Search

The aim of this study was to determine the trophic level, trophic breadth, and dietary overlap of two species of otariids (Zalophus californianus and Arctocephalus philippii townsendi) at the San Benito Islands in Baja California, Mexico, using scat analysis, and stable isotope analyses (SIA) of carbon and nitrogen in pup hair and dental collagen. Scat samples were collected during summer and winter. The most important prey for both species during summer was squid, comprising 74% of the A. p. townsendi diet and 45% of that of Z. californianus. In winter, squid was 87% of the A. p. townsendi diet, whereas fish was the predominant food (76%) for Z. californianus. Both species were specialist feeders and reached maximum trophic overlap during the summer (CH = 0.9). SIA of the hair of otariid pups showed significant differences in δ15N (P = 0.001), coinciding with A. p. townsendi's greater consumption of squid. The average δ13C value was significantly lower for A. p. townsendi (−16.3‰ ± 0.2‰), reflecting the species' consumption of squid from pelagic habitats, whereas that of Z. californianus (−15.9‰ ± 0.3‰) was significantly higher, reflecting their consumption of benthic fish. Difference in habitat use was confirmed in δ13C values of dental collagen.

RATIONALE Tissues with different turnover rates have different isotope compositions and reflect the different periods in an animal's life when the isotopes are incorporated during the growth of tissues. Bone is one of the most used tissues for reconstruction of an animal's diet; however, the time of isotope integration remains unknown for many species. METHODS The δ15N and δ13C values in tooth dentine and bone tissue from the maxilla and mandible of 21 stranded northern elephant seals, Mirounga angustirostris, collected on the San Benito and Magdalena Islands, Mexico, between 2000 and 2008 were compared. Bone and dentine samples from each growth layer within the tooth were analyzed using a PDZ Europa ANCA-GSL elemental analyzer interfaced with a PDZ Europa 20-20 continuous flow gas source mass spectrometer. RESULTS The δ15N and δ13C values were not different between bone structures, indicating similar turnover rates, metabolic activity and amino acid compositions. The differences in the observed δ13C values between tissues are probably indicative of differences in their amino acid compositions, although the similarities in δ15N values indicated less variation from different amino acids. Correlation of the analyses between isotopic values of tissues suggests that the maxilla and/or mandible of M. angustirostris might reflect the δ15N signal incorporated during the last 5 years of life of the individuals. CONCLUSIONS This study demonstrated the usefulness of the applied approach for providing a best approximation of the timing of isotopic integration into the skull of a marine mammal, thereby reducing uncertainty in exploring historic changes in the species' feeding behavior. Copyright © 2013 John Wiley & Sons, Ltd.

The 2008 IUCN (International Union for Conservation of Nature) review of the status of the world’s mammals identified marine mammals (IUCN 2008) as disproportionally threatened and data poor compared to their terrestrial counterparts, and their status was noted as a particular concern (IUCN 2008, Mace et al. 2008, Schipper et al. 2008). The threats faced by marine mammals were identified as being different, with accidental mortality and pollution being dominant threats that superseded habitat loss, which was identified as the principal concern for land mammals. It was suggested that harvesting remained a major threat for half of the marine mammal species in the world. The distribution of marine mammals was described as being concentrated in tropical and temperate coastal platforms and associated with high levels of primary productivity. Threat levels were highest for marine mammals living in the North Atlantic, North Pacific and Southeast Asia. Range size was described as generally declining toward both poles. Although these generalities are of course correct for all marine mammals, it must be recognized that the larger number of species of cetaceans compared to other marine mammal groups (85 cetaceans, the polar bear Ursus maritimus, 4 extant sirenians and 36 pinniped species) biases the conclusions in the assessment for marine mammals to such a degree that a separate summary for the pinnipeds of the world is warranted. Additionally, a species-level assessment masks some important conservation concerns for pinnipeds with species classified as Least Concern actually containing threatened subspecies ( e.g., ringed seals). Finally, and perhaps most importantly, climate change issues were not specifically addressed in the vast majority of marine mammal assessments done for the 2008 IUCN review (IUCN 2008). We present here a complete assessment of the status and threats facing pinnipeds at the subspecies level, integrating the expected impacts of changing patterns of threat due to global climate change.

The Guadalupe fur seal population at San Benito Islands is experiencing an exponential increase (r = 18.9%, r2 = 0.96) in this first phase of population growth. A similar pattern (r = 13.3%) was observed for Guadalupe fur seals from Gudalupe Island. The relatively rapid growth of the fur seal population on the San Benito Islands is due mainly to an increase in the number of females during the first 2 yrs and of juveniles during the last 2 yrs. The number of males and pups has been fairly constant over time. Apparently this first phase of population growth has been largely the result of the emigration of animals from GI; the only other breeding colony of the species.

Summary 1. Understanding spatial structure and identifying subpopulations are critical for estimating population growth rates and extinction risk, and as such essential for e!ective conservation planning. However, movement and spatiotemporal environmental data are often unavailable, limiting our abilitytodirectlydefine subpopulations and theirlevel 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 di!erent subpopulation numbers and configurations. This technique is applied to the population of CaliforniasealionsZalophuscalifornianusintheGulfofCalifornia, Mexico, distributedacross13breeding sites. 3. The abundanceof California sea lions in the Gulfof California has declinedover the lastdecade, though not all areas have been equally a!ected. In light of this variation, it is important to understandthepopulation structuretoensureaccurateviabilityassessmentsand e!ectivemanagement. 4. Ourdatasupportthehypothesisthatthe GulfofCaliforniasealion population hasfoursubpopulations, each with 2‐5 breeding sites. The dynamics between several adjacent subpopulations were correlated, suggesting that they experience similar environmental variation. For each subpopulation,weestimatedlong-termgrowthrates,aswell astheenvironmental andobservationvariation. 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 theirprojectedriskof severedeclineoverthe next 50 years. 6. Synthesis and applications. We illustrate a new multivariate state-space modelling technique that usestimeseriesofabundancetoquantifythedatasupportfordi!erentsubpopulationconfigurations. OuranalysisoftheCaliforniasealionpopulationintheGulfofCaliforniaindicatesthatthepopulationisspatiallystructuredintofoursubpopulations,eachexhibitingdistinctrisksofextinction.Based on our results, we recommend that conservation and management e!orts in the Gulf of California focus on the two subpopulations with high probabilities of extinction within the next 50 years (NorthernMidri!,SouthernMidri!).Multivariatestate-spacemodelsprovideapracticalapproach to determine the spatial structure of virtually any species; they may be particularly useful for species ofconservationconcernforwhichdataondispersalandenvironmentaldriversarelikelytobescarce.

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 δ 15 N and δ 13 C values among rookeries during the breeding season. Mean δ 15 N and δ 13 C values varied from 20.2‰ to 22.4‰ and from -15.4%o 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 δ 15 N 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.

A pressing need exists to develop new approaches for obtaining information on demographic rates without causing further threats to imperiled animal populations. In this paper, we illustrate and apply a data-fitting technique based on quadratic programming that uses stage-specific abundance data to estimate demographic rates and asymptotic population growth rates (lambda). We used data from seven breeding colonies of California sea lions (Zalophus californianus) in the Gulf of California, Mexico. Estimates of lambda were similar to those from previous studies relying on a diffusion approximation using trends in total abundance. On average, predicted abundances were within 24% of the observed value for the inverse estimation method and within 29% of the observed value for the diffusion approximation. Our results suggest that three of the seven populations are declining (lambda1), but as many as six may be at risk. Elasticity and sensitivity analyses suggest that population management in most sites should focus on the protection of adults, whose survival generally contributes the most to lambda. The quadratic programming approach is a promising noninvasive technique for estimating demographic rates and assessing the viability of populations of imperiled species.

Estimates of demographic parameters are essential for assessing the status of populations and assigning conservation priority. In light of the difficulties associated with obtaining such estimates, vital rates are rarely available even for well-studied species.We present the first estimates of age-specific birth rates for female California sea lions (Zalophus californianus) >10 yr of age. These rates were estimated from the reproductive histories of five cohorts of animals branded as pups between 1980 and 1984 at Los Islotes colony in the Gulf of California, Mexico. Age-specific birth rates varied among age classes and ranged between 0.06 and 0.80. The highest birth rates were observed for females between 10 and 15 yr of age, with decreased birth rates among older females. The effect of age, year, and resighting effort were explored using logistic regression analysis. Based on Akaike Information Criteria, birth rates were best explained by female age, while year and resighting effort did not have a significant effect. The odds ratio of producing a pup decreased with age but did not change significantly for middle-aged females. Our estimates of age-specific birth rates are consistent with general patterns observed for other large vertebrates

Commercial and subsistence fisheries pressure is increasing in the Gulf of California, Mexico. One consequence often associated with high levels of fishing pressure is an increase in bycatch of marine mammals and birds. Fisheries bycatch has contributed to declines in several pinniped species and may be affecting the California sea lion (Zalophus californianus) population in the Gulf of California. We used data on fisheries and sea lion entanglement in gill nets to estimate current fishing pressure and fishing rates under which viable sea lion populations could be sustained at 11 breeding sites in the Gulf of California. We used 3 models to estimate sustainable bycatch rates: a simple population-growth model, a demographic model, and an estimate of the potential biological removal. All models were based on life history and census data collected for sea lions in the Gulf of California. We estimated the current level of fishing pressure and the acceptable level of fishing required to maintain viable sea lion populations as the number of fishing days (1 fisher/boat setting and retrieving 1 day's worth of nets) per year. Estimates of current fishing pressure ranged from 101 (0-405) fishing days around the Los Machos breeding site to 1887 (842-3140) around the Los Islotes rookery. To maintain viable sea lion populations at each site, the current level of fishing permissible could be augmented at some sites and should be reduced at other sites. For example, the area around San Esteban could support up to 1428 (935-2337) additional fishing days, whereas fishing around Lobos should be reduced by at least 165 days (107-268). Our results provide conservation practitioners with site-specific guidelines for maintaining sustainable sea lion populations and provide a method to estimate fishing pressure and sustainable bycatch rates that could be used for other marine mammals and birds.

In recent years, more than 130 extinctions have been estimated to have occurred in the marine realm. Here we review this body of evidence and show that this figure may actually be overestimated by as much as 50%. We argue that previous estimates have not fully taken into account critical uncertainties such as naturally variable geographical distributions, and have misinterpreted documentary evidence. However, current evidence indicates that some sharks, rays and reef-associated species, although not necessarily geographically restricted, are particularly vulnerable to anthropogenic impacts and now occur in very low numbers. Overestimating extinctions is of concern because it could reduce confidence in the credibility of the ‘extinct’ category in threatened species lists and, ultimately, be used to question the integrity of conservation and management policies. We suggest that when integrating future checklists of marine extinct species, there needs to be a more rigorous use of the terminology of extinction, and participation by specialists in each of the particular taxonomic groups involved.

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.

Female northern elephant seals, Mirounga angustirostris, from Ano 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 δ13C and δ15N 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 δ13C and δ15N 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.

The Gulf of California harbors about 15% of the total California sea lion (Zalophus californianus californianus) population. We studied pup morphometrics from eight Gulf of California rookeries during the 1996 and 1997 reproductive seasons to describe sex differences in body size and body condition indices. Newborn pup body size was not different from previous reports. Male pups were heavier and larger than female pups in terms of all linear dimensions. Morphometric relationships, however, showed that males were 3%-4% denser, and that after removing the effects of length, they were about 2% heavier than females. Sculp depth adjusted for length was 12% larger in female than in male pups. Our data provide further evidence that male otariid pups may allocate a larger fraction of milk energy to muscular and skeletal growth compared to female pups.