Seals Feeding Research Articles

Do monk seals exert top‐down pressure in subphotic ecosystems?

AbstractPatterns of subphotic fish assemblages on seamounts in the Northwestern Hawaiian Islands were identified and compared for potential structuring influences, including the bottom‐up effects of regional oceanic productivity and top‐down predation pressure exerted by visiting monk seals (Monachus schauinslandi). Patterns in fish size, density, and biomass were evaluated at the deep extreme (350–500 m) of the seals feeding range to avoid confounding effects of diverse shallow habitats (e.g., coral reefs). Fish number and size were used to calculate biomass density of the seamount fish assemblages that were then compared to the independent variables of summit depth, substrate type, relief, oceanic productivity, distance to seal colonies, and seal colony population. Only the variables of distance to seal colony and seal colony population were retained in a multiple regression model that explained 31% of the variance. Despite the presence of obvious regional differences in oceanic productivity, the overall patterns in the subphotic fish assemblages are better explained by the top‐down hypothesis of predation pressure from monk seals.

Impaired cellular immune response in harbour seals (Phoca vitulina) feeding on environmentally contaminated herring.

In a 2.5-year immunotoxicological study, two groups of captive harbour seals (Phoca vitulina) were fed herring from the heavily polluted Baltic Sea or from the relatively uncontaminated Atlantic Ocean. Blood samples were collected at regular intervals, and functional immunological parameters were monitored. T cell mitogen and mixed lymphocyte-induced proliferative responses of peripheral blood mononuclear cells (PBMC) obtained from seals fed Baltic herring were significantly reduced over the course of experiment. Upon immunization with rabies virus antigen (RV) and tetanus toxoid (TT), specific proliferative responses of PBMC from the seals fed Baltic herring were also significantly reduced. Impairment of T cell-mediated immune responses became especially apparent during the second year on the respective diets, and correlated significantly to 2,3,7,8-tetrachloro-dibenzo-p-dioxin toxic equivalent levels in blubber biopsies taken from the seals after 2 years on the respective diets. Humoral immune responses, including lipopolysaccharide (LPS)-induced lymphoproliferative responses, in vitro immunoglobulin production by PBMC, as well as RV-, TT-and poliovirus-specific serum antibody responses following immunization, remained largely unaffected. We conclude that suppression of the cellular immune response in the seals fed Baltic herring was induced by the chronic exposure to immunotoxic environmental contaminants accumulated through the food chain. Since cellular immune responses are known to be of crucial importance in the clearance of morbillivirus infections, these results suggest that environmental pollution-related immunosuppression may have contributed to the severity and extent of recent morbillivirus-related mass mortalities among marine mammals.

The effects of El Niño–La Niña on reproductive parameters of elephant seals feeding in the Bellingshausen Sea

AbstractAim To assess the impacts of El Niño–La Niña events on the pup weaning mass and diet of female southern elephant seals (Mirounga leonina) feeding in the Bellingshausen Sea, Antarctica, and to understand the ecological processes that drive these impacts.Location Atlantic southern elephant seal weaning mass and diet were measured at King George Island (62º14′ S, 58º30′ W). Feeding areas for pregnant female seals from King George Island are located west of Alexander Island in the Bellingshausen Sea.Methods Data on weaning mass were collected between 1985 and 1994 during the breeding season (September–November). Moulting females were anaesthetized and cephalopod beaks were isolated and identified from stomach contents obtained from stomach lavages. Sea‐surface temperature anomaly (SSTA) data for the ‘El Niño 3.4’ geographical region (5º N–5º S, 120º W–170º W) were used to define El Niño–Southern Oscillation (ENSO) event years (grouped as El Niño, La Niña and Neutral) as well as the strength of each ENSO event year. Using data from the US National Center for Environmental Prediction, temperature, sea ice concentration and atmospheric pressure anomalies in the Bellingshausen Sea were calculated from March to August, corresponding to the feeding period of pregnant female seals.Results Positive temperature anomalies and negative pressure anomalies in the Bellingshausen Sea were observed during La Niña years and negative temperature anomalies and positive pressure anomalies during El Niño years. These data correlate with sea ice concentration anomalies, which are highly negative during La Niña years and highly positive during El Niño years. Warm temperature conditions in the Bellingshausen Sea during La Niña years are strongly related to both higher weaning mass in elephant seals and to an increase in squid beaks in the stomach contents of females.Main conclusions It is possible that higher elephant seal weaning masses in La Niña years correlate with warmer waters in the Bellingshausen Sea leading to the rapid growth of squid and their more frequent descents to depths frequented by elephant seals. This results in increased predation by pregnant females, leading to a greater mass among weaned pups. This hypothesis may guide future research about interactions between climate and the marine biosphere.

LINKING MOVEMENT, DIVING, AND HABITAT TO FORAGING SUCCESS IN A LARGE MARINE PREDATOR

Establishing where and when predators forage is essential to understanding trophic interactions, yet foraging behavior remains poorly understood in large marine carnivores. We investigated the factors leading to foraging success in gray seals (Halichoerus grypus) in the Northwest Atlantic in the first study to use simultaneous deployments of satellite transmitters, time depth recorders, and stomach-temperature loggers on a free-ranging marine mammal. Thirty-two seals were each fitted with the three types of instrumentation; however, complete records from all three instruments were obtained from only 13 individuals, underscoring the difficulty of such a multi-instrument approach. Our goal was to determine the characteristics of diving, habitat, and movement that predict feeding. We linked diving behavior to foraging success at two temporal scales: trips (days) and bouts (hours) to test models of optimal diving, which indicate that feeding can be predicted by time spent at the bottom of a dive. Using an information-theoretic approach, a Generalized Linear Mixed Model with trip duration and accumulated bottom time per day best explained the number of feeding events per trip, whereas the best predictor of the number of feeding events per bout was accumulated bottom time. We then tested whether characteristics of movement were predictive of feeding. Significant predictors of the number of feeding events per trip were angular variance (i.e., path tortuosity) and distance traveled per day. Finally, we integrated measures of diving, movement, and habitat at four temporal scales to determine overall predictors of feeding. At the 3-h scale, mean bottom time and distance traveled were the most important predictors of feeding frequency, whereas at the 6-h and 24-h time scales, distance traveled alone was most important. Bathymetry was the most significant predictor of feeding at the 12-h interval, with feeding more likely to occur at deeper depths. Our findings indicate that several factors predict feeding in gray seals, but predictor variables differ across temporal scales such that environmental variation becomes important at some scales and not others. Overall, our results illustrate the value of simultaneously recording and integrating multiple types of information to better understand the circumstances leading to foraging success.

THE BEHAVIORAL BASIS FOR NONLINEAR FUNCTIONAL RESPONSES AND OPTIMAL FORAGING IN ANTARCTIC FUR SEALS

This study investigated the degree to which the behavior of a predator in the marine environment can be used to indicate the availability of prey. It examined this in Antarctic fur seals (Arctocephalus gazella) feeding on krill, with a view to understanding how behavioral choices could lead to a nonlinear functional response. The study showed how fur seals adjusted their time budgets to maximize the mean rate of energy intake in response to varying prey abundance. The behavioral indicator of prey abundance was the amount of time spent by fur seals at the bottom of dives after adjustment for dive depth. This indicator was well correlated with independent measures of prey abundance. The indicator was calibrated in terms of net rate of energy intake from information about energy expenditures in Antarctic fur seals and was used within the framework of optimal foraging theory to examine how female fur seals organized their foraging activity within and between bouts. Foraging in Antarctic fur seals conformed to many of the predictions from energy rate-maximizing behavior. For example, the apparent net rate of energy intake by fur seals declined with time spent in a patch, and the rate of energy intake on leaving a patch declined as apparent food availability in the environment declined. In addition, the time spent in patches increased as apparent food availability declined. Antarctic fur seals were able to adjust their behavior to track highly variable prey distributions and densities. The simple decision about when to leave a patch in relation to current environmental prey availability appears to be responsible for allowing fur seals to maintain saturation of the foraging rate as food density declines.

Interannual variation in the summer diets of harbour seals Phocavitulina at Mousa, Shetland (UK)

The main prey species in the summer (July–September) diets of harbour seals (Phoca vitulina) on the Island of Mousa (Shetland, UK), 1994–1997, were whiting (Merlangius merlangus), herring (Clupea harengus), sandeel (Ammodytidae) and garfish (Belone belone). Norway pout (Trisopterus esmarkii) was numerically important but comprised no more than 11% of prey weight in any year. There were marked between-year fluctuations in the relative importance of these prey, with whiting comprising 16–34% (by weight) of the diet, herring 12–28%, sandeels 7–18% and garfish 7–22%. Additional data on spring (April–June) diet were available for 1995–1997: sandeels were the most important prey (% weight) in all three years (51–60% of the diet), while herring (8–48%) and gadids (2–22%) varied in importance. The average size of fish eaten was larger than that reported in comparable studies from other areas. Harbour seals appear to have selected larger sandeels, whiting and Norway pout than the average size available in the area, as indicated by survey trawls, although between-year changes in the size of Norway pout in the diet did to some extent reflect availability. Interannual variation in the importance of Norway pout in the diet appeared to track trends in abundance, although the short time series precluded detection of a statistically significant correlation. Thus, some of our results are consistent with harbour seals feeding opportunistically while others point to selectivity, particularly for prey size. Estimated consumption of fish by the 500 harbour seals on Mousa during summer ranged from 152–195 tonnes.

Diurnal and seasonal variations in the duration and depth of the longest dives in southern elephant seals (Mirounga leonina): possible physiological and behavioural constraints.

This study seeks to understand how the physiological constraints of diving may change on a daily and seasonal basis. Dive data were obtained from southern elephant seals (Mirounga leonina) from South Georgia using satellite relay data loggers. We analysed the longest (95th percentile) dive durations as proxies for physiological dive limits. A strong, significant relationship existed between the duration of these dives and the time of day and week of year in which they were performed. The depth of the deepest dives also showed a significant, but far less consistent, relationship with local time of day and season. Changes in the duration of the longest dives occurred irrespective of their depth. Dives were longest in the morning (04:00-12:00 h) and shortest in the evening (16:00-00:00 h). The size of the fluctuation varied among animals from 4.0 to 20.0 min. The daily pattern in dive depth was phase-shifted in relation to the diurnal rhythm in dive duration. Dives were deeper at midday and shallower around midnight. Greater daily changes in duration occurred in seals feeding in the open ocean than in those foraging on the continental shelf. The seasonal peak in the duration of the longest dives coincided with austral midwinter. The size of the increase in dive duration from autumn/spring to winter ranged from 11.5 to 30.0 min. Changes in depth of the longest dives were not consistently associated with particular times of year. The substantial diurnal and seasonal fluctuations in maximum dive duration may be a result of changes in the physiological capacity to remain submerged, in addition to temporal changes in the ecological constraints on dive behaviour. We speculate about the role of melatonin as a hormonal mediator of diving capability.

Immunotoxicity in invertebrates: measurement and ecotoxicological relevance.

Concern is growing regarding the impact of chemicals suspected of altering the function of the immune system in humans and wildlife. There are numerous examples of links between pollution and increased susceptibility to disease in wildlife species, including immunosuppression in harbour seals feeding on fish from contaminated sites, altered immune function in riverine fish and decreased host resistance in birds exposed to pollutants. Laboratory tests have identified potential immunological hazards posed by a range of anthropogenic chemicals in mammals and higher vertebrates. However, few reports have considered the ecological relevance of pollution-induced immunosuppression in invertebrate phyla, which constitute around 95% of all animal species and occupy key structural and functional roles in ecosystems. In this paper effects of chemicals on immune function in invertebrates are briefly reviewed and biomarkers of immunotoxicity are identified. Examples of new approaches for the measurement of immunological inflammatory reactions and stress in molluscan haemocytes are detailed. The relevance of defining the immune system as a target organ of toxicity in invertebrates is discussed and an integrated approach for the use of immunological biomarkers in environment management is proposed, combining measures of immune function and organismal viability at the biochemical, cellular and population level.

Prey selection by harbour seals, Phoca vitulina, in relation to variations in prey abundance

This study compared the composition of the diet of harbour seals (Phoca vitulina) feeding in the Moray Firth, Scotland, with the abundance of their fish prey estimated from dedicated fishery surveys in January 1992 and 1994 and June 1992. Prey-size selection was also examined in these three time periods and in January 1991. In each period, the most abundant fish species contributed most to the diet. However, the relative abundances of the remaining species in the sea showed little similarity to their contribution to the seals' diet. Diet composition was almost totally dominated by either pelagic species or species dwelling on or strongly associated with the seabed, depending upon the relative abundance of pelagic schooling prey. Most fish consumed were 10–16 cm in length, although larger cod and herring were taken. With the exception of cod, the extent of size selection was dependent upon the use of correction factors that accounted for otolith erosion due to digestion.

Suppression of natural killer cell activity in harbour seals ( Phoca vitulina) fed Baltic Sea herring

Mass mortalities among marine mammal populations in recent years have raised questions about a possible contributory role of contaminants accumulated through the marine food chain. While viruses were shown to be the primary cause of the outbreaks, an immunotoxic action by organochlorine chemicals in affected animals could not be ruled out. We carried out a 2 1 2 - year immunotoxicological experiment in which two groups of 11 harbour seals each were fed herring from either the relatively contaminated Baltic Sea or the relatively uncontaminated Atlantic Ocean. Seals in the Baltic Sea group accumulated 3–4 times higher levels of Ah-receptor-mediated 2,3,7,8-TCDD Toxic Equivalents in blubber than did their Atlantic counterparts following 2 years on the respective diets. Blood was sampled a total of 17 times during the course of the experiment for immunological evaluation, during which time the natural cytotoxic activity of peripheral blood mononuclear cells isolated from seals fed Baltic Sea herring declined to a level approximately 25% lower than that observed in seals fed Atlantic herring ( P < 0.01). Natural killer (NK) cell activity has not been previously described for a marine mammal species. We characterized the natural cytotoxic activity of harbour seal peripheral blood mononuclear cells (PBMC), and found this to be interleukin-2 (IL-2) responsive, sensitive to antibody anti-asialo GM1, and it was higher against a virus-infected target cell, like NK cells described for other mammals. As NK cells are leukocytes which play an important role in the first line of defence against viruses, the observed impairment of NK cell activity in the seals feeding on the Baltic Sea herring suggests that exposure to contaminants may have an adverse effect on the defence against virus infections in seals inhabiting polluted waters in Europe. This may therefore have affected the severity of the infections, the survival rates and the spread of infections during recent epizootics.

Predation of Adélie penguins (Pygoscelis adeliae) by leopard seals (Hydrurga leptonyx) in Prydz Bay, Antarctica

The behaviour of leopard seals, Hydrurga leptonyx, feeding on Adélie penguins, Pygoscelis adeliae, was investigated between November 1993 and January 1994 in Prydz Bay, Antarctica. The seals were distributed along the fast-ice edge in locations where departing penguins congregated. Five different hunting techniques were observed, four of which were used through most of the summer. Individual leopard seals favoured specific hunting techniques. Hitherto, penguin hunting was believed to be carried out primarily by a few male seals; in this study, however, the predation observed involved many different individuals of both sexes. It is estimated that six leopard seals feeding in this area over a period of 120 days would consume 2.7% of the adult penguin population.

AMBIO

textabstractDisease outbreaks with high mortality rates among seals and dolphins have recently attracted considerable public and scientific interest. Allhouyh in most cases morbillivirus infections were shown to be the primary cause of the disease outbreaks, it was speculated that pollution-induced immunosuppression had playat:! a contributory role. Here we present results of a prospective study under semifield conditions, in which two groups of harbor seals (Phoca vitulina) were fed herring from marine regions with different contamination levels; the highly polluted Baltic Sea and the relatively unpolluted Atlantic Ocean. During a period of 93 weeks, parameters related to immune function were monitored and compared between the two groups. We found that natural killer-cell activity and mitogen-induced proliferative T -cell responses from the seals feeding on herring from the Baltic Sea were significantly lower. In addition, we observed higher levels of circulating polymorphonuclear granulocytes in these animals, which may indicate an increase in the occurrence of bacterial infections. This is the first demonstration of impaired immunological functions in mammals associated with chronic exposure to environmental contaminants accumulated through the marine food chain.

Key fish species, northern fur seals, Callorhinus ursinus, and fisheries interactions involving walleye pollock, Theragra chalcogramma, in the eastern Bering Sea

The recent decline of the northern fur seal population breeding on the Pribilof Islands has not yet been explained. This study estimates the amounts and sizes of walleye pollock (the fur seal's main prey) removed by predatory fish, fur seals and commercial fisheries in the main fur seal feeding area in the eastern Bering Sea during summer 1985. Fur seals relied primarily on age‐1 pollock during 1985, while predatory fish consumed pollock mainly of ages 0‐1. The fishery took pollock older than age‐3. This indicated no direct effect of fishery removal on fur seal prey abundance, at least during the study period. In the short term, the 1985 pollock fishery may have indirectly affected fur seal prey abundance in a positive manner by removing adult pollock that compete with fur seals for age‐1 pollock.

Reproductive failure in common seals feeding on fish from polluted coastal waters.

The population of common seal Phoca vitulina in the westernmost part of the Wadden Sea, The Netherlands, has collapsed during the past few decades. Between 1950 and 1975 the population dropped from more than 3,000 to less than 500 animals. Comparative studies of common seal populations from different parts of the Wadden Sea reveal that pup production has declined sharply only in the western (Dutch) part. A comparative toxicological study on the levels of heavy metals and organochlorines in tissues of seals from the western and northern parts of the Wadden Sea shows that only the polychlorinated biphenyl (PCB) levels differ significantly. This is predominantly a result of PCB pollution from the river Rhine, which mainly affects the western (Dutch) part. PCBs are thought to be responsible for the low rate of reproduction in Dutch common seals on the basis of epidemiological and experimental data on the ability of PCBs to interfere with mammalian reproduction. Here I report that reproductive failure in common seals from the Dutch Wadden Sea is related to feeding on fish from that polluted area. This is the first demonstration of a causal relationship between naturally occurring levels of pollutants and a physiological response in marine mammals.

Interpretation of otoliths in stomach content analyses of phocid seals: quantifying fish consumption

Thirteen captive phocid seals (10 grey seals (Halichoerus grypus), 2 harp seals (Phoca groenlandica), and 1 ringed seal (P. hispida)) were fed Atlantic herring (Clupea harengus harengus) in experiments designed to examine the use of otoliths recovered in stomach contents to interpret food consumption of wild seals. The percentage of ingested otoliths recovered in the stomach contents decreased with the time elapsed after feeding; 100% of otoliths were recovered between 0 and 3 h after feeding and 0% were recovered by 12.9 h after feeding. Absence of otoliths in the large intestine of seals having fed 3 to 6 h previous indicated that unrecovered otoliths had been digested (i.e., a complete disappearance of whole otoliths) while in the stomach. A significant relationship was also evident between the state of digestion of a seal's stomach contents, as measured by the proportion of otoliths remaining in skull cases (skull-recovered otoliths), and the duration of time since it had fed. These relationships have potential application in the estimation of daily fish consumption of seals feeding in the wild.

Visual Orientation and Feeding in Seals and Sea Lions

AT least some, and perhaps most, pinnipeds of the families Phocidae and Otariidae are nocturnal feeders1. The eyes of these predators, being sensitive to very low light intensities2, are suited for use at night and in deep water. During the day the visual acuity of the California sea lion (Zalophus californianus) allows this animal to discriminate a size difference ratio as small as 1.06 : 1 (ref. 3). A great deal of vocalizing occurs among pinnipeds, both above and below the surface of the water1,4–6. This fact, along with experimental work such as that of Mohl with the harbour seal (Phoca vitulina)7, indicates that hearing is generally well developed. Poulter suggested that the California sea lion, and probably other species, is able to capture prey in darkness by utilizing an echo-ranging sense similar to that known to occur among cetaceans4. However, subsequent work5,6 has failed to support Poulter's theory. Nevertheless, as “individuals totally blind but in a good state of health have been reported”1, the possibility that such a sense may bo developed when needed should not be dismissed lightly.