We examine the utility of stable carbon and nitrogen isotope variations to characterize the length of the nursing/lactation period and age at weaning for two northern Pacific otariid species, the northern fur seal (Callorhinus ursinus) and California sea lion (Zalophus californianus). We used two sampling strategies to measure ontogenetic trends in isotope value, and compared our results to observational data on the reproductive strategies used by these otariids. For Zalophus, we found evidence for 15 N enrichment and 13 C-depletion in bone collagen representing the first and second year of growth, which is consistent with the ∼12-14-mo weaning age in this population after a suitable turnover rate for bone collagen is considered. Analysis of individual tooth annuli from a different suite of Zalophus specimens suggests that half of the individuals were weaned at ∼12 mo of age, and half were dependent on milk for a portion of their second year. For Callorhinus, bone collagen for age classes that contain pre-weaned individuals were 15 N-enriched, but values were significantly lower in specimens between 6 and 20 mo of age. These 15 N-enriched values, presumably acquired during nursing between 0 and 4 mo of age in Callorhinus, were not present in specimens older than 12 mo of age. Thus complete bone collagen turnover in young-of-the-year occurs in 8-10 mo. 15 N enrichment is evident in the first annulus of female Callorhinus individuals, but is not detectable in males. Analyses of Callorhinus tooth annuli show no ontogenetic trends in ␦ 13 C values. Our study indicates that nitrogen, and in some cases carbon, isotopes can be used to assess reproductive strategies in marine mammals. When coupled with accurate age estimates based on bone growth regressions, this isotopic technique can be applied to historical or fossil otariids to gain insight into the flexibility of maternal strategies within and across species. 556 NEWSOME ET AL.: INVESTIGATION OF MATERNAL STRATEGIES 557

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Three fundamental foraging patterns in air-breathing marine vertebrates have been described: epipelagic, mesopelagic and benthic. Many sea lion species with access to extensive continental shelves have been described as benthic foragers. Coincidently these species are considered threatened. The Galapagos sea lion Zalophus wollebaeki, a top predator in the Galapagos Islands, is also considered threatened in this ecosystem. Sea lions at the central part of the archipelago have access to a vast continental shelf. For this reason we hypothesized that sea lions within this region would dive benthically. In addition, effective protection and conservation of this species requires knowledge of their foraging patterns and habitat utilization. We investigated the diving behaviour and habitat utilization of female Z. wollebaeki of a centrally located colony situated inside the highest density area of the population using time-depth recorders and satellite telemetry. Three distinct foraging patterns were found and described (shallow, deep and bottom divers), and individuals utilizing each pattern foraged in different locations. Epipelagic, mesopelagic and benthic dives were exhibited in the sea lions' diving behaviour, but these dive types were not exclusively associated with a foraging pattern. Between foraging trips females hauled out more frequently on other islands than they did on their breeding colony. The finding of 3 distinct foraging patterns that differ spatially has direct implications for management, particularly with regard to fisheries interactions. Marine protected areas can be implemented in the regions described as Z. wollebaeki foraging areas. Z. wollebaeki's wide foraging range coupled with their use of multiple haul-out sites should be considered in future studies when determining foraging trip lengths and habitat utilization since presence/absence from the colony does not reflect foraging trip length. KEY WORDS: Diving behaviour · Foraging behaviour · Galapagos Islands · Habitat utilization · Individual specialization · Galapagos sea lion · Zalophus wollebaeki Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 363: [299][300][301][302][303][304][305][306][307][308][309] 2008 lions, sea turtles like loggerhead Caretta caretta and olive ridley Lepidochelys olivacea and seabirds such as cormorants (Phalacrocoracidae) (Cooper 1986, Thompson et al. 1998, Costa & Gales 2000, HopkinsMurphy et al. 2003, McMahon et al. 2007). Diving patterns have been described for all extant sea lion species (Kooyman & Trillmich 1986, Feldkamp et al. 1989, Merrick & Loughlin 1997, Thompson et al. 1998, Costa & Gales 2000. However, diving behaviour of the Galapagos sea lion Zalophus wollebaeki has been investigated in only 4 individuals (Kooyman & Trillmich 1986).Zalophus wollebaeki is endemic to the Galapagos Islands. The population is widely distributed among this archipelago, with the highest density of individuals at the central and southern islands (Salazar 2005). ...

Summary 1.Understanding spatial structure and identifying subpopulations are critical for estimating population growth rates and extinction risk, and as such essential for effective conservation planning. However, movement and spatiotemporal environmental data are often unavailable, limiting our ability to directly define subpopulations and their level of asynchrony. 2. This study applies a recently developed statistical technique using time-series analysis of abundance data to identify subpopulations. The approach uses multivariate state-space models and Akaike's Information Criterion-based model selection to quantify the data support for different subpopulation numbers and configurations. This technique is applied to the population of California sea lions Zalophus californianus in the Gulf of California, Mexico, distributed across 13 breeding sites. 3. The abundance of California sea lions in the Gulf of California has declined over the last decade, though not all areas have been equally affected. In light of this variation, it is important to understand the population structure to ensure accurate viability assessments and effective management. 4. Our data support the hypothesis that the Gulf of California sea lion population has four subpopulations, each with 2-5 breeding sites. The dynamics between several adjacent subpopulations were correlated, suggesting that they experience similar environmental variation. For each subpopulation, we estimated long-term growth rates, as well as the environmental and observation variation. 5. For most of the subpopulations, our estimates of growth rates were considerably lower than those previously reported. In addition, we found considerable variability across subpopulations in their projected risk of severe decline over the next 50 years. 6. Synthesis and applications. We illustrate a new multivariate state-space modelling technique that uses time series of abundance to quantify the data support for different subpopulation configurations. Our analysis of the California sea lion population in the Gulf of California indicates that the population is spatially structured into four subpopulations, each exhibiting distinct risks of extinction. Based on our results, we recommend that conservation and management efforts in the Gulf of California focus on the two subpopulations with high probabilities of extinction within the next 50 years (Northern Midriff, Southern Midriff). Multivariate state-space models provide a practical approach to determine the spatial structure of virtually any species; they may be particularly useful for species of conservation concern for which data on dispersal and environmental drivers are likely to be scarce.

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Stable isotope and scat analyses were used in concert to determine trophic level and dietary overlap among California sea lions from different rookeries in the Gulf of California. Isotopic analysis of the fur of sea lion pups revealed differences in δ15N and δ13C values among rookeries during the breeding season. Mean δ15N and δ13C values varied from 20.2‰ to 22.4‰ and from −15.4‰ to −14.0‰, respectively. The pattern of differences among rookeries was similar between years in most cases. Isotopic variations among rookeries were associated with differences in prey consumption. There was a significant correlation between δ15N value and trophic level, as determined by scat analysis. Joint application of isotopic and scat analyses allowed us to identify how the feeding habits of sea lions vary with location. Our results suggest the presence of spatial structure in available prey as well as the localized use of prey by sea lions across the Gulf of California.

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