Sea Lion Diet Research Articles

Salmon farming, overfishing and southern sea lion: Not so opportunistic responses of a top predator to human perturbations in the Patagonian Fjords

Pinnipeds are considered generalist predators that prey opportunistically on more abundant prey. Thus, their diet could be affected by natural and anthropogenic stressors, such as declines in native prey and increases in exotic prey such as escaped salmon. Using stable isotopes, we evaluated temporal and spatial variability in the diet of southern sea lion (Otaria flavescens), and assessed its response to seasonal and stochastic variability in native prey and escaped salmon. Demersal fishes (median contribution range = 50–79 %) and escaped salmon (6–44 %) were the most important prey, in all colonies and seasons examined. The median estimated contributions of caged salmon, Munida gregaria and Sprattus fuegensis were consistently lower than 10% in all colonies and seasons. Variability in the relative availability of stocked and escaped salmon, as well as in Merluccius australis and S. fuegensis, showed limited influence on the contribution of these prey to the diet. Thus, although a noticeable response was evident after a large-scale escape of farmed Atlantic salmon that occurred during the study period, the opportunistic behaviour of southern sea lion seems questionable. A better understanding of dietary responses to induced changes in prey availability may have management implications in marine habitats affected by overfishing and farming of exotic species.

ЭКОЛОГИЯ ПИТАНИЯ СИВУЧА С РЕПРОДУКТИВНОГО ЛЕЖБИЩА НА МЫСЕ КОЗЛОВА (ВОСТОЧНАЯ КАМЧАТКА)

We studied the Steller's sea lion Eumetopias jubatus feeding ecology based on undigested food items found in feces at the Kozlova Cape rookery, Eastern coast of Kamchatka Peninsula, in summers (2004 through 2008). Atka mackerel Pleurogrammus monopterygius and walleye pollock Theragra chalcogramma were dominant in a sea lion diet hauled out at Kozlova Cape rookery in terms of frequency of occurrence. Less frequently occurred were Pacific sand lance Ammodytes hexapterus, Threespine stickleback Gasterosteus aculaeatus, Smelt Osmerus mordax, Sculpins Cottidae, Pacific sandfish Trichodon trichodon, Flatfishes Pleuronectidae, Salmon Oncorhynchus sp., Pacific cod Gadus macrocephalus, and Squid and Octopus Cephalopoda. The contribution of the other food items was not significant. Diet diversity median during sea lion single foraging trip was two prey items, range being one to nine Atka mackerel and Walleye pollock as the major prey objects. Most of sea lion food items were referred to species featuring a marked seasonal presence near the shore or on the shelf. Sea lions feed near the rookery close to the shore on seasonal food aggregations common to the area. Seasonal prey aggregations can be extremely important for breeding animals due to increasing the efficiency of food search and capture, which in turn can affect the reproductive success of animals and their survival. In this regard, the stability of the food resource near Kozlova Cape Rookery may influence sea lion population status there.

Steller sea lion diet in the Eastern Kamchatka

We studied the diet of Steller sea lion (Eumetopias jubatus) on the eastern coast of Kamchatka based on the remains of undigested food parts found in feces at terrestrial sites near Cape Kozlova (rookery) and Cape Kekurny (haulout) in the summer. A total of 276 fecal samples were analyzed from 2004 to 2008. Forty eight food items of 17 fish families and one family of cephalopod mollusks were identified. Steller sea lion diet was diverse yet dominated by only several species. The pollock Gadus chalcogrammus, Atka mackerel Pleurogrammus monopterygius, Pacific sand lance Ammodytes hexapterus and Hemilepidotus sp. were dominant by frequency of occurrence in decreasing order at both sites. Differences were found in the Steller sea lion diet in the studied sites. Prey diversity at Kekurny Cape within scat samples ranged from one to twelve objects within the median of four prey item per a sample. The median of prey diversity at Kozlov Cape within scat samples was two food objects within a range of one to nine prey types per sample. Of the 37 scats (24,5 %) collected at Kozlov Cape that contained a single prey item, 75,7% contained Atka mackerel. Among scats collected at Kekurny Cape that contained a single prey species (n=10, 8,0 %), 50,0 % contained Atka mackerel and 40,0 % pollock. In contrast to the high diet diversity at Kekurny Cape haulout, breeding Steller sea lions at Kozlova Cape rookery fed on a narrow set of prey items. Interannual changes in the diet structure at each site were not statistically significant.

Diet composition of an opportunistic predator from an upwelling area in the Southeastern Pacific

AbstractThe aim of this study was to use scat analysis to analyse the dietary composition of the South American sea lion (SASL, Otaria byronia) over a short temporal scale and in different but closely located colonies in the Humboldt Current System of northern Chile. Scat samples were collected at Arica (18°34′S), Iquique (20°48′S) and Mejillones (23°04′S) during the summer (January to March) and winter (July) of 2015. A total of 16 595 individual prey items from nine species were found in 194 of the 215 scat samples. In general, squat lobster (Pleuroncodes monodon) was the most important prey item for the sea lion diet, both by number and according to the index of relative importance (%IRI), followed by anchovy (Engraulis ringens), which was the most frequent prey item in the scats and the most important by weight. Patagonian squid (Loligo gahi) ranked third in importance in the sea lion diet. However, some spatiotemporal variations in the consumption of prey were found. Squat lobster was the most important prey for sea lions in Arica and Iquique, while anchovy was the dominant prey in Mejillones. Regarding seasonal variation, squat lobster was more important in winter than in summer in the three localities, while anchovy was also more important during winter, except in Mejillones where the levels of consumption of this prey were similar in both seasons. These results show that SASL adjust their dietary preferences, modifying the contribution of different prey both spatially and temporally. Spatiotemporal variations in the SASL diet can be explained by temporal changes in the distribution and abundance of their prey.

A re-examination of the relationship between Steller sea lion (Eumetopias jubatus) diet and population trend using data from the Aleutian Islands

Prey diversity and energy density have been linked to each other and to population trends in many studies of bird and mammal diets. We re-examined these relationships in Steller sea lions (Eumetopias jubatus (Schreber, 1776)) using data collected from the Aleutian Islands, where there has been a strong longitudinal gradient in population trend. Diet diversity and energy density metrics were similar in the western Aleutians, where sea lion counts declined consistently, and in the easternmost Aleutian area, where population trends improved significantly. We compared traditional deterministic diet diversity metrics with diversity scores based on an occupancy model that accounts for differences in sample size and uncertainty in prey group detection. This analysis indicated that there was no significant change in diet diversity over the 23-year study period or any significant differences across the Aleutian Islands. These results are consistent with prey abundance data from nine groundfish bottom trawl surveys conducted over the same period. While diet studies detail what Steller sea lions eat and provide an estimate of their energy intake, they provide only limited information on the energy expended to obtain their food or the consequences of their diet and foraging ecology on individual or population fitness.

Small-scale variation in the diet of the South American Sea lion (Otaria flavescens) in northern Patagonia (Argentina)

An understanding of the mechanisms that shape animal-population dynamics is a fundamental consideration in conservation biology and ecology. Populations of the species of South-American sea lions (SASLs) Otaria flavescens reveal different growth rates, which variations could be linked to changes in the accessibility of nutritional resources in any given location. The available information on differences in the SASL diet on a local scale, however, is relatively scarce. We therefore searched for evidence of geographical variations in the diet of the SASLs in Northern Patagonia—namely, in the Golfo San Matías (GSM) and the Golfo Nuevo (GN), both of which bays are quite different regarding their physiography, oceanography, and anthropic pressures. From 2005–2013, stomach samples ( nGSM=39; nGN=31) were collected from animals found dead on the coast or accidentally caught in fishing nets. The dietary composition differed between the two gulfs with respect to the most common species, the zoological groups, and the sizes of the primary prey. In the GSM, the SASLs fed on demersal–pelagic fish, while in the GN the diet exhibited a high contribution of benthic species. Differences in the items and sizes of prey consumed at each area are discussed in terms of differential prey availability along with the opportunistic feeding behavior of this predator. These differences, in addition to the species’s main prey as targets for major fisheries, must be taken into account in order to understand the different population dynamics of this apex predator and thus ensure its conservation.

The diet of free-ranging male Steller sea lions (Eumetopias jubatus) in the eastern Bering Sea: a retrospective analysis based on stomach contents of an endangered pinniped

This study illuminates historical diet and foraging locations of endangered western U.S. stock Steller sea lions (Eumetopias jubatus (Schreber, 1776)). Prey were identified from stomachs of 22 males collected in the eastern Bering Sea from the ice edge in March 1985 and nearshore St. Paul Island in September–October 1985 and 1986. Percent frequency of occurrence (PFO) and percent number (PN) were highest for walleye pollock (Gadus chalcogrammus Pallas, 1814; PFO 69%, PN 15%, mean length 17 cm), Pacific herring (Clupea pallasii Valenciennes in Cuvier and Valenciennes, 1847; PFO 62%, PN 16%, mean length 26 cm), shorthorn sculpin (Myoxocephalus scorpius (Linnaeus, 1758); PFO 54%, PN 30%), and Pacific giant octopus (Enteroctopus dofleini (Wülker, 1910); PFO 39%, PN 8%, mean weight 31 kg) in spring, and northern rock sole (Lepidopsetta polyxystra Orr and Matarese, 2000; PFO 78%, PN 47%, mean length 35 cm), Pacific cod (Gadus macrocephalus Tilesius, 1810; PFO 56%, PN 12%, mean length 62 cm), walleye pollock (PFO 44%, PN 7%, mean length 49 cm), and red Irish lord (Hemilepidotus hemilepidotus (Tilesius, 1811); PFO 11%, PN 9%) in fall. Species of Cryptacanthidae, Liparidae, and Zoarcidae were highly represented and exclusive to spring collections. Predictable seasonal concentrations and movements of mature prey along frontal boundaries of the continental shelf and ice edge may be critical to male Steller sea lion fitness during the non-breeding season.

Changes in California sea lion diet during a period of substantial climate variability

The California Current Ecosystem (CCE) is a productive eastern boundary upwelling system that supports a wide variety of forage stocks. Decadal and interannual variability in the environment influence forage species, which in turn affects predators. The recent diet of California sea lions (Zalophus californianus; CSL) from Central California was determined using identification of hard parts from scat samples (n = 785) collected on Ano Nuevo Island (37.1083°N 122.3378°W) in 2010 and 2012–2016. Comparisons were made with previously reported data from the late 1990’s and with prey availability as measured by fishery-independent surveys. A significant shift in diet was seen between the two decades where diet from the 1990’s was dominated by Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax). By 2010, diet was more diverse, characterized by rockfishes, Pacific hake, and market squid. There were also strong interannual differences in diet during the most recent decade, a time of substantial climate variability in the North Pacific Ocean. In general, prey were consumed in relation to what was available in the environment.

Changes in the feeding ecology of South American sea lions on the southern Brazilian coast over the last two decades of excessive fishing exploration

In the last decades, an increasing fishing effort and a decreasing trend in fish catches have been observed in southern Brazil. Considering that marine mammals and fisheries usually compete for the same resources, it is reasonable to presume that the feeding ecology of these predators is affected by the current scenario. To evaluate this hypothesis, long-term variation in the diet of the South American sea lion (Otaria flavescens) relative to fisheries exploitation was analyzed for two periods (1993–2003 versus 2004–2014). The degree of overlap between the relative biomass of the sea lions’ diet and the target species of six types of local fishery was analyzed. An increase in prey overlap between sea lions and fisheries was observed in the more recent sampling period, along with an increase in prey diversity, richness, and niche breadth of the sea lions’ diet. These results suggest that the overfishing scenario could partly explain the modified feeding ecology of the sea lions. In this context, we recommend a review and better regulation of the current fishing effort in the region, which we believe will be an important step to maintain the fish stocks and minimize the impact of fishing on marine top predators.

Spatial, temporal, age, and sex related variation in the diet of South American sea lions in southern Chile

AbstractWe analyzed spatial, temporal, and age/sex variation in the diet of the South American sea lions (SASL, Otaria byronia) in southern Chile via δ13C and δ15N isotope analysis, with emphasis on exotic farmed and feral salmonids as a source of prey for this generalist predator. Isotope mixing models show that the primary prey consumed by SASL were demersal and benthopelagic fishes. Individuals sampled in summer and in the outer coast had higher δ15N values than those from winter or from the inner coast due to consumption of high trophic level demersal fish. We also found evidence of an ontogenetic shift in isotopic niche width as measured by δ13C and δ15N standard ellipse areas, with juveniles showing a restricted isotopic niche width (1.0‰2) in comparison to subadult (2.2‰2) and adult males (1.9‰2). The proportion of salmonids in the diet of the SASL was negligible and lower than reported in previous studies, which may be related to a recent drop in the production of salmon aquaculture in the region. Our study suggests that the SASL are able to adapt to shifts in spatiotemporal variations in the abundance of potential prey, including feral and farmed salmonids.

A fully coupled ecosystem model to predict the foraging ecology of apex predators in the California Current

MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Vol. 556: 273–285, 2016 doi: 10.3354/meps11849 Published September 8 A fully coupled ecosystem model to predict the foraging ecology of apex predators in the California Current J. Fiechter 1, *, L. A. Huckstadt 2 , K. A. Rose 3 , D. P. Costa 2 Institute of Marine Sciences, University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA Department of Ecology and Evolutionary Biology, University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA Department of Oceanography and Coastal Sciences, Louisiana State University, 1002-Y Energy, Coast & Environment Building, Baton Rouge, LA 70803, USA ABSTRACT: Results from a fully coupled end-to-end ecosystem model for the California Current Large Marine Ecosystem are used to describe the impact of environmental variability and prey availability on the foraging ecology of its most abundant apex predator, the California sea lion Zalophus californianus. The ecosystem model consists of a biogeochemical submodel embedded in a regional ocean circulation submodel, both coupled with a multi-species individual-based sub- model for forage fish (sardine and anchovy) and California sea lions. Sardine and anchovy are explicitly represented in the model as they are commonly found in the diet of sea lions and exhibit significant interannual and decadal variability in population abundances that reflect variations in their environment and lower trophic level prey. Output from a 20 yr run (1989−2008) of the model demonstrates how different physical and biological processes control habitat utilization and forag- ing success of California sea lions on interannual time scales, with the dominant modes of variabil- ity linked to sardine abundance and coastal upwelling intensity. The results also illustrate how variability in environmental conditions, forage fish distribution, and prey assemblage affect sea lion feeding success. While specifically focusing on the foraging ecology of sea lions, the modeling framework has the ability to provide a more complete understanding of the physical and biologi- cal mechanisms impacting trophic interactions in the California Current, or other regions where similar fully coupled ecosystem models may be implemented. KEY WORDS: Ecosystem model · Foraging ecology · California Current · Upwelling · Sea lions · Marine predators · Zalophus californianus Resale or republication not permitted without written consent of the publisher Determining the relationships between environ- mental variability, prey field dynamics, and the foraging ecology of marine mammals and other apex predators has been an on-going research topic for several decades (Fiedler et al. 1998). However, re - cent pro gress in animal tagging has shed new light on the mechanisms through which changing environmental conditions may impact the habitat selection of pelagic predators (Block et al. 2011, Irvine et al. 2014). Such information is critical to identify the underlying physical and biological processes that control the foraging and migratory behavior of higher trophic level species (Weise et al. 2006, Bailey et al. 2009). Understanding the linkages among environmental variability, prey availability, and predator distribution should ulti- mately lead to a better characterization of biologi- cal ‘hot spots’ in the ocean and how these hot spots may respond to changing climate conditions (Pala- cios et al. 2006). *Corresponding author: fiechter@ucsc.edu © Inter-Research 2016 · www.int-res.com INTRODUCTION

Assessment of Competition between Fisheries and Steller Sea Lions in Alaska Based on Estimated Prey Biomass, Fisheries Removals and Predator Foraging Behaviour.

A leading hypothesis to explain the dramatic decline of Steller sea lions (Eumetopias jubatus) in western Alaska during the latter part of the 20th century is a change in prey availability due to commercial fisheries. We tested this hypothesis by exploring the relationships between sea lion population trends, fishery catches, and the prey biomass accessible to sea lions around 33 rookeries between 2000 and 2008. We focused on three commercially important species that have dominated the sea lion diet during the population decline: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estimate the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass. Of 304 comparisons, we found 3 statistically significant relationships, all suggesting that sea lion populations increased with increasing prey accessibility. Given that the majority of comparisons showed no significant effect, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s.

Shifting niches of marine predators due to human exploitation: the diet of the South American sea lion (Otaria flavescens) since the late Holocene as a case study

Abstract—Stable isotope ratios of carbon and nitrogen in archaeological and modern bone samples have been used to reconstruct the dietary changes of the South American sea lionOtaria flavescensfrom the late Holocene to the present in the southwestern Atlantic. We sampled bones from archaeological sites in northern-central and southern Patagonia, Argentina, and bones housed in modern scientific collections. Additionally, we analyzed the stable isotope ratios in ancient and modern shells of intertidal molluscs to explore changes in the isotope baseline and allow comparison between bone samples from different periods after correction for baseline shifts. Results confirmed the trophic plasticity of the South American sea lion, demonstrated the much larger impact of modern exploitation of marine resources as compared with that of hunter-gatherers, and underscored the dissimilarity between the past and modern niches of exploited species. These conclusions are supported by the rather stable diet of South American sea lions during several millennia of aboriginal exploitation, in both northern-central and southern Patagonia, and the dramatic increase in trophic level observed during the twentieth century. The recent increase in trophic level might be related to the smaller population size resulting from modern sealing and the resulting reduced intraspecific competition. These results demonstrate how much can be learned about the ecology of modern species thanks to retrospective studies beyond the current, anthropogenically modified setting where ecosystem structure is totally different from that in the pristine environments where current species evolved.

Dietary consistency of male South American sea lions (Otaria flavescens) in southern Brazil during three decades inferred from stable isotope analysis

Marine predators may undergo remarkable dietary changes through time as a result of both anthropogenic and natural changes in the environment, but this variability is often difficult to tackle and seldom incorporated into ecosystem models. This paper uses the stable isotope ratios of carbon and nitrogen in skeletal material of South American sea lions from Brazilian scientific collections to investigate whether these animals modified their diet from 1986 to 2009, as reported for other marine predators in the region. Stable isotope ratios indicated that demersal potential prey were always enriched in 13C as compared with pelagic prey. Accordingly, the absence of any statistically significant correlation between stranding year and the δ13C values of adult males indicated no major increase in the consumption of pelagic prey from 1986 to 2009. Likewise, the results of the mixing model SIAR revealed a mixed diet including pelagic and demersal prey, with a central role for demersal fishes throughout the whole period. Furthermore, SIAR suggested no major changes in the proportion of pelagic and demersal prey in the diet of adult male South American sea lions during the past three decades. Demersal fishes were also relevant prey for juvenile South American sea lions during the whole period, but they always consumed a larger proportion of pelagic prey than the adults did. These results suggest no major changes in the diet of male South American sea lions during the past three decades in southern Brazil, contrary to what has been reported for other to predators in the regions and for the species in northern Patagonia.

Fine‐scale diet of the Australian sea lion (Neophoca cinerea) using DNA‐based analysis of faeces

AbstractWe applied DNA‐based faecal analysis to determine the diet of female Australian sea lions (n = 12) from two breeding colonies in South Australia. DNA dietary components of fish and cephalopods were amplified using the polymerase chain reaction and mitochondrial DNA primers targeting the short (~100 base pair) section of the 16S gene region. Prey diversity was determined by sequencing ~50 amplicons generated from clone libraries developed for each individual. Faecal DNA was also combined and cloned from multiple individuals at each colony and fish diversity determined. Diets varied between individuals and sites. Overall, DNA analysis identified a broad diversity of prey comprising 23 fish and five cephalopod taxa, including many species not previously described as prey of the Australian sea lion. Labridae (wrasse), Monacanthidae (leatherjackets) and Mullidae (goat fish) were important fish prey taxa. Commonly identified cephalopods were Octopodidae (octopus), Loliginidae (calamary squid) and Sepiidae (cuttlefish). Comparisons of fish prey diversity determined by pooling faecal DNA from several samples provided a reasonable but incomplete resemblance (55–71%) to the total fish diversity identified across individual diets at each site. Interpretation of diet based on the recovery of prey hard‐parts identified one cephalopod beak (Octopus sp.) and one fish otolith (Parapriacanthus elongatus). The present study highlights the value of DNA‐based analyses and their capabilities to enhance information of trophic interactions.

Estimating size of opalfish from otoliths: implications for analyses of New Zealand sea lion diet

Anomalies can arise when species-specific otolith equations are applied to congeneric species in analyses of seal prey remains. Opalfishes, Hemerocoetes, two spp., are the most numerous fish in the diet of New Zealand sea lions, Phocarctos hookeri, at their population base at the Auckland Islands. Opalfish identified from otoliths in sea lion prey remains accounted for one-third of prey items in this and two previous diet studies. This highlighted the need to estimate size of opalfish accurately. Equations to estimate fish size from otolith length differed between the two species because otoliths of H. morelandi were relatively smaller than otoliths of H. artus. However, otoliths of these species were indistinguishable and this precluded definitive estimates of opalfish length and mass from otoliths. In particular, misidentification of otoliths could create untenably large estimates for fish size. A combination of the equations for otolith size to fish size from the two species avoided this anomaly and superseded previously published equations.

Regional variation in trophic ecology of adult female Australian sea lions inferred from stable isotopes in whiskers

Context The primary selective forces responsible for shaping life-history traits come from the physical and biological environment in which a species resides. Consequently, the limits of a species range may provide a useful measure of adaptive potential to environmental change. The proximity of foraging grounds to terrestrial nursing habitat constrains central-place foragers such as otariid seals in selecting breeding locations. The Australian sea lion (Neophoca cinerea) is an endangered otariid endemic to Australia, whose northern-range extent occurs at a temperate–tropical transition zone on the western coast of Western Australia (WA). Aims Currently, there is a complete absence of data on the foraging ecology of Australian sea lions in WA. We sought to address this critical knowledge gap and provide data on the foraging ecology of adult female Australian sea lions at three isolated breeding colonies in western WA. Methods We used stable-isotope ratios of carbon (δ13C) and nitrogen (δ15N) in the whiskers of pups as proxies to characterise feeding behaviour of 10–28% of all adult female Australian sea lions at each colony. We then compared these geographic data to (1) conspecifics at similar latitude in South Australia (SA) and (2) isotopic data collated from other studies on seabirds that inhabit the region, to place foraging behaviour of adult female Australian sea lions into context. Key results At the southernmost colonies in WA, individual animals were members of one of two distinct isotopic clusters that could be described by differences in δ15N and δ13C values. Individuals at the northernmost colony displayed δ15N values similar to those of seabirds in the same region. Across the study, isotope ratios of adult female Australian sea lions in western WA were between 3‰ and 5‰ lower than those observed at a colony at similar latitude in SA. Conclusions Gross differences in the physical oceanography between WA and SA may in part explain the differences in isotope ratios of individuals between the regions, with lower δ15N and δ13C values in WA probably reflecting the relatively depauperate conditions of the Leeuwin Current. Implications Potential regional differences in trophic structure should be considered when developing appropriate management plans for Australian sea lions and regional variation in the diet of Australian sea lion warrants further investigation.

The diet of the South American sea lion (Otaria flavescens) at Río Negro, Patagonia, Argentina, during the winter-spring period

The South American sea lion, Otaria flavescens (Shaw, 1800) population is steadily expanding along the Patagonian coast of Argentina in the last decades. However, little is known about the feeding ecology of the species in the area. The aim of this study was to analyze the food habits of O. flavescens from 91 scats collected at Río Negro province, during the winter and spring of 2005. Fish occurred in 96% of scats containing prey remains, followed by cephalopods (26%). Raneya brasiliensis (Kaup, 1856) was the most frequent and abundant species occurring in 58.6% of samples and constituting almost 50% of fish predated. Second in importance were Porichthys porosissimus (Cuvier, 1829) and Cynoscion guatucupa (Cuvier, 1830) in terms of occurrence (%FO 20.7) and numbers (29.6%) respectively. The squid Loligo gahi (d'Orbigny, 1835) was the most frequent cephalopod prey (42.1%), whereas Octopus tehuelchus (d'Orbigny, 1834) was the most abundant (77%). The higher amount and diversity of prey found in the spring in comparison with the winter season might be related to a higher feeding activity of seals or to a seasonal increase in food availability in the area.

Autumn diet of recolonising female New Zealand sea lions based at Otago Peninsula, South Island, New Zealand

New Zealand (NZ) sea lions (Phocarctos hookeri) are slowly recolonising the Otago coast, South Island, New Zealand. The increase in their numbers may lead to resource competition with other marine predators and fisheries. We determined the diet of female NZ sea lions at Otago during autumn. In total, 571 scats and 110 regurgitations were collected on Otago Peninsula during 2008 and 2009. Barracouta (Thyrsites atun) and jack mackerel (Trachurus sp.) were the two main prey species and accounted for 26% and 31% of the reconstituted biomass, respectively. This was consistent between two years. Only five other species contributed > 5% of the diet by biomass in either year. Prey species are all found on the narrow continental shelf surrounding Otago Peninsula. The main prey species of Otago NZ sea lions may be of higher energy content than prey in the Auckland Islands (remnant breeding area). Resource overlap with other marine predators and fisheries appears to occur around Otago Peninsula. A marine trophic model of the area off Otago Peninsula would help understanding potential competition between marine predators and fisheries in this area.

Intraspecific comparison of diet of California sea lions (Zalophus californianus) assessed using fecal and stable isotope analyses

The diet of juvenile and adult female California sea lions ( Zalophus californianus (Lesson, 1828)) at San Miguel Island, California, was estimated and compared using fecal and stable isotope analyses to determine dietary differences by age. Fecal samples were collected during 2002–2006 and prey remains were identified. Stable carbon (δ13C) and stable nitrogen (δ15N) isotope values were determined from plasma and fur obtained from yearlings, 2- to 3-year-old juveniles, and adult females during 2005 and 2006. Juveniles ate more than 15 prey taxa, whereas adult females consumed more than 33 taxa. Relative importance of prey was determined using percent frequency of occurrence (%FO). Engraulis mordax Girard, 1854, Sardinops sagax (Jenyns, 1842), Merluccius productus (Ayres, 1855), genus Sebastes Cuvier, 1829, and Loligo opalescens Berry, 1911 were the most frequently occurring (%FO > 10%) prey in the feces of both juvenile and adult female sea lions, although their importance varied between age groups. Only yearlings had significantly different isotopic values than older conspecifics, indicating that older juveniles were feeding at a similar trophic level and in similar habitats as adult females. Whereas each method had biases, combining the two provided a better understanding of the diet of California sea lions and intraspecific differences.