Harbour seals target juvenile salmon of conservation concern

Abstract

Knowing the species and life stages of prey that predators consume is important for understanding the impacts that predation may have on prey populations, but traditional methods for determining diets often cannot provide sufficient detail. We combined data from two methods of scat analysis (DNA metabarcoding and morphological prey ID) to quantify the species and life stages of salmon (Oncorhynchus spp.) consumed by harbour seals (Phoca vitulina) in the Strait of Georgia, Canada, where juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon survival is poor. Harbour seals primarily consumed adult salmon of lesser conservation concern in the fall (August–November): chum (Oncorhynchus keta: 18.4%), pink (Oncorhynchus gorbuscha: 12.6%), sockeye (Oncorhynchus nerka: 7.4%), Chinook (7.1%), and coho (1.8%). However, the opposite species trend occurred during the spring when seals preferred juvenile salmon of greater conservation concern (April–July): coho (2.9%), Chinook (2.9%), sockeye (2.5%), pink (1.4%), and chum (0.8%) — percentages that can equate to many individuals consumed. Our data suggest that harbour seals select juveniles of salmon species that out-migrate at ages >1 year and provide evidence of a potential causal relationship between harbour seal predation and juvenile salmon survival trends.

Ecological implications of changing hatchery practices for Chinook salmon in the Salish Sea

“…Size overlap of hatchery (gray) and natural (black) juvenile Chinook salmon with hypothesized predation windows for avian (blue), marine mammal (red), and fish (green) predators, based on published studies. The central tendency measures associated with Thomas et al (2017) are based on estimated lengths during the months where their respective proportions in the harbor seal diet were the highest. The length distributions for hatchery Chinook are based on Regional Mark Information System data for releases between 2010 and 2017.…”

“…8). Harbor seals do appear to target juvenile Chinook salmon, but typically in mid-summer (Lance et al 2012, Thomas et al 2017, when fish are larger (average size of >100 mm). Seals tagged with GPS devices exhibited increased feeding behavior in the estuary shortly after releases of coho salmon (mean release size: 130 mm) but did not respond to far more abundant, smaller Chinook (release size: 80 mm) released only a few weeks later.…”

“…This is also the case with fishes that prey on juvenile salmon (Beamish et al 1992, Emmett andKrutzikowsky 2008), including larger individuals of the same species (Beauchamp and Duffy 2011). Additionally, research describing harbor seal (Phoca vitulina) predation on juvenile fish provides further evidence of size-selective predation (Tollit et al 1997, Allegue 2017, Thomas et al 2017. We evaluated the potential implications of current hatchery practices for predation on juvenile Chinook, given the existence of a predation window implied by existing studies of these known ❖ www.esajournals.org predators.…”

“…Additionally, research describing harbor seal (Phoca vitulina) predation on juvenile fish provides further evidence of size-selective predation (Tollit et al 1997, Allegue 2017, Thomas et al 2017. Measures of central tendency were summarized for preferred prey size in harbor seals (Tollit et al 1997, Lance et al 2012, Thomas et al 2017, fish predators (Duffy and Beauchamp 2008, Emmett and Krutzikowsky 2008, Beauchamp and Duffy 2011, and avian predators (Collis et al 2002, Schrimpf et al 2012, Tucker et al 2016. Measures of central tendency were summarized for preferred prey size in harbor seals (Tollit et al 1997, Lance et al 2012, Thomas et al 2017, fish predators (Duffy and Beauchamp 2008, Emmett and Krutzikowsky 2008, Beauchamp and Duffy 2011, and avian predators (Collis et al 2002, Schrimpf et al 2012, Tucker et al 2016.…”

Combining hard-part and DNA analyses of scats with biologging and stable isotopes can reveal different diet compositions and feeding strategies within a fur seal population

“…It is non-invasive, has the advantages of being quick and relatively inexpensive, and is more sensitive than hard-part remains methods (Deagle et al 2009). For example, DNA analyses detected prey to a finer taxonomic level than hard-parts analyses in Steller sea lions Eumetopia jubatus and harbour seals Phoca vitulina (Tollit et al 2009, Thomas et al 2017. The quantitative capabilities of DNA metabarcoding (i.e.…”