Tanya Graham

 

The leatherback sea turtle is a critically endangered species which occurs seasonally off central California. Individuals migrate from Papua, Indonesia across the Pacific Ocean basin to the coast of Oregon and California to take advantage of dense swarms of Scyphozoan jellyfish that occur from spring through fall. The predictable arrival of adults of both sexes to central California, particularly Monterey Bay, provides a unique opportunity to study the foraging ecology of this species. The objectives of my study is to assess availability, vertical and horizontal distribution, and nutritional content of the prey consumed by leatherback turtles in Monterey Bay

Most of our knowledge of leatherbacks, and sea turtles in general, is of nesting populations. Nesting individuals are easier to access than pelagic individuals and are somewhat predictable in their occurrence. However, this highly-migratory species, which can be found from 60°N to 42°S in the Pacific (Stinson 1984), spends nearly its entire life in pelagic waters. Gaps in our understanding of the migratory and foraging behavior of this species are broad, and very little is known about non-breeding females or males at any stage beyond hatchling. Stomach content analysis and opportunistic observations have shown that adult leatherbacks feed on scyphomedusae (jellyfish), salps, pyrosomas, and other soft-bodied invertebrates (Carr 1967; FAO Species Catalogue, 1990; Holland et al. 1990; Work and Balazs, 2002). However, there is little understanding of the factors affecting prey selection.

Small groups of individual leatherback turtles feed seasonally in coastal waters (FAO Species Catalogue, 1990; Starbird et al. 1993; James & Herman 2001). Inshore waters off California, between Pt. Conception and Pt. Arena, are visited annually by approximately 150 to 170 leatherback turtles (Benson et al. 2003), with the greatest numbers occurring during early fall (Starbird et al. 1993; Benson et al. 2003). Leatherbacks foraging in Monterey Bay, CA, are exclusively individuals from western Pacific nesting beaches (Dutton & Benson, unpub. data). They appear in the area to feed on dense aggregations of scyphomedusae that occur during this period (Starbird et al. 1993). In coastal Oregon, Shenker (1984) found these swarms of jellyfish can contain nearly 80% as much carbon as the densest copepod populations, providing a rich food source for predators such as the leatherback.

Three species of large scyphomedusae are common in surface waters of Monterey Bay —Chrysaora fuscescens (brown sea nettle), Aurelia labiata (moon jelly), and Pelagia colorata (purple stripe jelly), all three of which are consumed by leatherback turtles. Very little is known about their vertical distribution within Monterey Bay, with only slightly greater understanding of the horizontal distribution. Aerial surveys have indicated larger, more frequent surface aggregations of scyphomedusae have occurred in the northern portion of the bay (S. Benson, unpub. data). This may be due to surface currents (Shenker 1984; Suchman & Brodeur 2005) or the presence of nutrient-rich water advected southward by the California Current from the Año Nuevo upwelling center into the northern portion of Monterey Bay. Distribution at depth likely follows similar patterns, but nothing is known at this time. Net tows and hydroacoustic sampling of scyphomedusae would provide useful information to fill these gaps in our understanding of their horizontal and vertical distribution.

Opportunistic observations of regurgitated stomach contents and direct observations of foraging behavior indicate that C. fuscesens appears to be selected even when other species of scyphomedusae are present and more abundant (S. Benson, pers. comm.; J. Harvey, Moss Landing Marine Labs, pers. comm.). This selectivity may be a result of the nutritional value among species. A study of the nutritional content (as indicated by carbon and nitrogen content and stable isotopes) of scyphomedusae from Monterey Bay throughout the leatherback turtle foraging season will help determine whether the selective foraging observed is related to nutritional content.

Similarly, foraging leatherbacks in Monterey Bay have been observed to consume only the tentacles gonads of sexually mature C. fuscesens. This may be due to the nutritional content of the components of the body (bell or tentacles and gonads). Shenker (1985) found that although gonadal tissue had the lowest dry weight (1.86% of the wet weight; WW), it had greater carbon content (0.58% WW) than either the bell or the oral arm. However, these results are not from C. fuscesens within Monterey Bay, so a complete study of the nutritional content of C. fuscesens body components within the Monterey Bay leatherback turtle foraging area would address this question and help characterize the foraging requirements of this endangered species.