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When 2 closely related species co-occur, each exhibiting sex-specific differences in size, resource partitioning is expected. We studied sex-specific foraging behaviour of 2 sympatric seabird species in the Gulf of California to disentangle the respective influence of species and sex, but also mass and size of individuals, on observed foraging behaviour. We used highly accurate data loggers to study movements, diving behaviour and activity of brown and blue-footed boobies rearing young chicks. Interspecific differences were limited; brown boobies had longer foraging trips and spent less time on the water than blue-footed boobies. The major differences observed were sex-specific; females of each species tended to have longer foraging trips, foraged farther from the colony, flew greater distances and had larger zones of area-restricted search. These sex-specific differences were more prominent in brown than in blue-footed boobies. Diet and stable isotope analyses showed that, during the study period, both species fed mainly on sardines, at similar trophic levels and in similar zones; outside the breeding season, the carbon and nitrogen signatures from feathers were also similar on average. In these sympatric species that feed on a superabundant prey, sex-specific differences appear to have a greater role than species-specific differences. We suggest that sex-specific differences may be mainly related to breeding involvement, as males are more involved in nest attendance and defence and females are greater provisioners. However, we show that several sex-specific differences in observed foraging behaviour were partly or totally explained by body size (flight speeds, foraging range, flapping frequency) or by body mass (depths attained during diving, duration of dives), which are parameters influenced by biomechanical constraints such as flight and diving.

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California sea lions breed along the Pacific Coast of the United States, south to Baja California, Mexico, and throughout the Gulf of California. Although the population in the United States has been increasing over the last 15 years, the status in the Gulf of California is unknown. The last published census in 1994 yielded an estimate of 31,393 animals, but some rookeries have subsequently declined in abundance. The aim of this study is to provide a new estimate of population size for California sea lions in the Gulf of California and to examine the relative risk of extinction among thirteen sites using census data from 1970-2004. Our initial population estimate for 2004 is 17,484 including 4,299 pups, and the total number of sea lions when correction factors were applied was between 24,062 and 31,159. Four of the thirteen rookeries exhibited increasing trends: the two northernmost (Consag and San Jorge), the southernmost (Los Islotes), and San Esteban in the central gulf. The remaining rookeries were either stable or declining in abundance. During our analysis of total population and pup production trends, we identified a group of sea lion rookeries that are growing in total numbers and pup production, and exhibit low probability of extinction. Another group shows total abundances decreasing, pup production declining, negative population growth, and a high probability of extinction. Finally, a third group of rookeries exhibit high fluctuations in abundance and no clear Sea Lions of the World Alaska Sea Grant College Program • AK- SG-06-0, 2006 population trend. Rookeries with maximum probability of extinction and declining trends are situated at or near Angel de la Guarda Island (central gulf). We discuss several possible causes of these declines, including the influence of sardine in the diet of sea lions.

Female northern elephant seals, Mirounga angustirostris, from Año Nuevo (AN) in central California feed offshore in mid-latitude waters (40 • -55 • N). Migratory patterns and foraging locations of seals from Mexico are unknown. Rookeries on San Benitos (SB) islands in Baja California Sur, Mexico, are ∼1,170 km south of AN. Although the colonies are similar in size, seals from SB begin breeding earlier and have an earlier breeding birthing peak than seals from AN. To determine if the foraging location of seals from Mexico was similar to that of seals from California, we measured ␦ 13 C and ␦ 15 N values in the hair of 48 suckling pups at SB and 37 from AN, assuming that their isotopic signatures reflected those of mothers' milk, their exclusive diet. The mean ␦ 13 C and ␦ 15 N values for SB pups (−16.1‰ ± 0.9‰ and 17.7‰ ± 0.9‰, respectively) were significantly higher than those for AN pups (−17.6‰ ± 0.4‰ and 15.6‰ ± 1.0‰, respectively). From data on environmental isotope gradients and known behavior of SB and AN populations, we hypothesize that the isotope differences are due to females in the SB colony foraging ∼8 • south of seals from AN. This hypothesis can be tested by deployment of satellite tags on adult females from the SB colony.

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In recent years, population viability analysis has become a popular tool to assess the relative risk of extinction among populations. Viability estimates for spatially structured populations require movement data that are often unavailable. In this paper, a diffusion approximation model was used to explore the effects of different spatial scenarios resulting from assumptions about movement rates. Census data for 13 breeding islands occupied by California sea lions Zalophus californianus californianus in the Gulf of California were used to explore three potential scenarios: unlimited movement between sites (panmictic population), limited movement (several clusters of populations) and no movement between islands (isolated islands). Predicted viability estimates were different for each scenario, but contrary to expectations, the mean extinction risk estimates were generally lowest when movement was unlimited (panmictic scenario). However, despite an extensive dataset, the confidence of the viability predictions for each scenario was low. In some cases, uncertainty in predictions within a scenario was greater than differences between scenarios. Therefore, it is recommended that in situations where movement rates and spatial structure are unknown, extinction risk estimates should reflect both the confidence intervals for each risk estimate and the uncertainty resulting from different spatial scenarios. This study also provides the first estimate of population viability (considering spatial structure) for California sea lions in the Gulf of California and an evaluation of population status based on the IUCN criteria for species listing.

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