Interannual variation in the at‐sea behavior of California sea lions (Zalophus californianus)

To be successful, marine predators must alter their foraging behavior in response to changes in their environment. To understand the impact and severity of environmental change on a population it is necessary to first describe typical foraging patterns and identify the underlying variability that exists in foraging behavior. Therefore, we characterized the at‐sea behavior of adult female California sea lions (n = 32) over three years (2003, 2004, and 2005) using satellite transmitters and time‐depth recorders and examined how foraging behavior varied among years. In all years, sea lions traveled on average 84.7 ± 11.1 km from the rookery during foraging trips that were 3.2 ± 0.3 d. Sea lions spent 42.7% ± 1.9% of their time at sea diving and displayed short (2.2 ± 0.2 min), shallow dives (58.5 ± 8.5 m). Among individuals, there was significant variation in both dive behavior and movement patterns, which was found in all years. Among years, differences were found in trip durations, distances traveled, and some dive variables (e.g., dive duration and bottom time) as sea lions faced moderate variability in their foraging habitat (increased sea‐surface temperatures, decreased upwelling, and potential decreased prey abundance). The flexibility we found in the foraging behavior of California sea lions may be a mechanism to cope with environmental variability among years and could be linked to the continuing growth of sea lion populations.     

The influence of depth on a breath-hold diver: Predicting the diving metabolism of Steller sea lions (Eumetopias jubatus)

Diving animals must endeavor to increase their dive depths and prolong the time they spend exploiting resources at depth. Results from captive and wild studies suggest that many diving animals extend their foraging bouts by decreasing their metabolisms while submerged. We measured metabolic rates of Steller sea lions (Eumetopias jubatus) trained to dive to depth in the open ocean to investigate the relationships between diving behaviour and the energetic costs of diving. We also constructed a general linear model to predict the oxygen consumption of sea lions diving in the wild. The resultant model suggests that swimming distance and depth of dives significantly influence the oxygen consumption of diving Steller sea lions. The predictive power of the model was tested using a cross-validation approach, whereby models reconstructed using data from pairs of sea lions were found to accurately predict the oxygen consumption of the third diving animal. Predicted oxygen consumption during dives to depth ranged from 3.37 L min^− 1 at 10 m, to 1.40 L min^− 1 at 300 m over a standardized swimming distance of 600 m. This equated to an estimated metabolic rate of 97.54 and 40.52 MJ day^− 1, and an estimated daily feeding requirement of 18.92 and 7.96 kg day^− 1 for dives between 10 and 300 m, respectively. The model thereby provides information on the potential energetic consequences that alterations in foraging strategies due to changes in prey availability could have on wild populations of sea lions.     

Individual Foraging Strategies Reveal Niche Overlap between Endangered Galapagos Pinnipeds

Most competition studies between species are conducted from a population-level approach. Few studies have examined inter-specific competition in conjunction with intra-specific competition, with an individual-based approach. To our knowledge, none has been conducted on marine top predators. Sympatric Galapagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki) share similar geographic habitats and potentially compete. We studied their foraging niche overlap at Cabo Douglas, Fernandina Island from simultaneously collected dive and movement data to examine spatial and temporal inter- and intra-specific competition. Sea lions exhibited 3 foraging strategies (shallow, intermediate and deep) indicating intra-specific competition. Fur seals exhibited one foraging strategy, diving predominantly at night, between 0–80 m depth and mostly at 19–22 h. Most sea lion dives also occurred at night (63%), between 0–40 m, within fur seals'' diving depth range. 34% of sea lions night dives occurred at 19–22 h, when fur seals dived the most, but most of them occurred at dawn and dusk, when fur seals exhibited the least amount of dives. Fur seals and sea lions foraging behavior overlapped at 19 and 21 h between 0–30 m depths. Sea lions from the deep diving strategy exhibited the greatest foraging overlap with fur seals, in time (19 h), depth during overlapping time (21–24 m), and foraging range (37.7%). Fur seals foraging range was larger. Cabo Douglas northwest coastal area, region of highest diving density, is a foraging “hot spot” for both species. Fur seals and sea lions foraging niche overlap occurred, but segregation also occurred; fur seals primarily dived at night, while sea lions exhibited night and day diving. Both species exploited depths and areas exclusive to their species. Niche breadth generally increases with environmental uncertainty and decreased productivity. Potential competition between these species could be greater during warmer periods when prey availability is reduced.     

Variation in energy expenditure among black-legged kittiwakes: effects of activity-specific metabolic rates and activity budgets

We sought to determine the effect of variation in time-activity budgets (TABs) and foraging behavior on energy expenditure rates of parent black-legged kittiwakes (Rissa tridactyla). We quantified TABs using direct observations of radio-tagged adults and simultaneously measured field metabolic rates (FMR) of these same individuals (n=20) using the doubly labeled water technique. Estimated metabolic rates of kittiwakes attending their brood at the nest or loafing near the colony were similar (ca. 1.3 x basal metabolic rate [BMR]), although loafing during foraging trips was more costly (2.9 x BMR). Metabolic rates during commuting flight (7.3 x BMR) and prey-searching flight (6.2 x BMR) were similar, while metabolic rates during plunge diving were much higher (ca. 47 x BMR). The proportion of the measurement interval spent foraging had a positive effect on FMR (R2=0.68), while the combined proportion of time engaged in nest attendance and loafing near the colony had a negative effect on FMR (R2=0.72). Thus, more than two-thirds of the variation in kittiwake FMR could be explained by the allocation of time among various activities. The high energetic cost of plunge diving relative to straight flight and searching flight suggests that kittiwakes can optimize their foraging strategy under conditions of low food availability by commuting long distances to feed in areas where gross foraging efficiency is high.     

Energetic cost of foraging in free-diving emperor penguins.

Hypothesizing that emperor penguins (Aptenodytes forsteri) would have higher daily energy expenditures when foraging for their food than when being hand-fed and that the increased expenditure could represent their foraging cost, we measured field metabolic rates (FMR; using doubly labeled water) over 4-d periods when 10 penguins either foraged under sea ice or were not allowed to dive but were fed fish by hand. Surprisingly, penguins did not have higher rates of energy expenditure when they dove and captured their own food than when they did not forage but were given food. Analysis of time-activity and energy budgets indicated that FMR was about 1.7 x BMR (basal metabolic rate) during the 12 h d(-1) that penguins were lying on sea ice. During the remaining 12 h d(-1), which we termed their "foraging period" of the day, the birds were alert and active (standing, preening, walking, and either free diving or being hand-fed), and their FMR was about 4.1 x BMR. This is the lowest cost of foraging estimated to date among the eight penguin species studied. The calculated aerobic diving limit (ADL(C)), determined with the foraging period metabolic rate of 4.1 x BMR and known O(2) stores, was only 2.6 min, which is far less than the 6-min ADL previously measured with postdive lactate analyses in emperors diving under similar conditions. This indicates that calculating ADL(C) from an at-sea or foraging-period metabolic rate in penguins is not appropriate. The relatively low foraging cost for emperor penguins contributes to their relatively low total daily FMR (2.9 x BMR). The allometric relationship for FMR in eight penguin species, including the smallest and largest living representatives, is kJ d(-1)=1,185 kg(0.705).     

The relationship between foraging behaviour and energy expenditure in Antarctic fur seals

By using time-depth recorders to measure diving activity and the doubly-labelled water method to determine energy expenditure, the relationship between foraging behaviour and energy expenditure was investigated in nine Antarctic fur seal females rearing pups. At-sea metabolic rate (MR) (mean of 6.34 ± 0.4 W. kg^-1; 4.6 times predicted BMR) was positively correlated to foraging trip duration (mean of 4.21 ± 0.54 days; r²= 0.5, P < 0.04). There were no relationships between MR and the total number of dives, the total time spent diving or the total vertical distance travelled during the foraging trip. There was, however, a close negative sigmoidal relationship (r²= 0.93) between at-sea MR and the proportion of time at sea spent diving. This measure of diving behaviour may provide a useful, inexpensive means of estimating foraging energy expenditure in this species and possibly in other otariids. The rate of diving (m.h^-1) was also negatively related to at-sea MR (r²= 0.69, P < 0.005). Body mass gain during a foraging trip had a positive relationship to the time spent at sea (r²= 0.58, P < 0.02) and the total amount of energy expended while at sea (r²= 0.72, P < 0.004) such that, while females undertaking long trips have higher metabolic rates, the energetic efficiency with which females gain mass is independent of the time spent at sea. Therefore, within the range of conditions observed, there is no apparent energetic advantage for females in undertaking foraging trips of any particular duration.     

Latitudinal range influences the seasonal variation in the foraging behavior of marine top predators

Non-migratory resident species should be capable of modifying their foraging behavior to accommodate changes in prey abundance and availability associated with a changing environment. Populations that are better adapted to change will have higher foraging success and greater potential for survival in the face of climate change. We studied two species of resident central place foragers from temperate and equatorial regions with differing population trends and prey availability associated to season, the California sea lion (Zalophus californianus) (CSL) whose population is increasing and the endangered Galapagos sea lion (Zalophus wollebaeki) (GSL) whose population is declining. To determine their response to environmental change, we studied and compared their diving behavior using time-depth recorders and satellite location tags and their diet by measuring C and N isotope ratios during a warm and a cold season. Based on latitudinal differences in oceanographic productivity, we hypothesized that the seasonal variation in foraging behavior would differ for these two species. CSL exhibited greater seasonal variability in their foraging behavior as seen in changes to their diving behavior, foraging areas and diet between seasons. Conversely, GSL did not change their diving behavior between seasons, presenting three foraging strategies (shallow, deep and bottom divers) during both. GSL exhibited greater dive and foraging effort than CSL. We suggest that during the warm and less productive season a greater range of foraging behaviors in CSL was associated with greater competition for prey, which relaxed during the cold season when resource availability was greater. GSL foraging specialization suggests that resources are limited throughout the year due to lower primary production and lower seasonal variation in productivity compared to CSL. These latitudinal differences influence their foraging success, pup survival and population growth reflected in contrasting population trends in which CSL are more successful and potentially more resilient to climate change.     

Effects of increased swimming costs on foraging behavior and efficiency of captive Steller sea lions: Evidence for behavioral plasticity in the recovery phase of dives

A significant component of foraging energetics is the cost of locomotion, which for marine animals, is the cost of swimming. Increases in the cost of swimming may have significant impacts on foraging efficiency. Minimizing the cost of swimming can contribute to the optimization of foraging strategies by reducing the energetic cost of foraging. Results of several field studies suggest that an increase in the cost of locomotion may have comparable effects on foraging behavior and efficiency to a decrease in prey availability. We tested the hypothesis that an increased cost of swimming, brought on by increased hydrodynamic drag, has the same effect on dive behavior and efficiency as reduced prey availability under standard locomotion. Experiments were performed using two adult female Steller sea lions at the Alaska SeaLife Center in Seward, AK, using the same animals and general experimental design previously used to test the effects of reduced prey encounter rate on dive behavior and efficiency. Animals were fitted with a drag-inducing harness for half of the 500 simulated foraging dives in order to increase the cost of swimming. Individual dive duration and foraging time were significantly reduced in all cost-increased dives, comparable to the effects of reduced prey encounter rate. However, on a bout-by-bout basis, dive and foraging efficiency were only slightly reduced, which is likely due to an average 50% reduction in post-dive surface recovery duration during cost-increased dives. Increased heat flux across the body surface measured in a parallel study confirmed a significant increase in work during drag-increased dives. These results suggest that sea lions are able to compensate for changes in the cost of foraging and maintain their foraging efficiency by altering their dive strategy over an entire bout of dives when operating well within their aerobic scope.     

Trial Implantation of Heart Rate Data Loggers in Pinnipeds

ABSTRACT Pinnipeds are major consumers in marine ecosystems, and understanding their energy budgets is essential to determining their role in food webs, particularly where there is competition with fisheries. Food consumption and energy expenditure have been evaluated in pinnipeds using different methods, but the use of heart rate to estimate energy expenditure is potentially a very powerful tool suited to the life history of these animals. We tested a procedure for the subcutaneous implantation of heart rate data loggers to determine whether heart rate could be recorded for ≥1 year in free-ranging pinnipeds, as it has been in birds. We implanted 3 captive California sea lions (Zalophus californianus) and 3 captive northern elephant seals (Mirounga angustirostris) with heart rate data loggers and monitored their recovery and behavior in a controlled environment. In both species, the implantation site allowed for excellent detection of the electrocardiogram, and we observed heart rate signatures characteristic of behaviors such as resting and diving. Although all 3 sea lions recovered well from the implantation surgery, all 3 elephant seals showed a substantial inflammatory response for unknown reasons, and we removed the implanted data loggers. Subcutaneous implantation of data loggers is a powerful technique to study physiology, energetics, and behavior in California sea lions, but more work is required to realize the potential of this technique in northern elephant seals.     

Oceanic thermal structure mediates dive sequences in a foraging seabird

1. Changes in marine ecosystems are easier to detect in upper‐level predators, like seabirds, which integrate trophic interactions throughout the food web.
2. Here, we examined whether diving parameters and complexity in the temporal organization of diving behavior of little penguins (Eudyptula minor) are influenced by sea surface temperature (SST), water stratification, and wind speed—three oceanographic features influencing prey abundance and distribution in the water column.
3. Using fractal time series analysis, we found that foraging complexity, expressed as the degree of long‐range correlations or memory in the dive series, was associated with SST and water stratification throughout the breeding season, but not with wind speed. Little penguins foraging in warmer/more‐stratified waters exhibited greater determinism (memory) in foraging sequences, likely as a response to prey aggregations near the thermocline. They also showed higher foraging efficiency, performed more dives and dove to shallower depths than those foraging in colder/less‐stratified waters.
4. Reductions in the long‐term memory of dive sequences, or in other words increases in behavioral stochasticity, may suggest different strategies concerning the exploration–exploitation trade‐off under contrasting environmental conditions.

     

Characterizing habitat suitability for a central‐place forager in a dynamic marine environment

Characterizing habitat suitability for a marine predator requires an understanding of the environmental heterogeneity and variability over the range in which a population moves during a particular life cycle. Female California sea lions (Zalophus californianus) are central‐place foragers and are particularly constrained while provisioning their young. During this time, habitat selection is a function of prey availability and proximity to the rookery, which has important implications for reproductive and population success. We explore how lactating females may select habitat and respond to environmental variability over broad spatial and temporal scales within the California Current System. We combine near‐real‐time remotely sensed satellite oceanography, animal tracking data (n = 72) from November to February over multiple years (2003–2009) and Generalized Additive Mixed Models (GAMMs) to determine the probability of sea lion occurrence based on environmental covariates. Results indicate that sea lion presence is associated with cool ( <14°C ), productive waters, shallow depths, increased eddy activity, and positive sea‐level anomalies. Predictive habitat maps generated from these biophysical associations suggest winter foraging areas are spatially consistent in the nearshore and offshore environments, except during the 2004–2005 winter, which coincided with an El Niño event. Here, we show how a species distribution model can provide broadscale information on the distribution of female California sea lions during an important life history stage and its implications for population dynamics and spatial management.     

Responses of Spotted Hyenas to Lions Reflect Individual Differences in Behavior

Competition among mammalian carnivores can be particularly intense and can influence population dynamics at lower trophic levels. One strategy employed by carnivores to minimize potentially costly interspecific competition is avoidance of dominant species. Recent research has highlighted the importance of consistent individual differences in behavior (i.e. temperament traits) in understanding behavioral variation during predator–prey interactions and intraspecific interactions. However, the importance of such individual differences to interspecific competition has received little attention. Here, we examined the responses of spotted hyenas (Crocuta crocuta) to their primary competitors, African lions (Panthera leo), to (1) determine whether hyenas avoid lions and (2) evaluate the potential importance of individual differences in behavior during interspecific competition. Spotted hyenas and lions co‐occur throughout much of Africa and are vigorous competitors. Whereas lions sometimes kill hyenas and steal their food, lions also represent a source of food for hyenas via scavenging. Using audio playback experiments, we found that hyenas do not uniformly avoid potential encounters with lions. Indeed, we noted considerable variation among individuals in their responses to lion roars, and this variation reflected consistent individual differences in risk‐taking and vigilance tendencies. Individual differences in vigilance behavior were specific to interactions with lions. We conclude that individual differences in behavior have the potential to play an important role in determining the nature and outcome of interspecific competition.     

Divergent diving behavior during short and long trips of a bimodal forager,the little auk Alle alle

The purpose of this study was to characterize for the first time seabird diving behavior during bimodal foraging. Little auks Alle alle, small zooplanktivorous Alcids of the High Arctic, have recently been shown to make foraging trips of short and long duration. Because short (ST) and long trips (LT) are thought to occur in different locations and serve different purposes (chick‐ and self‐feeding, respectively) we hypothesized that foraging differences would be apparent, both in terms of water temperature and diving characteristics. Using Time Depth Recorders (TDRs), we tested this hypothesis at three colonies along the Greenland Sea with contrasting oceanographic conditions. We found that diving behavior generally differed between ST and LT. However, the magnitude of the disparity in diving characteristics depended on local foraging conditions. At the study site where conditions were favorable, diving behavior differed only to a small degree between LT and ST. Together with a lack of difference in diving depth and ocean temperature, this indicates that these birds did not increase their foraging effort during ST nor did they travel long distances to seek out more profitable prey. In contrast, where local foraging conditions were poor, birds increased their diving effort substantially to collect a chick meal during ST as indicated by longer, more U‐shaped dives with slower ascent rates and shorter resting times (post‐dive intervals and extended surface pauses). In addition, large differences in diving depth and ocean temperature indicate that birds forage on different prey species and utilize different foraging areas during LT, which may be up to 200 km away from the colony. Continued warming and deteriorating near‐colony foraging conditions may have energetic consequences for little auks breeding in the eastern Greenland Sea.     

Seasonal variation in blood and muscle oxygen stores attributed to diving behavior, environmental temperature and pregnancy in a marine predator, the California sea lion

Survival depends on an animal's ability to find and acquire prey. In diving vertebrates, this ability is directly related to their physiological capability (e.g. oxygen stores). We studied the seasonal variation in oxygen stores, body temperature and body condition in California sea lions (Zalophus californianus) (CSL) as a function of seasonal variation in temperature, primary productivity, diving behavior and reproductive stage. During summer, blood oxygen stores were significantly greater and muscle oxygen stores were significantly lower than in winter. Total oxygen stores, body condition and body temperature did not change between seasons but variations in body temperature were greater during summer. Changes in oxygen stores are partly attributed to diving behavior, temperature and pregnancy that could increase oxygen consumption. Blood and muscle oxygen stores appear to be influenced by reproductive state. Blood oxygen stores are more likely influenced by diving behavior and temperature than muscle oxygen stores.     

Aerial hearing thresholds and detection of hearing loss in male California sea lions (Zalophus californianus) using auditory evoked potentials

Auditory evoked potential (AEP) measurements are useful for describing the variability of hearing among individuals in marine mammal populations, an important consideration in terms of basic biology and the design of noise mitigation criteria. In this study, hearing thresholds were measured for 16 male California sea lions at frequencies ranging from 0.5 to 32 kHz using the auditory steady state‐response (ASSR), a frequency‐specific AEP. Audiograms for most sea lions were grossly similar to previously reported psychophysical data in that hearing sensitivity increased with increasing frequency up to a steep reduction in sensitivity between 16 and 32 kHz. Average thresholds were not different from AEP thresholds previously reported for male and female California sea lions. Two sea lions from the current study exhibited abnormal audiograms: a 26‐yr‐old sea lion had impaired hearing with a high‐frequency hearing limit (HFHL) between 8 and 16 kHz, and an 8‐yr‐old sea lion displayed elevated thresholds across most tested frequencies. The auditory brainstem responses (ABRs) for these two individuals and an additional 26‐yr‐old sea lion were aberrant compared to those of other sea lions. Hearing loss may have fitness implications for sea lions that rely on sound during foraging and reproductive activities.     

Seasonal Variation in Parental Care Drives Sex-Specific Foraging by a Monomorphic Seabird

Evidence of sex-specific foraging in monomorphic seabirds is increasing though the underlying mechanisms remain poorly understood. We investigate differential parental care as a mechanism for sex-specific foraging in monomorphic Common Murres (Uria aalge), where the male parent alone provisions the chick after colony departure. Using a combination of geolocation-immersion loggers and stable isotopes, we assess two hypotheses: the reproductive role specialization hypothesis and the energetic constraint hypothesis. We compare the foraging behavior of females (n = 15) and males (n = 9) during bi-parental at the colony, post-fledging male-only parental care and winter when parental care is absent. As predicted by the reproductive role specialization hypothesis, we found evidence of sex-specific foraging during post-fledging only, the stage with the greatest divergence in parental care roles. Single-parenting males spent almost twice as much time diving per day and foraged at lower quality prey patches relative to independent females. This implies a potential energetic constraint for males during the estimated 62.8 ± 8.9 days of offspring dependence at sea. Contrary to the predictions of the energetic constraint hypothesis, we found no evidence of sex-specific foraging during biparental care, suggesting that male parents did not forage for their own benefit before colony departure in anticipation of post-fledging energy constraints. We hypothesize that unpredictable prey conditions at Newfoundland colonies in recent years may limit male parental ability to allocate additional time and energy to self-feeding during biparental care, without compromising chick survival. Our findings support differential parental care as a mechanism for sex-specific foraging in monomorphic murres, and highlight the need to consider ecological context in the interpretation of sex-specific foraging behavior.     

Ontogeny of diving behaviour in the Australian sea lion: trials of adolescence in a late bloomer

1. 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.     

Linking seasonal distribution patterns with prey availability in a central-place forager, the Steller sea lion

Aim We used a novel approach to infer foraging areas of a central‐place forager, the Steller sea lion (Eumetopias jubatus), by assessing changes in the temporal and spatial distribution patterns of sea lions at terrestrial sites. Specifically, our objectives were (1) to classify seasonal distribution patterns of Steller sea lions and (2) to determine to what extent the seasonal distribution of Steller sea lions is explained by seasonal concentrations of prey. Location Southeast Alaska, USA. Methods Steller sea lions of all age classes were counted monthly (2001–04) by aerial surveys at 28 terrestrial sites. Hierarchical cluster analysis and principal components analysis were used to classify seasonal distribution patterns of Steller sea lions at these terrestrial sites. We estimated the proportion of sea lions in the study area that were associated with each seasonal distribution pattern. Results Multivariate ordination techniques revealed four distinct seasonal distributional patterns. During December, 55% of the sea lions in the study area were found at Type 1 sites, located near over‐wintering herring aggregations. During May, 56% of sea lions were found at Type 2 sites, near aggregations of spring‐spawning forage fish. In July, 78% of sea lions were found at Type 3 sites, near summer migratory corridors of salmon. During September, 44% of sea lions were found at Type 4 sites, near autumn migratory corridors of salmon. Main conclusions Seasonal attendance patterns of sea lions were commonly associated with the seasonal availability of prey species near terrestrial sites and reflected seasonal foraging patterns of Steller sea lions in Southeast Alaska. A reasonable annual foraging strategy for Steller sea lions is to forage on herring (Clupea pallasii) aggregations in winter, spawning aggregations of forage fish in spring, salmon (Oncorhynchus spp.) in summer and autumn, and pollock (Theragra chalcogramma) and Pacific hake (Merluccius productus) throughout the year. The seasonal use of haulouts by sea lions and ultimately haulout‐specific foraging patterns of Steller sea lions depend in part upon seasonally available prey species in each region.     

Predator‐induced modifications to diving behavior vary with foraging mode

Breath‐hold divers are strongly interacting species whose top–down influence on aquatic communities is shaped by factors governing their diving decisions. Although some of these factors (e.g. physiological constraints, energetic needs) have been scrutinized, the possibility that predation risk influences diving behavior has been largely overlooked, and no study to date has asked if anti‐predator responses by divers depend on foraging mode. We contrasted dive cycle changes by herbivorous dugongs Dugong dugon using two foraging tactics – cropping, which always permits anti‐predator vigilance, and excavation, which limits surveillance at depth – in response to temporal variation in tiger shark Galeocerdo cuvier abundance. Dugongs responded to increasing shark abundance (one component of predation risk) by diving more frequently without changing their surface times and thereby spending a greater proportion of time at the surface, but only while excavating. When threatened, in other words, excavating dugongs sacrificed foraging time at depth to facilitate shark detection. In contrast, cropping dugongs at risk from sharks were able to continue diving and foraging normally. By implication, future studies should consider the influence of predation risk on diving decisions, even by large‐bodied species, and the possibility that behavioral responses by divers to predators may vary with foraging mode.     

A life in the fast lane: energetics and foraging strategies of the great cormorant

Body insulation is critically important for diving marine endotherms. However,cormorants have a wettable plumage, which leads to poor insulation. Despitethis, these birds are apparently highly successful predatorsin most aquatic ecosystems. We studied the theoretical influenceof water temperature, dive depth, foraging techniques, and preyavailability on the energetic costs of diving, prey search time,daily food intake, and survival in foraging, nonbreeding greatcormorants (Phalacrocorax carbo). Our model was based on fieldmeasurements and on data taken from the literature. Water temperatureand dive depth influenced diving costs drastically, with predicted increasesof up to 250% and 258% in males and females, respectively. Changes inwater temperature and depth conditions may lead to an increaseof daily food intake of 500-800 g in males and 440-780 g infemales. However, the model predicts that cormorant foragingparameters are most strongly influenced by prey availability,so that even limited reduction in prey density makes birds unableto balance energy needs and may thus limit their influence onprey stocks. We discuss the ramifications of these results withregard to foraging strategies, dispersal, population dynamics,and intraspecific competition in this avian predator and pointout the importance of this model species for our understandingof foraging energetics in diving endotherms.