The DNA of prey present in animal scats may provide a valuable source of information for dietary studies. We conducted a captive feeding trial to test whether prey DNA could be reliably detected in scat samples from Steller sea lions (Eumetopias jubatus). Two sea lions were fed a diet of fish (five species) and squid (one species), and DNA was extracted from the soft component of collected scats. Most of the DNA obtained came from the predator, but prey DNA could be amplified using prey-specific primers. The four prey species fed in consistent daily proportions throughout the trial were detected in more than 90% of the scat DNA extractions. Squid and sockeye salmon, which were fed as a relatively small percentage of the daily diet, were detected as reliably as the more abundant diet items. Prey detection was erratic in scats collected when the daily diet was fed in two meals that differed in prey composition, suggesting that prey DNA is passed in meal specific pulses. Prey items that were removed from the diet following one day of feeding were only detected in scats collected within 48 h of ingestion. Proportions of fish DNA present in eight scat samples (evaluated through the screening of clone libraries) were roughly proportional to the mass of prey items consumed, raising the possibility that DNA quantification methods could provide semi-quantitative diet composition data. This study should be of broad interest to researchers studying diet since it highlights an approach that can accurately identify prey species and is not dependent on prey hard parts surviving digestion.

This research examines the foraging energetics and diving behavior of the Australian sea lion, Neophoca cinerea. We examine whether the foraging ecology of the Australian sea lion is typical for an animal that has evolved to exploit benthic habitats. Such a strategy is in marked contrast to those utilized by some seabirds and other pinnipeds that feed in the midwater, where travel and search components of the time at sea become more important. Onshore and at‐sea field metabolic rates (FMR) were measured using doubly labeled water in lactating sea lions at Kangaroo Island, South Australia, during the winter of 1988 (early lactation, breeding season 1) and the summer of 1990 (early lactation, breeding season 2). Dive behavior was also measured with dataloggers during these seasons, as well as in the summer of 1991 (late lactation, breeding season 2). The foraging behavior of Neophoca cinerea indicated that it works hard to exploit benthic habitats in the waters around its breeding site. Sea lions maximized time spent at or near the benthos, with 61% of each dive and 35% of their time at sea being spent at the deepest 20% of the dives. The dive pattern was characterized by almost continuous diving when at sea, with 57.9% of their time at sea spent at depths ≥6 m, and dive rates of 10.7 dives/h. Mean surface intervals (1.0–1.9 min) accounted for only 42% of mean dive durations (2.2–4.1 min). Mean dive depths ranged from 41.5 m to 83.1 m, with maximum dives ranging from 60 m to 105 m. The energetic costs of this strategy are high when compared with those of other otariids: the mean at‐sea FMR was 7.05 ± 0.99 W/kg. We report seasonal variability in foraging energetics and dive behavior that is likely to be sensitive to regional oceanography, the maintenance costs of female sea lions and their offspring, and the distribution and behavior of their prey. Further, we note that Australian sea lions are functionally adapted to benefit from benthic foraging strategies because their larger size and insulating blubber convey an advantage over the generally smaller sympatric fur seals that would have a decrease in air/fur insulation with compression at deeper depths and would experience greater water infiltration of the fur with longer dives. Corresponding Editor: C. Martínez del Rio.

Unique DNA sequences are present in all species and can be used as biomarkers for the detection of cells from that species. These DNA sequences can most easily be detected using the polymerase chain reaction (PCR), which allows very small quantities of target DNA sequence to be amplified even when the target is mixed with large amounts of nontarget DNA. PCR amplification of DNA markers that are present in a wide range of species has proven very useful for studies of species diversity in environmental samples. The taxonomic range of species to be identified from environmental samples may often need to be restricted to simplify downstream analyses and to ensure that less abundant sequences are amplified. Group-specific PCR primer sets are one means of specifying the range of taxa that produce an amplicon in a PCR. We have developed a range of group-specific PCR primers for studying the prey diversity found in predator stomach contents and scats. These primers, their design and their application to studying prey diversity and identity in predator diet are described.

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The diving behaviour of 14 female New Zealand sea lions (Phocarctos hookeri) was recorded during early lactation in January and February 1995 on the Auckland Islands, New Zealand. During 73 trips to sea, 19 720 dives were recorded. The average duration of a foraging cycle was 2.9 days (range 1.4–4.8 days), of which 1.7 days (57%) (range 1.1–3.4 days) were spent at sea and 1.2 days (43%) (range 0.8–2.3 days) ashore. At sea the sea lions dived almost continuously at a rate of 7.5 dives/h and spent a mean of 45% of the time submerged (≥ 2 m). Dive behaviour varied among individuals but showed no diel pattern overall. The dive depth for all dives ≥ 6 m was 123 ± 87 m (mean ± SD) (median 124 m, maximum 474 m) and ranged among individuals from 79 ± 85 to 187 ± 166 m. About half of the dives were in the 101- to 180-m range. The duration of all dives was 3.9 ± 1.8 min (median 4.33 min, maximum 11.3 min); about half (51%) of the dive durations were between 4 and 6 min. Surface interval was 4.5 ± 15.8 min (median 1.9 min). Almost half (44%) of all dives exceeded the calculated aerobic dive limit of each sea lion (range 16–73% for individuals). Most dive profiles were flat bottomed and, we believe, are to the benthos. A mean of 51.5% of all dive time was spent in the deepest 85% of the dive. Prey remains found during this study were primarily of benthic and demersal organisms. Phocarctos hookeri is the deepest and longest diving of any of the otariids recorded to date. We suggest that the dive behaviour may reflect either successful physiological adaptation to exploiting benthic prey and (or) a marginal foraging environment in which diving behaviour is close to physiological limits.

Summary1. Foraging behaviours of the Australian sea lion ( Neophoca cinerea ) reflect an animal working hard to exploit benthic habitats. Lactating females demonstrate almost continuous diving, maximize bottom time, exhibit elevated field metabolism and frequently exceed their calculated aerobic dive limit. Given that larger animals have disproportionately greater diving capabilities, we wanted to examine how pups and juveniles forage successfully. 2. Time/depth recorders were deployed on pups, juveniles and adult females at Seal Bay Conservation Park, Kangaroo Island, South Australia. Ten different mother/pup pairs were equipped at three stages of development (6, 15 and 23 months) to record the diving behaviours of 51 (nine instruments failed) animals. 3. Dive depth and duration increased with age. However, development was slow. At 6 months, pups demonstrated minimal diving activity and the mean depth for 23-month-old juveniles was only 44 ± 4 m, or 62% of adult mean depth. 4. Although pups and juveniles did not reach adult depths or durations, dive records for young sea lions indicate benthic diving with mean bottom times (2·0 ± 0·2 min) similar to those of females (2·1 ± 0·2 min). This was accomplished by spending higher proportions of each dive and total time at sea on or near the bottom than adults. Immature sea lions also spent a higher percentage of time at sea diving. 5. Juveniles may have to work harder because they are weaned before reaching full diving capability. For benthic foragers, reduced diving ability limits available foraging habitat. Furthermore, as juveniles appear to operate close to their physiological maximum, they would have a difficult time increasing foraging effort in response to reductions in prey. Although benthic prey are less influenced by seasonal fluctuations and oceanographic perturbations than epipelagic prey, demersal fishery trawls may impact juvenile survival by disrupting habitat and removing larger size classes of prey. These issues may be an important factor as to why the Australian sea lion population is currently at risk.

Abstract. In animal ecology, a question of key interest for aquatic species is how changes in movement behavior are related in the horizontal and vertical dimensions when individuals forage. Alternative theoretical models and inconsistent empirical findings mean that this question remains unresolved. Here we tested expectations by incorporating the vertical dimension (dive information) when predicting switching between movement states (''resident'' or ''directed'') within a state-space model. We integrated telemetry-based tracking and diving data available for four seal species (southern elephant, Weddell, antarctic fur, and crabeater) in East Antarctica. Where possible, we included dive variables derived from the relationships between (1) dive duration and depth (as a measure of effort), and (2) dive duration and the postdive surface interval (as a physiological measure of cost). Our results varied within and across species, but there was a general tendency for the probability of switching into ''resident'' state to be positively associated with shorter dive durations (for a given depth) and longer postdive surface intervals (for a given dive duration). Our results add to a growing body of literature suggesting that simplistic interpretations of optimal foraging theory based only on horizontal movements do not directly translate into the vertical dimension in dynamic marine environments. Analyses that incorporate at least two dimensions can test more sophisticated models of foraging behavior.

Accurate identification of species that are consumed by vertebrate predators is necessary for understanding marine food webs. Morphological methods for identifying prey components after consumption often fail to make accurate identifications of invertebrates because prey morphology becomes damaged during capture, ingestion and digestion. Another disadvantage of morphological methods for prey identification is that they often involve sampling procedures that are disruptive for the predator, such as stomach flushing or lethal collection. We have developed a DNA-based method for identifying species of krill (Crustacea: Malacostraca), an enormously abundant group of invertebrates that are directly consumed by many groups of marine vertebrates. The DNA-based approach allows identification of krill species present in samples of vertebrate stomach contents, vomit, and, more importantly, faeces. Utilizing samples of faeces from vertebrate predators minimizes the impact of dietary studies on the subject animals. We demonstrate our method first on samples of Adelie penguin (Pygoscelis adeliae) stomach contents, where DNA-based species identification can be confirmed by prey morphology. We then apply the method to faeces of Adelie penguins and to faeces of the endangered pygmy blue whale (Balaenoptera musculus brevicauda). In each of these cases, krill species consumed by the predators could be identified from their DNA present in faeces or stomach contents.