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Nutritional Constraints on the Southern Sea Otter in the Monterey Bay National Marine Sanctuary

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Nutritional Constraints on the Southern Sea Otter in the Monterey Bay National Marine Sanctuary NUTRITIONAL CONSTRAINTS ON THE SOUTHERN SEA OTTER IN THE MONTEREY BAY NATIONAL MARINE SANCTUARY and a comparison to sea otter populations at San Nicolas Island, California and Glacier Bay, Alaska. Joint Final Report to Monterey Bay National Marine Sanctuary (and Monterey Bay Sanctuary Foundation) and the Marine Mammal Commission. Prepared by: Olav T. Oftedal PhD, Nutrition Laboratory, Conservation Ecology Center, Smithsonian’s National Zoological Park, Washington DC Katherine Ralls PhD, Center for Conservation and Evolutionary Genetics, Smithsonian’s National Zoological Park, Washington, DC M. Tim Tinker PhD, University of California Santa Cruz, Santa Cruz, California Alice Green, PhD candidate, Nutritional Biology, University of California Davis, Davis, California With assistance and contributions from: Seth Newsome PhD, Carnegie Institute of Washington, Washington, DC Jim Bodkin MS, Alaska Science Center, US Geological Survey, Anchorage, Alaska Gena Bentall MS, University of California Santa Cruz, Santa Cruz, California Nutritional constraints in the southern sea otter. Oftedal, Ralls, Tinker and Green, 2007 NUTRITIONAL CONSTRAINTS ON THE SOUTHERN SEA OTTER IN THE MONTEREY BAY NATIONAL MARINE SANCTUARY and a comparison to sea otter populations at San Nicolas Island, California and Glacier Bay, Alaska. O. Oftedal, K. Ralls, M.T. Tinker and A. Green with assistance and contributions from S. Newsome, J. Bodkin and G. Bentall Executive Summary The Monterey Bay National Marine Sanctuary in California, which extends from Marin in the north to Cambria in the south, encompasses much of the range of the threatened southern sea otter, Enhydra lutris nereis. The growth of this otter population has slowed and/or stalled due to increased mortality, and many otters die of disease. We reasoned that nutrition might constrain this population if availability of high quality food resources was limited. Sea otters are well known for their ability to locally reduce the abundance of preferred prey such as abalone and sea urchins. Nutritional deficits might affect body condition, reproductive performance, immune function, susceptibility to disease and ability to withstand contaminant or toxin loads, and may ultimately contribute to reduced survival. However, sea otters in MBNMS feed exclusively on large marine invertebrates, and the nutritional consequences of this diet had not been studied prior to the work described in this report. This project examined the nutritional constraints faced by southern sea otters in and adjacent to the sanctuary via a combined analysis of foraging data and nutritional assessment of all major prey species. These two components involved extensive synergy, as the foraging observations indicated which prey taxa to target for detailed nutritional analysis, while assessment of the edible portion, biomass, and energy content of prey allowed more accurate assessment of the energy returns per unit of foraging time. Estimated nutritional quality of diets was compared among otters in MBNMS, and between otter populations in MBNMS, San Nicolas Island, CA and Glacier Bay, AK. The latter two were selected as examples of expanding populations in apparently good condition. We also compared prey and diet composition to recommended nutrient levels for domestic carnivores, the closest available model for sea otter nutrient needs. ii Nutritional constraints in the southern sea otter. Oftedal, Ralls, Tinker and Green, 2007 From the comparison to the San Nicolas and Glacier Bay populations, we concluded that sea otters in MBNMS are faced with food and/or nutritional shortages. Sea otters at San Nicolas and Glacier Bay have access to abundant prey resources and are characterized by large body size and good body condition, higher rates of energy gain when foraging, reduced foraging times per 24-hour day, and low dietary diversity at the population level. The central California sea otter population, in an environment where populations of some preferred prey items have been reduced, is characterized by smaller body size, poor body condition, lower rates of energy gain when foraging, increased foraging times per 24-hour day, and high dietary diversity at the population level. Diets at the population level are composed of the diets of many individuals. Although dietary specialization is always advantageous to sea otters due to increased efficiency in handling prey, individuals within a population may tend to specialize on the same or different suites of prey. Theory predicts that individuals within a population will tend to specialize on different suites of prey as resources become less abundant. At San Nicolas, all individuals (n = 11) fed predominantly on urchins, with smaller amounts of crab, lobster and snails. Although the otters in Glacier Bay were not individually identifiable, the population-level diet consisted largely of clams, suggesting that there was relatively little dietary specialization among individuals. In contrast, individual otters in MBNMS (n = 63) had extremely varied diet that could be grouped into several distinct types. These diet types could be categorized by 3 prey sizes (large = 1, medium =2, small = 3) and 6 predominant prey types: abalone and crabs (Type 1a), Cancer crabs (Type 1b), kelp crabs and rocky bottom prey (Type 2a), urchins and mussels (Type 2b), clams and sandy bottom prey (Type 2c) and trochid snails (Type 3a). While these types could be clearly distinguished statistically, some prey (such as crabs and urchins) were found in most diet types. A survey of prey nutrient composition in California and Alaska (more than 700 samples of 76 taxa) revealed that prey energy content (on a dry mass basis) was primarily determined by two factors: the amounts of mineral matter (ash) and fat in the edible portion. Across a wide range of bivalves and gastropods, fat content was uniformly low, so that for these taxa ash was the primary determinant of energy, with a strong negative correlation between ash and energy. Some bivalves had remarkably high ash content, iii Nutritional constraints in the southern sea otter. Oftedal, Ralls, Tinker and Green, 2007 apparently due to accumulation of sand or sediment; these species were low in calcium. Among gastropods, high ash was largely due to the inclusion of shell fragments when the prey were crushed; such samples had very high calcium content. Decapods were also high in calcium and ash, presumably due to calcification of the exoskeleton, but some species (such as sand crabs) accumulated modest amounts of fat at least seasonally. The edible portion (gonads, ceca, viscera) of echinoderms (such as urchins and sea stars) was typically high in fat, regardless of season. Our analysis indicated that fat may be a limiting resource for sea otters for several reasons. Many prey taxa are very low in fat, resulting in low energy content and the need for otters to consume large amounts of prey to meet energy requirements. Diets high in snails (3a) were particularly low in fat and energy on a dry basis. Low fat is also associated with low levels of fat-soluble vitamin A and of essential fatty acids of the omega6 family. Estimated levels of vitamin A and omega6 fatty acids were so low in some MBNMS diet types that one would predict that these could produce nutritional deficiency. MBNMS diets were also apparently lower in vitamin A than diets at Glacier Bay. The possible consequences of low dietary vitamin A in sea otter diets warrant investigation given the importance of vitamin A for proper immune system function. However, carotenoids such as β- carotene and γ-carotene were abundant in most sea otter prey, and it is not known whether sea otters can utilize carotenoids as a source of vitamin A. Other nutrients were also of concern in MBNMS diets. The B vitamin thiamin was low to marginal in all MBNMS diet types, although further study is needed because vitamin losses may have occurred during analysis. Estimated thiamin levels were higher in San Nicolas and Glacier Bay diets. High levels of calcium were prevalent in many prey types (especially gastropods and decapods). The high calcium levels found in the diets of MBMNS otters (from 4.8% in the urchin and mussel diet type to 19% in the snail diet type) could produce malabsorption problems for other minerals such as phosphorus and zinc. This is of particular concern in MBNMS diet types that were deemed marginal or low in phosphorus and/or zinc, such as the kelp crab (2a), urchin and mussel (2b) and snail (3a) diets. Otters at MBNMS consumed significantly more calcium, on average, than otters at San Nicolas, and the Glacier Bay diet was lower still (1.9% calcium). The iv Nutritional constraints in the southern sea otter. Oftedal, Ralls, Tinker and Green, 2007 normal calcium:phosphorus ratio (2:1) at Glacier Bay was in stark contrast to the MBNMS ratio that averaged 8:1 but was as high as 40:1 in the snail diet (3a). Our analysis suggests that the six specialized diets adopted by MBNMS otters are not all of equal nutritional value. In particular, the snail diet (3a) adopted by some otters appeared to be low, marginal or excessive in many nutrients, including fat, omega6 fatty acids, calcium, phosphorus, iron, zinc, thiamin, vitamin B6, and vitamin A. Any one or a combination of these nutritional imbalances could impair reproductive performance, immune function, resistance to disease, or ability to deal with toxin or contaminant loads. We believe that additional research is needed on the health status and fate of otters of differing diet histories. Such an analysis may be able to take advantage of new indicators of the diet history of individual animals, such as stable isotope analysis and fatty acid analysis, in addition to or instead of traditional observational and radiotelemetry techniques. We have demonstrated differences in stable isotope profiles of sea otters in different populations and in the estimated fatty acid composition of different otter diet types. These techniques offer the promise of being able to indicate diet type of otters captured live or obtained postmortem and warrant further study as means of monitoring sea otter populations in the MBNMS.
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