Changes in Prey

abstract
This study quantified the effect of changes in prey intake on the growth of individual Steller sea lions. Data from 12 female sea lions subject to various experimental episodes of restricted food intake were used to produce an overall model predicting changes in growth rates from different levels of unpredicted reductions in energy intake. The resulting equation was robust across different types and levels of restriction, seasons, and age classes. This predictive relationship between changes in food intake and growth is invaluable for incorporating into bioenergetic models estimating the effects of environmental changes on wild Steller sea lions.

keywords     Steller sea lion, food intake, growth, body mass, energy intake, bioenergetics

abstract
Pinnipeds have specific macronutrient (protein, lipid) requirements to satisfy physiological functions, yet little is known about how diet characteristics affect macronutrient digestibility. We measured relative and absolute lipid and protein digestibility in six female northern fur seals (Callorhinus ursinus (Linnaeus 1758)) fed eight experimental diets composed variously of four prey species (Pacific herring (Clupea pallasii Valenciennes in Cuvier and Valenciennes 1847), walleye pollock (Gadus chalcogrammus Pallas 1814, formerly Theragra chalcogrammus (Pallas 1814)), capelin (Mallotus villosus (Müller 1776) and magister armhook squid (Berryteuthis magisterial (Berry 1913)). We quantified how digestibility was affected by proximate composition of the diet (%lipid or protein), levels of food mass and macronutrient intake, and tested for any potential benefit of multi-species diets. Overall, digestibility of both protein and lipid were high across diets, although macronutrient retention of lipids (96.0–98.4%) was significantly higher than protein (95.7–96.7%) for all but the two highest protein diets. Increased levels of protein intake resulted in increased protein retention, but decreased lipid digestibility. There was no evidence that mixed-species diets provide greater macronutrient digestibility over single-species diets. The results suggest that high to moderate lipid diets are more beneficial to northern fur seals as they lead to increased levels of lipid retention without large decreases in protein digestibility. This raises concerns that dietary factors may be contributing to the population declines of northern fur seals in the Bering Sea.

keywords     Northern fur seal, Callorhinus ursinus, diet composition, macronutrients, lipid digestibility, protein digestibility

abstract
Breath-hold divers should adjust their dive behaviors to maximize the benefits and minimize the costs of foraging on prey patches of different densities at different depths. However, few studies have quantified how animals respond to changes in prey availability (depth and density), and how this affects their foraging efficiency. We tested the effects of changes in prey availability on the foraging behavior and efficiency of Steller sea lions (Eumetopias jubatus) by measuring diving metabolic rate, dive durations, and food intake of 4 trained sea lions diving in the open ocean on controlled prey patches of different densities at different depths. Sea lions completed bouts of 5 consecutive dives on high- or low-density prey patches at two depths (10m and 40m). We found that the rate of energy expenditure did not change under any of the imposed foraging conditions (meanąSD: 0.22ą0.02 kJ min−1 kg−1), but that the proportion of time spent consuming prey increased with prey patch density due to changes in diving patterns. At both depths, sea lions spent a greater proportion of the dive bout foraging on prey patches with high prey density, which led to high rates of energy gain (4.3 ą 0.96 kJ min−1 kg−1) and high foraging efficiency (cost:benefit was 1:20). In contrast, the sea lions spent a smaller proportion of their dive bout actively feeding on prey patches with low prey density, and consequently had a lower energetic gain (0.91 ą 0.29 kJ min−1 kg−1) and foraging efficiency (1:4). The 5-fold differences in foraging efficiency between the two types of prey patches were greater than the 3-fold differences that we expected based on differences in food availability. Our results suggest that sea lions faced with reduced prey availability forage less efficiently and therefore would have greater difficulty obtaining their daily energy requirements.

keywords     Dive behavior, Diving energetics, Foraging efficiency, Optimal foraging, Steller sea lion

abstract
Unexpected shortages of food may affect wildlife differently depending on the time of year it occurs. We imposed 48-hr fasts on six female northern fur seals (Callorhinus ursinus; ages 6 ? 24 months) to identify times of year when they might be particularly sensitive to interruptions in food supply. We monitored changes in their resting metabolic rates and their metabolic response to thermal challenges, and also examined potential bioenergetic causes for seasonal differences in body mass loss. Pre-fast metabolism of the fur seals while in ambient air or submerged in 4 ?C water was higher during summer (Jun-Sep) than winter (Nov-Mar), and submergence did not significantly increase metabolism indicating a lack of additional thermoregulatory costs. There was no evidence of metabolic depression following the fasting periods, nor did metabolism increase during the post-fast thermal challenge, suggesting that mass loss did not negatively impact thermoregulatory capacity. However, the fur seals lost mass at greater rates while fasting during the summer months when metabolism is normally high to facilitate faster growth rates (which would ordinarily have been supported by higher food intake levels). Our findings suggest that summer is a more critical time of year than winter for young northern fur seals to obtain adequate nutrition.

abstract
1. Numbers of Steller sea lions Eumetopias jubatus in the North Pacific have declined. According to the Nutritional Stress Hypothesis, this decline is due to reduced food availability. Data from studies conducted on pinnipeds in the laboratory are used here to test whether the Nutritional Stress Hypothesis can explain the decline of Steller sea lions. 2. Overall, there is strong evidence for biologically meaningful differences in the nutritional quality of major prey species. Steller sea lions can partly compensate for low-quality prey by increasing their food consumption. 3. There appear to be no detrimental effects of low-lipid prey on sea lion growth or body composition when sea lions can consume sufficient quantities of prey. However, the ability to increase consumption is physiologically limited, particularly in young animals. Overall, it is more difficult to maintain energy intake on a diet of low-quality prey than on a normal diet. 4. Under conditions of inadequate food intake (either due to decreased prey availability or quality, or increased energy requirements) the overall impacts of nutritional stress are complex, and are dependent upon season, prey quality, age, and the duration and intensity of the nutritional stress event. 5. Studies on pinnipeds in the laboratory have been instrumental in identifying the conditions under which changes in sea lion prey can result in nutritional stress, and the nature of the physiological impacts of nutritional stress events.