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The worldwide smallest cetacean, the vaquita (Phocoena sinus), is on the verge of extinction with fewer than 20 individuals left as of summer 2018. The main cause of mortality is bycatch by entanglement in illegal gillnets and environmental changes. Habitat disturbances have negatively impacted prey diversity, and therefore, the vaquita’s feeding success. We investigated the trophic niche of the vaquita and its variability from 1985 to 1993 by quantifying δ13C and δ15N values from bone samples (n = 33). We reconstructed part of vaquita’s refuge food web and identified some prey items. Between 1985 and 1993 the vaquita’s isotopic ellipses shifted moderately, δ13C and δ15N without significant differences, suggesting variations in habitat baseline values and probably prey composition. We observed a high overlap in the ellipses between males and females, suggesting that both sexes were feeding in the same geographic area over several years and with similar prey composition. Together, our results indicate that the vaquita is a generalist high-TL consumer, feeding on benthic and small pelagic prey it maintained similar mean δ15N values for years. Given these results, conservation efforts should intensify to protect the biodiversity of the Upper Gulf of California and the surviving vaquita.

The rate of energy expenditure and acquisition are fundamental components of an animals’ life history. Within mammals, the otariids (sea lions and fur seals) exhibit energetically expensive life styles, which can be challenging in equatorial regions where resources are particularly limited and unpredictable. To better understand how this energetically expensive life history pattern functions in an energetically challenging equatorial system, we concurrently measured the field metabolic rate (FMR) and foraging behavior of lactating Galapagos sea lions (GSL) rearing pups and yearlings. Females with pups tended to forage to the north, diving deeper, epi and mesopelagically compared to females with yearlings, which foraged to the west and performed dives to the sea bed that were shallower. FMR did not differ between females with pups or yearlings but, increased significantly with % time spent at-sea. Females with yearlings had higher water influx, suggesting greater food intake, but had lower body condition. The FMR (4.08±0.6 W/kg) of GSL is the lowest measured for any otariid, but is consistent with Galapagos fur seals which also exhibit low FMR. The observation that these two otariids have reduced energy requirements is consistent with an adaptation to the reduced prey availability of the Galapagos marine environment compared to other more productive marine systems.

Environmental conditions may influence mating behaviour in otariids breeding in areas with elevated temperatures and intense solar radiation. Although they have evolved morphological and physiological adaptations to prevent overheating while breeding on land, under extreme temperature conditions, they must adjust their behaviour in order to thermoregulate. The California sea lion mating system is based on male competition and displays that occur while defending their territories when females are present. We studied the agonistic behaviour of adult males at two breeding colonies in Mexico with contrasting environmental characteristics: Isla Santa Margarita (ISM) (Pacific coast) and Isla San Esteban (ISE) (Gulf of California). The goal of this study was to determine which variables influence where (i.e. on land or in the water) aggressive interactions between adult males occurred using logistic regression analysis. We analysed three scenarios: (1) both islands, (2) only ISM and (3) only ISE. The best model for the first scenario included the air temperature, density of females and type of aggression. The second scenario involved the density of females, and the third scenario included the rate of female interactions. Although the California sea lion mating strategy involves monopolizing critical resources, our results indicate that density of females and rate of female interactions have a significant impact on where male aggressive interactions occur. Our results highlight how changing environmental conditions affect the behavioural plasticity of this species' mating system. Most notably, males inhabiting high-temperature environments use the thermoregulatory strategy of defending territories adjacent to the coast while remaining immersed in the water. This strategy may result in decreased polygyny in the Gulf of California colonies relative to those on the Pacific coast.

The feeding behavior of marine predators varies with the level of competition to which they are exposed. Populations living in the same or overlapping geographic regions (sympatric) are generally subject to inter-specific competition, which can lead to the development of differing trophic strategies that maximize both nutritional and reproductive efficiency. The otariids of the islands in the western Galapagos Archipelago represent appropriate subjects for studying the response mechanisms of sympatric species exposed to strong competition, both trophic and spatial. The present study has focused on evaluating the possible differences in the trophic niches of the Galapagos sea lion ( Zalophus wollebaeki ) and the Galapagos fur seal ( Arctocephalus galapagoensis ) over time, based upon the analysis of stable isotope ratios of carbon and nitrogen (δ 13 C and δ 15 N), and observation of both species at sea. A comparative analysis of the isotopic signal of skin samples from pups of both species was performed for rookeries on Fernandina Island between 2003 and 2009. Analyses of the isotopic signal present in prey covering the entire trophic range of these predators were also performed, in order to relate this information to otariid sighting records collected during offshore cruises in 1993–1994 and 2000. The δ 13 C and δ 15 N values defined differences in the feeding zones and in the trophic level of each species; which were related with the interannual variability of the ecosystem, i.e. El Nino and La Nina events. Whereas Z. wollebaeki showed a coastal (inshore) foraging strategy, relying on the consumption of small epipelagic fishes, A. galapagoensis turned out to be a more oceanic (offshore) predator, with a preference for small squid. These results are in good agreement with the distribution of these animals at sea, as indicated by the sighting data. The findings of this study provide new insights to our understanding of how sympatric species exposed to strong inter-specific competition can develop foraging strategies leading to a decreasing level of food competition and facilitate their survival in a stochastic environment that is highly demanding in terms of the availability of resources.

Infections with Brucella ceti and pinnipedialis are prevalent in marine mammals worldwide. A total of 22 California sea lions (Zalophus californianus) were examined to determine their exposure to Brucella spp. at San Esteban Island in the Gulf of California, Mexico, in June and July 2011. Although samples of blood, vaginal mucus and milk cultured negative for these bacteria, the application of rose Bengal, agar gel immunodiffusion, PCR and modified fluorescence polarization assays found that five animals (22.7%) had evidence of exposure to Brucella strains. The data also suggested that in two of these five sea lions the strains involved were of terrestrial origin, a novel finding in marine mammals. Further work will be required to validate and determine the epidemiological significance of this finding.

The Gulf of California is unique because of its geographical location and conformation. It hosts diverse ecosystems and important fisheries that support industry and provide livelihood to coastal settlements. It is also the site of interests and