Michael A. Castellini

The diet of Weddell seals in McMurdo Sound, Antarctica as determined from scat collections and stable isotope analysis

Abstract The diet of adult and juvenile Weddell seals (Leptonychotes weddellii) in McMurdo Sound, Antarctica, was determined from both scat and stable isotope analyses, to ascertain if foraging behavior varied with age, season, or diving pattern. Scats were collected over 6 years and recovered hard parts identified. Stable carbon and nitrogen isotope values were determined for seal blood samples and potential prey items and used to identify primary prey species and assess trophic interactions. Pleuragramma antarcticum remains were recovered from between 70 and 100% of the scats, and there was little evidence for inter-annual or age-specific variation in foraging behavior. However, stable isotope and dive data analyses indicated that while most seals foraged predominantly on pelagic fish and squid, some juveniles concentrated on shallow benthic Trematomus spp. Combining these three methods permitted firm conclusions about diet and foraging behavior to be drawn.

Determinants of the Aerobic Dive Limit of Weddell Seals: Analysis of Diving Metabolic Rates, Postdive End Tidal Po2's, and Blood and Muscle Oxygen Stores

The mean aerobic dive limit (ADL) for Weddell seals was calculated from data collected on diving metabolic rates (V̇o2) and blood and muscle O₂ stores. Mean diving V̇o2 of adult seals during predominantly exploratory dive patterns was 4.5 mL O2 kg−1 min−1; mean V̇o2 of a subadult seal engaged in foraging dive bouts was 8.5 mL O2 kg−1 min−1. The adult value was 30% greater than that used in past ADL calculations. Mean plasma volume was 7% body mass (BM); blood volume calculated with the highest hematocrit (Hct) observed (66) was 21% BM. Hemoglobin concentration at such an Hct was 26% by weight. End tidal Po2 (pre-and postdive) justified the use of 95% and 20% arterial O2 saturations in the blood O2 store calculation. Total blood O2 stores were 50% greater than those used in past ADL calculations. Mean myoglobin concentration (5.4% by weight) and more recent anatomical estimates of muscle mass yielded a 35% increase in muscle O2 stores. The mean estimated ADL for a 450-kg seal calculated with these new data was 19.1 min, 2.3 min greater than in past calculations and only 1 min less than the 20-min inflection point of the curve of dive duration versus postdive lactic acid appearance. For the subadult engaged in foraging dives, the mean estimated ADL was about 9 min, again quite similar to past ADL calculations.

Physiological and behavioral determinants of the aerobic dive limit in Weddell seal (Leptonychotes weddellii) Pups

The aerobic dive limit, as defined by an increase in plasma lactate levels following dives, has to date only been determined in adult and juvenile Weddell seals (Leptonychotes weddellii). However, theoretical aerobic dive limits based on calculated total body oxygen stores, estimated metabolic rates, and dive duration frequencies have been published for several species. Using data collected over the past 3 years in McMurdo Sound. Antarctica, the aerobic dive limit of Weddell seal pups was determined by both the physiological and modeling methods. Time-depth diving recorders deployed on 36 pups between 2 and 14 weeks of age allowed the aerobic dive limit to be predicted from duration-frequency histograms. The aerobic dive limit was also calculated from estimates of total body oxygen stores and predicted diving metabolic rates. Finally, these two estimates were compared with aerobic dive limits determined from post-dive lactate levels in three pups between 5 and 7 weeks old. The aerobic dive limits of pups increased with age, but pup aerobic dive limits were still significantly shorter than those of yearlings and adults. In addition, the aerobic dive limits determined by the three methods were not equivalent for pups, yearlings, or adults, and indicate that care should be taken when modeling methods are used to estimate the aerobic dive limit in other species. Changes in hematocrit, plasma glucose, and plasma lactate levels during and between rest, diving, and recovery in pups were compared to known values for juveniles and adults. Plasma metabolite levels were more highly regulated in older pups, and together with the increasing aerobic dive limit, suggest that Weddell seal pups are not refined divers until after they are weaned, and that their diving ability continues to develop over several years.

Blood Chemistry Regulation during Repetitive Diving in Weddell Seals

During serial diving periods in Weddell seals, whole-blood lactate and glucose levels do not indicate a metabolic demand for extended recovery time at the surface. In fact, in some rare cases, the seals continue to dive even if blood lactate is elevated from a previous long dive. In addition, hematocrit (Hct) levels increase significantly and remain elevated until the dive bout has ended. These combined responses indicate that the seal will maximize its foraging time underwater by establishing a diving routine that does not significantly alter its blood chemistry. Instead, by minimizing blood glucose and lactate variations, occasionally continuing to dive in spite of excess blood lactate levels, and enhancing its blood oxygen carrying capacity, the seal can minimize time at the surface. These behavioral and metabolic correlates are unique observations on how seals can dive repetitively for hours or even days.

HEALTH STATUS OF YOUNG ALASKA STELLER SEA LION PUPS (EUMETOPIAS JUBATUS) AS INDICATED BY BLOOD CHEMISTRY AND HEMATOLOGY

Blood chemistry and hematology were examined in 238 Steller sea lion pups (Eumetopias jubatus) to assess the health status of pups <1 month of age. Failure of juvenile recruitment (possibly due to nutritionally or physiologically compromised pups) into breeding populations has been proposed as a cause of recent declines of this endangered species in Alaska. To identify potential correlations with areas of high population decline, blood chemistry data were considered for three areas: eastern Aleutian Islands (low rates of population decline to stable populations), Gulf of Alaska (high rates of decline), and Southeast Alaska (stable to increasing population). Southeast Alaska pups showed elevated ketone body concentrations (beta-hydroxybutyrate,(beta-HBA)) and depressed glucose levels than pups in the Gulf of Alaska. Over 40% of the pups from Southeast Alaska had elevated beta-HBA concentrations suggesting they underwent longer periods of fasting than seen in pups from other areas. Hematocrit (Hct), hemoglobin concentration (Hb) and water content of the blood exhibited typical mammalian relationships. In summary, blood chemistry and hematology data showed no indication that Steller sea lion pups <1 month old from areas of population decline were nutritionally compromised.

Fatty acid metabolism in fasting elephant seal pups.

SummaryThe turnover of two plasma free fatty acids (FFA) were measured in 5 northern elephant seal pups (Mirounga angustirostris) after approximately 2 months of post-weaning fasting. Turnover was determined using simultaneous bolus injections of14C-palmitate,3H-oleate and Evans blue (EB) administered via an indwelling extradural intravertebral catheter. At this time in their natural fast, the seals exhibited plasma FFA levels and turnover values higher than reported for other marine mammals and most terrestrial carnivores. There was no consistent difference between plasma FFA turnover measured by palmitate or oleate tracers. The results imply that FFA metabolism is the primary source of energy during fasting. This is interesting in light of previous observations of minimal ketoacid accumulation and low levels of glucose and protein energy production during fasting in these juvenile seals.

Glycolytic Enzyme Activities in Tissues of Marine and Terrestrial Mammals

To determine whether marine mammals possess exceptionally high levels of anaerobically poised enzymes in their tissues, the activities (units per gram wet weight of tissue at 37 C, pH 6.8) of two glycolytic enzymes, lactate dehydrogenase (LDH; assayed in the directions of lactate oxidation and pyruvate reduction) and pyruvate kinase (PK), were assayed in a variety of terrestrial and marine mammals. All major organs and several skeletal muscles were sampled from a range of species including seals, sea lions, porpoises, whales, sea otters, mice, rabbits, dogs, pigs, and beef. Contrary to the hypothesis that the diving abilities of marine mammals are based, in part, on high capacities for anaerobic glycolysis, muscle LDH and PK activities exhibited no relationship to diving ability. Brain enzyme activities were constant across all species examined, except in the gray whale. The enzyme profile of marine mammal lung and diaphragm could not be distinguished from terrestrial samples. Heart enzyme activities showed no consistent patterns, but LDH values were highest in the Weddell seal, an excellent diver. These high values in Weddell heart could reflect an adaptation for recovery from a diving period through the enhanced utilization of circulating lactate loads. Liver and kidney enzymes showed a small elevation in marine mammals but did not correlate with diving ability. Elevated marine mammal liver and kidney LDH could play an important role in postdive gluconeogenesis and in restoring circulating levels of glucose. We conclude that tissues of marine mammals do not possess unusually high capacities for anaerobic glycolysis, a conclusion consistent with recent evidence showing that marine mammals power most of their natural dives using aerobic metabolic pathways. The abilities of some marine mammals, notably Weddell seals, to remain submerged for extensive periods of time during which metabolism is largely anaerobic thus appear to be due to adaptations other than high activities of anaerobically poised enzymes.

Movements and diving behavior of weaned Weddell seal (Leptonychotes weddellii ) pups

Abstract Between 1993 and 1995, the diving behavior and movement patterns of 23 weaned Weddell seal pups (Leptonychotes weddellii) were tracked in the Ross Sea. Antarctica, using satellite-linked time-depth recorders. Regression analyses revealed that for seals of between 8 and 27 weeks old, age was poorly correlated with the dive depth, duration, or frequency. However, changes in dive parameters suggested that Weddell seal pups were attempting to maximize dive time, but the manner in which this was done depended on age and time of day. Movement patterns indicated that most Weddell seal pups left their natal area by the end of February, and traveled north along the Antarctic continent coastline. Several individuals returned to McMurdo Sound, but others were last located more than 400 km from McMurdo. Routes followed suggest that pups can use the pack ice habitat, but prefer to remain closer to the coastline than do adults.

Fuel homeostasis in the harbor seal during submerged swimming.

Summary1.The turnover rates and oxidation rates of plasma glucose, lactate, and free fatty acids (FFA) were measured in three harbor seals (average mass=40 kg) at rest or during voluntary submerged swimming in a water flume at 35% (1.3 m·s-1) and 50% (2 m·s-1) of maximum oxygen consumption (MO2max).2.For seals resting in water, the total turnover rates for glucose, lactate, and FFA were 23.2, 26.2, and 7.5 μmol·min-1·kg-1, respectively. Direct oxidation of these metabolites accounted for approximately 7%, 27%, and 33% of their turnover and 3%, 7%, and 18% of the total ATP production, respectively.3.For swimming seals,MO2max was achieved at a drag load equivalent to a speed of 3 m·s-1 and averaged 1.85 mmol O2·min-1·kg-1, which is 9-fold greater than resting metabolism in water at 18°C.4.At 35% and 50%MO2max, glucose turnover and oxidation rates did not change from resting levels. Glucose oxidation contributed about 1% of the total ATP production during swimming.5.At 50%MO2max, lactate turnover and anaerobic ATP production doubled, but the steady state plasma lactate concentration remained low at 1.1 mM. Lactate oxidation increased 63% but still contributed only 4% of the total ATP production. Anaerobic metabolism contributed about 1% of the total ATP production at rest and during swimming.6.The plasma FFA concentration and turnover rate inereased only 24% and 37% over resting levels, respectively, at 50%MO2max. However, the oxidation rate increased almost 3.5-fold and accounted for 85% of the turnover. The percentage of total ATP produced (21%) from FFA oxidation at 35% and 50%MO2max did not increase greatly over that at rest.7.Dive duration decreased from 78 s while resting in water to 28 s at 50%MO2max.8.The RQ ranged from 0.78 at rest to 0.74 at 50%MO2max, indicating that fat was an important source of energy during submerged swimming.9.By adjusting breath-hold duration during strenuous underwater swimming, harbor seals are able to maintain an aerobic, fat-based metabolism.

PLASMA HAPTOGLOBIN LEVELS IN THREATENED ALASKAN PINNIPED POPULATIONS

We evaluated the plasma concentration of the acute phase protein haptoglobin (Hp) from Steller sea lions (Eumetopias jubatus) and harbor seals (Phoca vitulina) in regions of Alaska (USA) where the populations of these pinnipeds were declining and compared the values with concentrations of Hp from the same species in areas where the populations were stable. Samples were collected from 1992 through 1994 at sites in Southeast Alaska, Prince William Sound, the Gulf of Alaska, and the Aleutian Islands. Significantly higher levels of Hp were found in the samples from the areas of decline compared to those from stable populations. Based on these findings, we propose that one may be able to distinguish these compromised pinniped populations using Hp as a biomedical indicator.