than a physiological limitation 

 of leatherback turtles. The broad shallow area of reten- 

 tion in the Gulf of the Farallones consistently exhibited 

 greater abundances of leatherback turtles during our 

 study. In contrast, few turtles were observed south of 

 Pt. Sur, where the shelf is extremely narrow and cooler 

 waters dominate along a nearly straight coastline where 

 there are limited retention zones. 



Many questions remain unanswered regarding the 

 role of physical and biological factors and their influence 

 on leatherback turtle abundance and distribution along 

 the U.S. West Coast. Upwelling shadows and relaxation 

 events probably affect leatherback turtle occurrence, but 

 directed studies are needed to establish trophic links. 

 Furthermore, the potential influence and role of the 

 San Francisco Bay outflow on this nearshore foraging 

 area is unknown. Results of this study provide a means 

 for designing finer-scale surveys in key index areas of 

 reliable leatherback turtle occurrence, such as the Gulf 

 of the Farallones and Monterey Bay. Aerial surveys of 

 these index areas should be coupled with telemetry 

 studies and investigations of environmental variables 

 that affect leatherback turtle foraging behavior to pro- 

 vide insights into the relevant trophic processes. 



Ultimately, successful conservation efforts for leath- 

 erback turtles must include both protection of nest- 

 ing beaches and mitigation of at-sea threats in forag- 

 ing areas and along migratory routes. This study has 

 demonstrated that waters off central California are a 

 critical foraging area for one of the largest remaining 

 Pacific nesting populations. Fortunately, threats such 

 as coastal gillnet and longline fisheries that may in- 

 cidentally catch leatherback turtles have largely been 

 eliminated within our nearshore study area although 

 pelagic driftnet and longline fisheries remain along the 

 migratory pathways to and from the coast (e.g., Spotila 

 et al., 1996; Carretta et al., 2005). Continued efforts 

 to identify and characterize Pacific foraging areas are 

 critical for mitigating at-sea threats, monitoring popula- 

 tion trends, and, ultimately, for the successful recovery 

 of Pacific leatherback turtle populations. 



Acknowledgments 



This study could not have been completed without the 

 dedicated support of many aerial observers, whom we 

 thank sincerely. S. Eckert originally noted the signifi- 

 cance of leatherback turtle sighting data collected during 

 these nearshore surveys and encouraged us to initiate 

 the analyses presented here. Aspen Helicopters, Inc. 

 provided aircraft and pilots, and Moss Landing Marine 

 Laboratories Marine Operations provided small boat 



support. Captain J. Douglas' exceptional boat handling 

 skills were key to the success of the telemetry stud- 

 ies. Suction-cup telemetry was conducted under ESA 

 permit no. 1227. Surveys were conducted under National 

 Marine Fisheries Service permit nos. 748 and 773-1437, 

 and National Marine Sanctuary permit nos. GFNMS / 

 MBNMS-03-93, MBNMS-11-93, GFNMS / MBNMS / 

 CINMS-09-94, GFNMS / MBNMS / CINMS-04-98-A1, 

 and MULTI-2002-003. Financial support and person- 

 nel were provided by the National Marine Fisheries 

 Service (Southwest Fisheries Science Center, Southwest 

 Region, and Office of Protected Resources), Monterey 

 Bay National Marine Sanctuary, and the California 

 Department of Fish and Game. We thank J. Barlow, E. 

 Calvert, A. Moles, J. Musick, F. Schwing, J. Seminoff, 

 and three anonymous reviewers for their helpful reviews 

 of this manuscript. 



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