Abstract
Food availability is a key concern for the conservation of marine top predators, particularly during a time when they face a rapidly changing environment and continued pressure from commercial fishing activities. Northern fur seals (Callorhinus ursinus) breeding on the Pribilof Islands in the eastern Bering Sea have experienced an unexplained population decline since the late-1990s. Dietary overlap with a large U.S. fishery for walleye pollock (Gadus chalcogrammus) in combination with changes in maternal foraging behavior and pup growth has led to the hypothesis that food limitation may be contributing to the population decline. We developed age- and sex-specific bioenergetic models to estimate fur seal energy intake from May–December in six target years, which were combined with diet data to quantify prey consumption. There was considerable sex- and age-specific variation in energy intake because of differences in body size, energetic costs, and behavior; net energy intake was lowest for juveniles (18.9 MJ sea-day–1, 1,409.4 MJ season–1) and highest for adult males (66.0 MJ sea-day–1, 7,651.7 MJ season–1). Population-level prey consumption ranged from 255,232 t (222,159 – 350,755 t, 95% CI) in 2006 to 500,039 t (453,720 – 555,205 t) in 1996, with pollock comprising between 41.4 and 76.5% of this biomass. Interannual variation in size-specific pollock consumption appeared largely driven by the availability of juvenile fish, with up to 81.6% of pollock biomass coming from mature pollock in years of poor age-1 recruitment. Relationships among metabolic rates, trip durations, pup growth rates, and energy intake of lactating females suggest the most feasible mechanism to increase pup growth rates is by increasing foraging efficiency through reductions in maternal foraging effort, which is unlikely to occur without increases in localized prey density. By quantifying year-specific fur seal consumption of pollock, our study provides a pathway to incorporate fur seals into multispecies pollock stock assessment models, which is critical for fur seal and fishery management given they were a significant source of mortality for both juvenile and mature pollock.
Highlights
Availability of food resources is a strong driver of population dynamics, and changes in the abundance, type, or distribution of resources can have wide-ranging impacts on wildlife populations (Sillett et al, 2000; Roth, 2002; Oro et al, 2004)
Identification of the specific ways that these mechanisms affect vital rates and population dynamics requires an understanding of energy requirements and prey consumption, which are necessary for quantifying species- and community-level interactions
We focused on 6 years that spanned the current population decline (1995–1996, 2004–2006, 2010) based on availability of fur seal data and because these years encompassed a variety of environmental conditions, primarily as it related to the extent of the cold pool (Supplementary Figure S2)
Summary
Availability of food resources is a strong driver of population dynamics, and changes in the abundance, type, or distribution of resources can have wide-ranging impacts on wildlife populations (Sillett et al, 2000; Roth, 2002; Oro et al, 2004). The development of bioenergetic models—data-driven estimates of individual- and population-level energy and prey requirements— is crucial for wildlife conservation, as it relates to understanding the role of food availability in population declines and management of commercially important prey species This is a pressing issue in marine environments, where food availability has been implicated as a contributing factor to current population declines of many seabirds and marine mammals (Becker and Beissinger, 2006; Ford et al, 2010; Grémillet et al, 2016), and fishing pressures may exacerbate the negative impacts of climate change on prey populations (Essington et al, 2015; Lindegren et al, 2018). George Island has recently stabilized and the population at Bogoslof Island continues to experience healthy population growth since its colonization in the 1980s (Muto et al, 2020)
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