1. Introduction
The nutritional ecology of marine predators is poorly understood compared to terrestrial predators. Classic foraging theory suggests that consumers should target prey that maximizes their net rate of energetic gain . As energetic intake was the focus, studies addressed consumers’ overall net energetic consumption as a factor in prey selection. However, more recent studies have focused on predators’ nutritional needs and how prey choice varies according to macronutrient composition . Consumers may target lower energy prey for various reasons, including reproductive status. For example, Machovsky‑Capuska et al. (2018) revealed that male Australasian gannets (Morus serrator ) consistently foraged for fish with higher protein-to-lipid ratios, whereas females foraged for fish with higher lipid-to-protein ratios. One potential explanation is the differential dietary response by female and male parents according to the changing needs of growing chicks. Similarly, female sea otters (Enhydra lutris ) will switch their foraging tactics when caring for a pup, which may represent a trade-off between maximizing potential energy return and meeting pup needs .
Sea otters are a dynamic species in which to study nutritional ecology because of their unique adaptations to the marine environment. Unlike most marine mammals, sea otters do not have blubber to keep them warm. Instead, sea otters maintain very high metabolisms . Various studies estimate sea otters consume anywhere from 19 to 39% of their body weight in food per day to sustain these elevated metabolic costs . Because of their voracious appetites, sea otters can exert large effects on the nearshore marine ecosystem within relatively short periods . These effects are particularly evident in kelp forests, where sea otters suppress the grazers, sea urchins (Strongylocentrotus spp.), which in turn relieves pressure on kelp. This relief of grazing pressure helps to increase kelp forest density, which has been shown to increase juvenile fish habitat and increase overall species diversity in the system.
Historical records show that sea otters once inhabited nearshore ecosystems of the Pacific Ocean from Japan to Baja California. However, by the late 19th Century, there were only 11 remnant populations within their once continuous distribution due to hunting for the lucrative fur markets in Russia and China. In 1911, sea otters were protected from hunting by the International Fur Seal Treaty. By this time, sea otters were extirpated from Southeast Alaska . To restore sea otters to their historical range, the Alaska Department of Fish and Game and the Atomic Energy Commission initiated a translocation program and, in the 1960s, relocated about 400 sea otters from the Aleutian Islands to six locations in Southeast Alaska. Since the translocation, sea otters have expanded their range and increased in numbers. The most recent range-wide sea otter aerial counts in 2010–2011 estimated that approximately 25,000 sea otters were present in Southeast Alaska . The expansion of sea otters from the six translocation sites in Southeast Alaska into unoccupied habitat over time allows for a ‘space-for-time’ substitution , in which the longer-term effects (positive, neutral, and negative) of sea otters on the nearshore ecosystem can be seen in areas of longer occupation.
Prince of Wales Island (POW), along with its neighboring islands, in southern Southeast Alaska has two original release locations. Hoyt (2015) studied sea otter diets around POW for three years (2010–2012) focusing on sea otter impacts on commercially important species. Hoyt (2015) found that the number of species consumed by sea otters increased as time since recolonization increased, and sea otters reduced the abundance of commercially important species. The sea cucumber (Apostichopus californicus ) fishery is an example of a commercial shellfishery impacted by sea otters. Previous studies showed that sea otter presence caused a decline of sea cucumbers in all regions where sea otters were present for > 15 years . Many sea cucumber fishery regions have been closed due to declining sea cucumber abundance after sea otters have recolonized the regions .
Measuring sea otter energetic intake rate is a widely used method to measure changes in diet and to assign quantitative values to sea otter nutritional needs . In Alaska, the Alaska Science Center, a part of the United States Geological Survey (USGS), maintains a database on species- and size-specific energetic values for sea otter prey items. Many of these values come from California invertebrate collections and published literature . These values are used to create energetic models and biomass estimates for each sea otter prey species; however, using prey values from other regions could lead to inaccurate consumption estimates for sea otters in Alaska. Similarly, a comprehensive analysis of the biochemical composition of sea otter prey was conducted in varying seasons in California but is absent for Alaskan prey . A preliminary study in Sitka, Alaska, showed that the preferred prey of sea otters (clams) was not highest in overall caloric content or lipid content when compared to all available prey items . This preliminary work is a driver for further investigation of sea otter diet analysis in Southeast Alaska.
Our goal for this study was to analyze the relationship between sea otter diet and prey nutritional composition. Our objectives were to: 1) investigate the macronutrient quality of sea otter prey in southern Southeast Alaska across seasons; and 2) examine diet composition according to prey macronutrient composition across different sea otter metrics: (i) time since recolonization, (ii) sea otter sex and reproductive class, and iii) season. This work increases our understanding of sea otter prey composition according to the energy and macronutrient content in Southeast Alaska, which, in turn, gives us a better understanding of the invertebrates removed from the nearshore system due to sea otter predation, and why. This increases the potential to predict future impacts of sea otters on nearshore ecosystems, including changes to kelp forest cover and reductions in commercial and subsistence species due to sea otter predation.