Energy-rich Prey Research Articles

Influence of Environmental Factors on Prey Discrimination of Bait-Attracted White Sharks from Gansbaai, South Africa.

The influence of environmental factors on prey discrimination of bait-attracted white sharks was studied over a six-year period (2008-2013) at Dyer Island Nature Reserve (Gansbaai, South Africa). Across 240 bait-attracted feeding events observed in this period, both immature and mature white sharks were attracted by the seal-shaped decoy rather than the tuna bait, except for the years 2008 and 2011. Tide ranges, underwater visibility, water temperature, and sea conditions were, in decreasing order, the factors which drove white sharks to select the seal-shaped decoy. High tide lowered the minimum depth from which sharks could approach seals close to the shore, while extended visibility helped the sharks in making predatory choices towards the more energy-rich prey source, the odorless seal-shaped decoy. On the contrary, warmer water is associated with an increase in phytoplankton that reduces underwater visibility and increases the diversity of teleosts including tuna-a known prey of white sharks-driving the sharks to favor the tuna bait. Overall, sea conditions were almost always slightly rough, ensuring a good average underwater visibility. Recommendations for future research work at this site are presented.

Do seasonal dietary shifts by Temminck's pangolins compensate for winter resource scarcity in a semi-arid environment?

In response to food scarcity, animals may increase foraging time or shift their diet by including more profitable prey items. Aardvarks (Orycteropus afer) in the semi-arid Kalahari region of southern Africa died, apparently of starvation, because they did not shift their diet when their primary food resource declined following a drought. Whether other myrmecophagous (ant- and termite-eating) mammals alter their diet in response to food scarcity is unclear. We investigated the feeding behaviour (through faecal analysis and direct observations) of six free-living, myrmecophagous Temminck's pangolins (Smutsia temminckii), for two years in the Kalahari. We also collected data on climate and ant and termite availability. Winter was characterised by low prey abundance, but pangolins predominantly selected three insect genera, namely Crematogaster ants, Anoplolepis ants, and Trinervitermes termites, regardless of food availability. Pangolins responded to winter resource scarcity by increasing the proportion of consumed Trinervitermes termites, but the shift did not prevent low energy intake during that period. During the winter of the drier of the two years, pangolins increased foraging time and fed on energy-rich prey. Whether the relatively low dietary flexibility of pangolins will allow them to meet their energy demands as insect populations decline with climate change is not known.

Spatial patterns in dry weight of nearshore Lake Michigan prey fishes

The ability for individuals to adapt to local food sources likely allows for regional persistence of aquatic populations. In the Laurentian Great Lakes, invasive species and changing physical conditions have vastly altered local and regional food webs, leading to the potential for variation in energy flow up to large predators. To assess the potential that prey fish condition varies spatially, we examined patterns in proportion dry weight of prey fishes collected from Lake Michigan nearshore areas (<16 m) in 2010 and 2011. Absolute pairwise differences in dry weight of fishes were as high as 9% between some of our sampling sites, but sites exhibiting relatively high or low values were patchily spread across the lake. The range of proportion dry weight values observed in our study do encompass previously-reported mean values; however, the values reported in our study are slightly lower than those previously reported for Lake Michigan. In contrast to previous studies, our analyses do not suggest that variation in prey fish condition exhibits a consistent spatial pattern around nearshore Lake Michigan, and high within-site variation of prey fish condition prevents strong conclusions about specific sites providing generally more (or less) energy-rich prey to predators.

Context Matters: Hawaiian Monk Seals Switch Between Feeding Strategies Depending on Ecological Context.

The ability to expand the behavioral repertoire past seemingly rigid morphological features enables animals to succeed in a variety of ecological contexts. The integration of morphology, performance, and behavior produces diverse animal feeding strategies. These different strategies reflect trade-offs between specialization, prey choice, and energetic expenditure, which have important consequences for understanding individual and population-level flexibility in response to environmental change. Here we examined the feeding strategies used by the Hawaiian monk seal (Neomonachus schauinslandi), an endangered marine predator. We tested how Hawaiian monk seal feeding strategies change in response to ecological context, specifically prey size and prey location at different depths. Seven captive Hawaiian monk seals were fed five prey types across a continuum of sizes, and prey were presented at three depths to represent surface, pelagic, and benthic feeding. Hawaiian monk seals used suction feeding and biting strategies, and these strategies were associated with significant differences in behavior and kinematic performance. Hawaiian monk seals used suction feeding most frequently when targeting small to medium prey (0-79% of the seal's head length) but switched to biting when consuming large prey (>80% of the seal's head length). These results demonstrate that prey size drives the transition between suction feeding and biting strategies. Seals also switched strategies based on prey position in the water column, primarily using suction feeding when prey were benthic and pelagic, and biting when prey were at the water's surface. Overall, suction feeding was three to five times faster than biting, required a smaller gape, and used fewer jaw movements, allowing seals to quickly consume numerous small to medium sized prey. In contrast, biting was slower but resulted in the ability to target larger, potentially more energy rich prey. Our results show that Hawaiian monk seals exhibit flexibility in their use of different feeding strategies, which likely facilitates increased foraging success when feeding in spatially and temporally dynamic marine environments.

The diet of adult and chick rock shags (Phalacrocorax magellanicus) inferred from combined pellet and stable isotope analyses

The current information about the diet composition of the rock shag (Phalacrocorax magellanicus) in the SW Atlantic coast comes mainly from conventional pellet or stomach content analysis from a few locations situated in northern Patagonia (Chubut Province, Argentina). In this work, we studied the diet of breeding rock shags over several years at a colony from southern Patagonia (Ría Deseado, Santa Cruz Province, Argentina) using a combined technique of conventional diet assessment (pellet analysis) and stable isotope analysis of carbon and nitrogen. Our results confirm the importance of benthic prey and the low inter-annual variability in the diet of the rock shag. These results coincide with previous research in relation to the exploitation of slow moving, predictable, but low-energy density prey. The stable isotope mixing models, which was informed with prior data obtained from pellet analysis, allowed for the detection of subtle differences between the diet of adults and chicks, consisting in the incorporation of higher proportions of cephalopods, an energy-rich prey, in the diet of chicks. By comparing our results with the diet of the red-legged cormorant, which breeds in sympatry in the Ría Deseado Estuary and whose diet composition is strongly pelagic, we suspect a certain level of trophic resource partitioning between these rock shag and red-legged cormorant.

Extreme climatic variability during migration invokes physiological and dietary plasticity among spring migrating ducks

Environmental stochasticity encountered during migration can have negative consequences for individuals and population demographics through direct reductions in survival or cross-seasonal impacts. We took advantage of substantial interannual variation in spring migration conditions over a 4 year field study to examine physiological and dietary variation among two species of migrant ducks. We collected female Lesser Scaup (Aythya affinis (Eyton, 1838)) and Blue-winged Teal (Spatula discors (Linnaeus, 1766)) during spring migration and measured lipid and protein reserves, an index of recent lipid metabolism based on concentrations of lipid metabolites in plasma, and diets. We documented systematic interannual variation among these metrics in both species, contrasting primarily the warmest, earliest spring and the coldest, latest spring. Lesser Scaup had reduced lipid and protein reserves and consumed less energy-rich prey during the coldest and latest spring but showed no interannual variation in the index of lipid metabolism. Blue-winged Teal similarly had reduced protein reserves in the cold, late spring but maintained constant lipid reserves among years, likely facilitated by increased consumption of energy-rich seeds reflected in diets and lipid metabolism. Our results reveal impacts of environmental stochasticity on migrants and suggest that recruitment may be impacted by variable conditions encountered during migration during extreme weather events.

Diel vertical feeding behaviour of Atlantic mackerel (Scomber scombrus) in the Irminger current

The main prey of the Atlantic mackerel undertake diel vertical migrations, however diel and vertical patterns in the feeding by mackerel remains poorly known thus limiting our understanding and quantification of ecosystem trophodynamics and potentially influencing stock assessment and ecosystem based management. By intensive monitoring of adult Atlantic mackerel and its prey during summer in the Irminger Sea, it was found that the diel feeding cycle is of high quantitative importance. Fresh stomach content analysis indicated a marked diel pattern with copepods as the main prey between 16 and 24 (solar time), and euphausiids/myctophids between 22 and 08. Compared with the zooplankton distribution and composition, there were no indications of feeding below the mixed layer. Mackerel selected for the largest and most energy rich prey, namely copepods with prosome lengths >2 mm, and then switched from copepods to euphausiids/myctophids around midnight, despite that the copepods were also aggregated during night. Stomach fullness increased significantly with length and condition, suggesting that individuals that were larger were more effective feeders. The stomach content by weight primarily consisted of copepods (65%, exclusively Calanus finmarchus), euphausiids (18%), and to a lesser extent teleosts (5%, mainly myctophids), hyperiids (4%) and cephalopods (2%). The results are likely widely applicable in the main feeding season and areas (the oceanic parts of the Nordic Seas during summer), because of similarities in the diet of the adult mackerel and in the prey’s diel vertical migration. In conclusion, it is imperative to account for the diel dynamics when quantifying the mackerel diet and its impact on prey populations.

Whale shark foraging on baitfish off Djibouti

In the Gulf of Tadjoura, Republic of Djibouti, whale sharks (Rhincodon typus Smith, 1828) are known to feed on a wide variety of zooplankton organisms. However, for the first time in this area, seven juvenile whale sharks were filmed feeding on a school of baitfish (anchovies). During the event, zooplankton samples were collected and biomass was calculated. Results support the hypothesis that when dense patches of zooplankton are not available in the Gulf of Tadjoura, whale sharks remain in the area and feed on alternative energy-rich prey, such as baitfish, to meet their energetic demands.

Transported zooplankton from the Okhotsk Sea facilitate feeding and growth of juvenile walleye pollock on a continental shelf along the Oyashio Current, western subarctic Pacific

The advective transport of zooplankton provides an important source of energy on continental shelves. On a continental shelf along the Oyashio Current, western subarctic Pacific (42°30′N, 144°00′E), the extant water mass is replaced biannually by inflows of water masses from the highly productive Okhotsk Sea. These inflows and accompanying zooplankton are hypothesized to enhance biological production on the shelf. In this study, seasonal feeding and growth of a juvenile planktivorous fish, walleye pollock (Gadus chalcogrammus), on the shelf were examined in relation to water mass exchanges from September 2003 to June 2005. Zooplankton were sampled from February to April in 2004 and 2005, and in April 2014 and 2015. Juvenile pollock frequently consumed an Arctic copepod, Calanus glacialis, and a boreal euphausiid, Thysanoessa inermis, after January when the shelf was occupied by cold water masses that originated from the southwestern Okhotsk Sea. Juvenile pollock whose diet contained higher contributions of these energy-rich prey items exhibited positive growth in February–March 2005, whereas fish with lower proportions of these prey exhibited little growth during this period in 2004. The winter distributions of the copepod and euphausiid extended from a continental slope area in the southwestern rim of the Okhotsk Sea to the shelf along the Oyashio Current. Together, these results suggest the transport of zooplankton from the Okhotsk Sea likely influences the biological production of the coastal Oyashio ecosystem by enhancing the availability of energy-rich prey for juvenile pollock.

Feeding ecology of the common dolphin (Delphinus delphis) in Western Iberian waters: has the decline in sardine (Sardina pilchardus) affected dolphin diet?

Predator diet is expected to respond to changes in the abundance of important prey items. We investigated whether common dolphin diet has changed with changes in the pelagic fish community off the Iberian Peninsula in recent years, in particular with reference to the decline of sardine (Sardina pilchardus) and the increase of Atlantic chub mackerel (Scomber colias) and scads (Trachurus spp.), and estimated how much fish was removed by common dolphins in coastal waters of mainland Portugal. Based on analysis of stomach contents of stranded animals from 2010 to 2013 (n = 150), we related diet composition to sardine abundance, dolphin size and sex, season and region. Despite the decline in sardine stocks in recent years, sardines were the most important prey of common dolphins, as was previously reported in the 1990s, followed by chub mackerel and scads. However, small sardines have disappeared from the diet, consistent with recent poor recruitment to the stock. The relative importance of these pelagic species in the diet increased significantly with increasing dolphin length. The dietary importance of chub mackerel also varied regionally, with higher intake on the southern coast. Finally, we compared the estimated biomass of fish removed by dolphins with that taken by fisheries. We found that even with the severe quota restrictions on sardine catches, total biomass removed by fisheries was over five times greater than the estimated removal by common dolphins. Our results confirm the previously reported preference for energy-rich prey, especially sardine, suggesting that common dolphins in Portuguese waters specialise in feeding on this species.

The most common diet results in low reproduction in a generalist seabird.

Dietary specialization has been described across a wide range of taxa in the animal kingdom. Fitness consequences are, however, not well documented. We examined the reproductive consequences of different dietary specializations in the herring gull Larus argentatus, an omnivorous seabird, using an extensive dataset which includes breeding and dietary data of 10 successive years. We hypothesized that pairs that focused on prey of higher energetic value would yield higher fledging rates. An alternative hypothesis is that pairs that relied on more resources simultaneously would reproduce better. The novelty of this study is that we used continuous measurements representing dietary composition and degree of specialization rather than restricting our analysis to predefined categories. By relating these two continuous measurements for diet to several proxies for reproductive success, we show clear consequences of dietary choice. Most pairs concentrated on bivalves, a prey type not particularly rich in energy. Pairs feeding on energy‐rich prey (e.g., “domestic refuse and fishery discards”) during chick rearing were found to have a higher reproductive success, supporting the first hypothesis. Pairs that used more resources did not clearly have a higher reproductive success. The majority of the pairs did not switch to energy‐rich prey during chick rearing, despite low breeding outcome. We discuss how trade‐offs between factors such as resource availability, predictability, and the time and energy needed to obtain certain prey species may influence resource selection.

Spatial and temporal changes in assemblage structure of zooplankton and pelagic fish in the eastern Bering Sea across varying climate conditions

Zooplankton and pelagic fish samples collected on the eastern Bering Sea shelf in late summer 2003-2010 were used to evaluate spatial and temporal changes in the plankton and nekton community structure. The zooplankton were sampled by vertical towing of a Juday net (168 μm mesh) and oblique towing of a Bongo net (505 μm mesh), and pelagic fish were caught by midwater rope trawl. The communities were compared across climate regimes (in relatively warm and cold years), by latitude (in the northern and southern parts of the shelf), and by water depth (in the inner, middle, and outer domains of the shelf). Zooplankton were dominated by the small copepod Oithona spp. in warm years but relatively larger copepods Pseudocalanus spp. and Acartia spp. in cold years. Notably, the large copepod Calanus spp., an important energy-rich prey for fish, were more abundant in cold years than warm years. Age-0 walleye pollock Gadus chalcogrammus were more abundant in warm years, while capelin Mallotus villosus were abundant within cold-year communities over the northern shelf. Latitudinal variations in communities were more prominent in the cold years, particularly in 2007 and 2010. Cross-shelf variations were evident, particularly for large zooplankton and fish, with communities corresponding to specific oceanographic domains. Outer shelf communities varied less than inner and middle shelf communities between warm and cold periods, suggesting that this region may be less impacted by climate variability. An understanding of the overlap of zooplankton (prey) and fish communities within specific shelf regions or climate regimes may provide information for ecosystem-based approaches to fisheries management.

The Presence of Common Frogs (Rana temporaria) Increases the Body Condition of Syntopic Alpine Newts (Ichthyosaura alpestris) in Oligotrophic High-Altitude Ponds: Benefits of High-Energy Prey in a Low-Productivity Habitat

In low-productivity, high-altitude habitats food can become a limiting factor for the occurring amphibians. Common frog eggs deposited in ponds and the developing tadpoles represent energy-rich prey for syntopic Alpine newts that should be advantageous for storing sufficient amounts of energy to survive hibernation and breed the following spring. In our study population in the Zillertal Alps, Austria at 2100 m a.s.l., in two consecutive years we found the body mass of Alpine newts to be significantly higher in ponds containing tadpoles of the common frog than in ponds without tadpoles. As the frogs forage mostly on land, we regard their eggs as an allochtonous nutrient subsidy for ultra-oligotrophic high-altitude ponds that may have a profound effect on the fitness and distribution of Alpine newts and common frogs.

Arctic Cod (&lt;i&gt;Boreogadus saida&lt;/i&gt;) as Prey: Fish Length-Energetics Relationships in the Beaufort Sea and Hudson Bay

Although Arctic cod (Boreogadus saida) is widely recognized as an important trophic link to top predators in Arctic marine ecosystems, the challenges of conducting fieldwork in the Arctic make this species difficult to study. We establish some basic relationships to improve prey energetics modeling when only in-field parameters (e.g., fork length) can be measured. We investigated the intraspecific relationships among energy density, fork length, mass, and water content for Arctic cod captured by Black Guillemots and Thick-billed Murres at two sites (Western Beaufort and Hudson Bay). Dry energy density was similar between sites (21.6 – 22.2 kJ g-1) and increased with fork length (Dry EDkJ/g = 0.028 (± 0.01) • Fork Lengthmm + 18.12 (± 1.33). Even though fish lost some water as they were transported to the nest by avian predators, wet energy density also increased with fork length. We suggest that environmental conditions had a similar effect on growth at these two locations although they occur in very different oceanographic regimes. Arctic cod, especially large cod, is one of the most energy-rich prey species in the Arctic. Our results highlight the importance of this valuable prey to Arctic ecosystems and the utility of using seabirds opportunistically as samplers of the marine environment.

The influence of body size on the foraging behaviour of European hake after settlement to the bottom

The trophic niche width and feeding strategy (FS) of juvenile European hake were studied during three trawl surveys in two different areas of the Mediterranean Sea. Both of these aspects of trophic behaviour were analysed as function of different biotic and abiotic factors. Prey were grouped by factors such as size, swimming behaviour and energy density. The trophic niche breadth values observed (2.25–2.89) were very similar regardless of the geographic area (geomorphology) and season. The FSs analysed were also quite similar regardless of the geographic area (geomorphology) and season, or predation environment (bottom or water column). Conversely, the FSs were linked with predator size. Increased predator size allowed changes in predatory behaviour through the development of preferences for larger and more energy-rich prey. The changes in FS from generalist to specialist began at a total length (TL) of approximately 12cm in both populations analysed. At larger sizes, the juveniles of both populations became more specialised, as confirmed by the reduction in the breadth of the individual trophic niche. The observed simultaneous changes in the FS, which occurred regardless of the geomorphology and seasonal characteristics, suggest that this shift is driven primarily by endogenous factors. The influence of body size on the foraging behaviour of European hake in the early life stages after bottom recruitment is a crucial information for the application of ecosystem mass-balance models.

Foraging behaviour in two Antarctic fur seal colonies with differing population recoveries

We compared Antarctic fur seals Arctocephalus gazella breeding at 2 contrasting sites on South Georgia: one high density colony at Bird Island and one lower density colony at Cooper Bay. The population at Cooper Bay was considerably smaller than that at Bird Island despite ample suitable breeding area being available. At Cooper Bay, female seals were longer but weighed less than those breeding at Bird Island and, whilst both maintained the same rate of female pup growth, male pups grew faster at Cooper Bay. Although Bird Island seals dived deeper, they dived less often than Cooper Bay seals so that both populations spent comparable amounts of time in the bottom phase of dives actively foraging. Longer distance oceanic foraging trips that were observed at Bird Island were almost entirely absent from Cooper Bay. Both populations fed on Antarctic krill, but there was an absence of myctophid prey in the diet of seals at Cooper Bay. Evidence suggests that the favoured myctophid prey of fur seals at South Georgia, Protomyctophum choriodon, are absent from the colder waters around the south-east of the island. We propose that, if these energy-rich prey are unavailable in this region then seals at Cooper Bay may find it hard to offset the increased costs of foraging trips with longer duration and distance. This potentially reduced niche width means that the Cooper Bay population may be less buffered against environmental variability. Although food resources appeared to be sufficient during the period of our study, the south-eastern region of South Georgia has increased variability in food resources that, coupled with a smaller area in which to forage, might explain the reduced population size compared to the north-eastern end of the island.

Aggregations of Sea Otters (Enhydra lutris kenyoni) Feeding on Fish Eggs and Kelp in Prince William Sound, Alaska

Dietary studies of sea otters often focus on the ecological implications of foraging on large, energy rich prey. Although fish eggs and kelp have been documented as part of sea otter diets in the past, their relative importance and ecological implications have often been overlooked. Evidence of a relatively large aggregation of sea otters foraging on fish eggs and kelp is presented in this study with the potential implications for local herring fishery recovery. A gradual increase in the number of individuals using this resource over time also indicated that sea otters were attracted to large feeding aggregations which may increase exploitation of previously under utilized resources.

Seasonal availability of abundant, energy-rich prey influences the abundance and diet of a marine predator, the Steller sea lion Eumetopias jubatus

Steller sea lions Eumetopias jubatus are central-place foragers that forage in the marine environment while using terrestrial sites to rest and care for young. Some terrestrial sites are used seasonally; however, the reasons for doing so are not fully understood. We addressed the hypothesis that seasonal availability of prey influences seasonal abundance and diet of sea lions. We quantified monthly prey availability and sea lion abundance and quarterly diet composition at Benjamin Island in SE Alaska (2001-2004). Large numbers of sea lions occupied Benjamin Island during the non- breeding season from October to April when Pacific herring Clupea pallasii biomass was highest. Herring was the most common species in sea lion diet (frequency of occurrence (FO) = 90%) and comprised over 81% of the available pelagic prey biomass and 96% of the energy encountered dur- ing pelagic surveys. Walleye pollock Theragra chalcogramma accounted for 19% of the available prey biomass but was only slightly less common in sea lion diet (FO = 88%) than herring. Herring bio- mass was correlated with the number of sea lions; in contrast, there was no relationship between pol- lock biomass and number of sea lions. Several fish species were found in nearshore areas, but were uncommon in sea lion diet. Sea lions consumed the available pelagic prey but little of the available nearshore prey. The FO of herring and pollock in sea lion diet did not differ significantly between seasons; however, the FO of other seasonal prey species differed between seasons. Seasonal occupa- tion of Benjamin Island by sea lions is influenced by seasonally available, densely aggregated, energy-rich prey.

Dietary resource partitioning among sympatric New Zealand and Australian fur seals

Adult male, female and juvenile New Zealand and Australian fur seals Arctocephalus forsteri and A. pusillus doriferus regularly return to colonies, creating the potential for intra- and inter-specific foraging competition in nearby waters. We hypothesise that the fur seals in this study utilise different prey, thereby reducing competition and facilitating coexistence. We analysed scats and regurgitates from adult male, female and juvenile New Zealand fur seals and adult male Aus- tralian fur seals and compared prey remains found in the samples. Most prey consumed by adult male and female fur seals occur over the continental shelf or shelf break, less than 200 km from Cape Gantheaume. Adult female fur seals utilised proportionally more low-energy prey such as large squid and medium-sized fish. The adult female diet reflected that of a generalist predator, dictated by prey abundance and their dependant pups' fasting abilities. In contrast, adult male New Zealand and Aus- tralian fur seals consumed proportionally more energy-rich prey such as large fish or birds, most likely because they could more efficiently access and/or handle such prey. Juvenile fur seals primar- ily consumed small fish that occur in pelagic waters, south of the shelf break, suggesting juveniles cannot efficiently utilise prey where adult fur seals forage. The age and sex groups in this study employ dramatically different strategies to maximise their survival and reproductive success and consequently the prey that they utilise reflect their different physiological constraints and metabolic requirements.

Availability to Steller sea lions (Eumetopias jubatus) of a seasonal prey resource: a prespawning aggregation of eulachon (Thaleichthys pacificus)

The availability of seasonally abundant energy-rich prey can be a significant factor for the survival and reproductive success of predator populations. Large numbers of Steller sea lions (Eumetopias jubatus) were attracted to a prespawning aggregation of eulachon (Thaleichthys pacificus) in Berners Bay in southeast Alaska during April–May in 2002 and 2003. Sea lion abundance increased as eulachon gathered in Berners Bay, peaked as eulachon abundance peaked, and decreased as the eulachon moved up-river. As sea lion abundance increased in Berners Bay, sea lion abundance decreased at Benjamin Island, a sea lion haulout located 22 km away. The eulachon provided an abundant, energy-rich, predictable prey source for the Steller sea lions: (i) eulachon energy density was 9.70 ± 0.24 kJ·g–1, much higher than that of any forage species reported in the North Pacific Ocean except northern lampfish (Stenobrachius leucopsarus); (ii) a large surplus of prey was available per sea lion while the eulachon aggregation was present; and (iii) the spawning run usually begins between late April and early May. The eulachon pulse may be critical to Steller sea lions during a period of high energetic demands.