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

Abstract

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.

Diet composition and biomass consumption of harbour seals in Telemark and Aust-Agder, Norwegian Skagerrak

“…They vary their diets according to season and geographical location, seemingly to prefer the regionally and seasonally most abundant of both benthic and pelagic species (e.g. Bromaghin et al 2012;Lance et al 2012;Geiger et al 2013;Luxa and Acevedo-Gutiérrez 2013;Schwarz et al 2018). In some areas, seals seem to favour a small number of prey species but also supplement their diet with other species (Lance et al 2012).…”

Determining diet from faeces: Selection of metabarcoding primers for the insectivore Pyrenean desman (Galemys pyrenaicus)

“…For instance, a cost-effective screening of multiple DNA metabarcodes in faeces detected a broad diversity of plants (99 taxa) in the diet of the vulnerable Italian hare ( Lepus corsicanus ), including items that leave no solid remains or that lack diagnostic taxonomic features [12]. Similarly, Schwarz et al [13] documented dietary differences consistent across site and year in the diet of male and female harbor seals ( Phoca vitulina ), likely affecting on commercial prey such as salmon. Moreover, the foraging ecology of the Alpine long-eared bat Plecotus macrobullaris was inferred from the molecular analysis of faeces and from the ecological requirements of prey, which would be impossible by traditional radio-tracking methods [14].…”

“…False positives and false negatives in dietary studies can have strong implications in the interpretation of predator ecology, as well as in ecosystem management [29]. Successful identification of relevant prey taxa is key to obtain sound conclusions about the ecological role, trophic specialization and conservation of any predator—or their consumed prey [13,30]—. Besides, the need for simultaneous identification of diet and predator identity from faeces makes methodological decisions more demanding, as a broader phylogenetic spectrum must be targeted.…”

Contemporary diets of walruses in Bristol Bay, Alaska suggest temporal variability in benthic community structure

“…Given the broadly diverse diet of walruses in Alaska, many different prey types would be needed in this type of experimental study, making it logistically challenging and somewhat problematic with the number of combinations of dietary proportions necessary to replicate that seen among wild walruses. Nevertheless, amplification of prey DNA in the scats of pinnipeds and other marine mammals has been shown to be a valuable tool for assessing relative variations in their diets (e.g., Deagle et al, 2005;Ford et al, 2016;Schwarz et al, 2018).…”