Benson et al.; Abundance, distribution, and habitat of Dermochelys conaceo off California 



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Figure 6 



Pattern of upwelling and retention along the central California coast, illustrated with monthly satellite-derived sea 

 surface temperature images for August. September, and October 1999. Leatherback turtles (Dermochelys coriacea) and 

 Scyphomedusae were primarily found in areas of retention (circled in middle panel). High resolution monthly composite 

 satellite images courtesy of NOAA CoastWatch, West Coast Node. 



be considered provisional, pending further TDR deploy- 

 ments and calibration experiments. 



Leatherback turtle populations at many Pacific nest- 

 ing beaches have decreased dramatically during the last 

 decade (Spotila et al., 2000), but decreases at Jamursba- 

 Medi have been less pronounced (Hitipeuw et al., 

 2007), and the abundance of turtles foraging off Cali- 

 fornia does not exhibit a trend between 1990 and 2003 

 (Fig. 3). The California study area is a dynamic up- 

 welling environment that exhibits great interannual 

 variability in oceanographic conditions (Chelton et al., 

 1982; McGowan et al., 1998) and distribution of marine 

 vertebrates (e.g., Ainley et al., 1993; Benson et al., 

 2002). Links have been proposed elsewhere between 

 large-scale environmental indices, such as the Southern 

 Oscillation Index and the North Atlantic Oscillation, 

 and sea turtles (Limpus and Nicholls, 1988; Rivalan, 

 2004) or their prey (Lynam et al., 2004). Li this study, 

 leatherback turtle abundance off California exhibited 

 a positive relationship with the average annual NOI 

 (Fig. 5). Positive NOI values correspond with condi- 

 tions favorable to upwelling along the California coast, 

 leading to increased zooplankton production (Schwing 

 et al., 2002) and the development of large aggregations 

 of gelatinous zooplankton (Graham, 1994), which are 

 known to be the primary prey of leatherback turtles 

 (Eisenberg and Frazier, 1983). 



Although we did not measure underlying physical and 

 biological processes, central California has been the fo- 

 cus of numerous oceanographic studies that shed light 

 on potential trophic links between physical processes 



and leatherback turtles. Strong northwest winds dur- 

 ing late spring and early summer lead to wind-driven 

 upwelling (Bakun et al., 1974), particularly near points 

 and headlands. These prominences can interact with 

 local hydrographic features to create localized reten- 

 tion areas (upwelling shadows; Graham, 1994), where 

 nutrient-rich, upwelling-modified water is entrained 

 nearshore, particularly during wind relaxation. This 

 process creates favorable conditions for phytoplankton 

 growth and increases retention of zooplankton, larval 

 fish, crabs, and gelatinous organisms (Wing et al., 1995; 

 Graham et al., 2001). Dense aggregations of jellyfish 

 (Scyphomedusae), primarily Chrysaora fuscescens, C. 

 colorata, and Aurelia spp., have been observed regu- 

 larly in these nearshore regions (Graham, 1994; this 

 study). Similar processes have been reported off Or- 

 egon, where Scyphomedusae become denser and larger 

 in size during summer, when the movement of surface 

 and near-surface waters concentrates plankton in near- 

 shore retention areas (Shenker, 1984). During our sur- 

 veys, Scyphomedusae were common in retention areas 

 between Pt. Reyes and Monterey Bay (Fig. 6), where 

 leatherback turtles were most frequently encountered 

 and observed feeding on C. fuscescens, C. colorata, and 

 Aurelia spp. (Starbird et al., 1993; this study). We hy- 

 pothesize that variability in the expression of these 

 physical and trophic processes leads to interannual 

 and seasonal variability in observed leatherback turtle 

 abundance off central California, with densities greatest 

 during periods of significant upwelling and subsequent 

 relaxation events. 



