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.