Stable isotope mixing models elucidate sex and size effects on the diet of a generalist marine predator

Bjorkland, Rhema; Pearson, Scott F.; Jeffries, Steve; Lance, Monique M.; Acevedo‐Gutiérrez, Alejandro; Ward, Eric J.

doi:10.3354/meps11230

Cited by 21 publications

(30 citation statements)

References 63 publications

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“…Our study adds to the evidence that harbor seals in the Salish Sea have some degree of foraging specialization that may occur over long time scales (Bjorkland et al., ; Bromaghin et al., ; Lance et al., ). They also suggest a complex food web between harbor seals and their prey and present a challenge for management (Bjorkland et al., ).…”

Section: Discussionsupporting

confidence: 56%

“…Pacific herring was a favorite prey item of male and female harbor seals, a finding consistent with prior diet studies in the Salish Sea (Bromaghin et al., ; Lance et al., ). We were also able to parse out the differential impact of males and females on specific prey items to discern the foraging ecology of both sexes and expand findings collected with a smaller sample size (Bjorkland et al., ). Overall, males had higher diet proportions of pelagic species, particularly Pacific hake and adult salmon than females while the latter had higher proportions of benthic and estuarine species in their diet than the former.…”

Section: Discussionmentioning

confidence: 99%

“…Female harbor seals not only consume fewer salmonids than males but also they prey upon sculpins and other species that are major predators of salmon eggs and juveniles (Berejikian, ; Mace, ; Tabor, Chan, & Hager, ). It is then possible that female harbor seal consumption of sculpins and similar prey improves conditions for salmon, while male seals may have an opposite effect (Bjorkland et al., ). Given the magnitude of harbor seal predation on Chinook salmon populations relative to fisheries and other marine mammal predators (Chasco et al., ,), it is critical to understand the differential role that female and male harbor seals may have in the community, including indirect effects on endangered southern resident killer whales.…”

Section: Discussionmentioning

confidence: 99%

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Large‐scale molecular diet analysis in a generalist marine mammal reveals male preference for prey of conservation concern

Schwarz

Spitzer

Thomas

et al. 2018

Ecology and Evolution

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Sex‐specific diet information is important in the determination of predator impacts on prey populations. Unfortunately, the diet of males and females can be difficult to describe, particularly when they are marine predators. We combined two molecular techniques to describe haul‐out use and prey preferences of male and female harbor seals (Phoca vitulina) from Comox and Cowichan Bay (Canada) during 2012–2013. DNA metabarcoding quantified the diet proportions comprised of prey species in harbor seal scat, and qPCR determined the sex of the individual that deposited each scat. Using 287 female and 260 male samples, we compared the monthly sex ratio with GLMs and analyzed prey consumption relative to sex, season, site, and year with PERMANOVA. The sex ratio between monthly samples differed widely in both years (range = 12%–79% males) and showed different patterns at each haul‐out site. Male and female diet differed across both years and sites: Females consumed a high proportion of demersal fish species while males consumed more salmonid species. Diet composition was related to both sex and season (PERMANOVA: R 2 = 27%, p < 0.001; R 2 = 24%, p < 0.001, respectively) and their interaction (PERMANOVA: R 2 = 11%, p < 0.001). Diet differences between males and females were consistent across site and year, suggesting fundamental foraging differences, including that males may have a larger impact on salmonids than females. Our novel combination of techniques allowed for both prey taxonomic and spatiotemporal resolution unprecedented in marine predators.

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Section: Discussionsupporting

confidence: 56%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Large‐scale molecular diet analysis in a generalist marine mammal reveals male preference for prey of conservation concern

Schwarz

Spitzer

Thomas

et al. 2018

Ecology and Evolution

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“…Prey contributions to Lake Trout diet were estimated using isotopic mixing models (Moore and Semmens , Parnell, Inger, Bearhop, & Jackson, ). Prey taxa were combined into coarse diet groups according to three criteria: major prey items previously identified in stomach analyses, environmental gradients reflecting shallow‐water habitat use (i.e., a littoral–pelagic axis), and by major taxonomic category (i.e., fish or invertebrates) (Bjorkland et al., ; Chavarie et al., ; Francis et al., ). Combining prey items accordingly produced a prey model consisting of littoral fish, littoral invertebrates, littoral shelled invertebrates, Mysis , juvenile Lake Trout, and Cisco (Table ).…”

Section: Methodsmentioning

confidence: 99%

Multiple generalist morphs of Lake Trout: Avoiding constraints on the evolution of intraspecific divergence?

Chavarie

Harford

Howland

et al. 2016

Ecology and Evolution

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A generalist strategy, as an adaptation to environmental heterogeneity, is common in Arctic freshwater systems, often accompanied, however, by intraspecific divergence that promotes specialization in niche use. To better understand how resources may be partitioned in a northern system that supports intraspecific diversity of Lake Trout, trophic niches were compared among four shallow‐water morphotypes in Great Bear Lake (N65° 56′ 39″, W120° 50′ 59″). Bayesian mixing model analyses of stable isotopes of carbon and nitrogen were conducted on adult Lake Trout. Major niche overlap in resource use among four Lake Trout morphotypes was found within littoral and pelagic zones, which raises the question of how such polymorphism can be sustained among opportunistic generalist morphotypes. Covariances of our morphological datasets were tested against δ13C and δ15N values. Patterns among morphotypes were mainly observed for δ15N. This link between ecological and morphological differentiation suggested that selection pressure(s) operate at the trophic level (δ15N), independent of habitat, rather than along the habitat‐foraging opportunity axis (δ13C). The spatial and temporal variability of resources in Arctic lakes, such as Great Bear Lake, may have favored the presence of multiple generalists showing different degrees of omnivory along a weak benthic–pelagic gradient. Morphs 1–3 had more generalist feeding habits using both benthic and pelagic habitats than Morph 4, which was a top‐predator specialist in the pelagic habitat. Evidence for frequent cannibalism in Great Bear Lake was found across all four morphotypes and may also contribute to polymorphism. We suggest that the multiple generalist morphs described here from Great Bear Lake are a unique expression of diversity due to the presumed constraints on the evolution of generalists and contrast with the development of multiple specialists, the standard response to intraspecific divergence.

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“…Specifically δ 15 N values increase 2–4‰ per trophic level in marine mammals . For marine mammal species, the majority of analyses over recent years have focused on single‐species niche breadth, comparative diet studies, and mixing models to determine diet composition . Unlike results of samples from stomach lavage or feces, stable isotope analysis of consumer tissues integrates diet over a longer time period than a single meal or day, and samples are more accessible from free‐ranging cetaceans .…”

Section: Introductionmentioning

confidence: 99%

Evidence for dietary time series in layers of cetacean skin using stable carbon and nitrogen isotope ratios

Wild

Chenoweth

Mueter

et al. 2018

Rapid Comm Mass Spectrometry

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Stable isotope mixing models elucidate sex and size effects on the diet of a generalist marine predator

Cited by 21 publications

References 63 publications

Large‐scale molecular diet analysis in a generalist marine mammal reveals male preference for prey of conservation concern

Large‐scale molecular diet analysis in a generalist marine mammal reveals male preference for prey of conservation concern

Multiple generalist morphs of Lake Trout: Avoiding constraints on the evolution of intraspecific divergence?

Evidence for dietary time series in layers of cetacean skin using stable carbon and nitrogen isotope ratios

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