Risk of extinction of a unique skate population due to predation by a recovering marine mammal

Abstract

Benefitting from reduced harvesting and an end to culling, many marine mammals are now recovering from past overexploitation. These recoveries represent important conservation successes but present a serious conservation problem when the recovering mammals are predators of species of conservation concern. Here, we examine the role of predation by recovering grey seals (Halichoerus grypus) in the near‐extinction of a unique skate population in the southern Gulf of St. Lawrence (sGSL) in Atlantic Canada. Winter skate (Leucoraja ocellata) in the sGSL are distinct from winter skate elsewhere and may represent an endemic species. Their adult abundance has declined by 98% since 1980, and these skates are now detectable in only a small fraction of their former range. Population modeling indicates that the ongoing collapse of this population is due to increases in the natural mortality of adults. Based on model projections, this population would be extinct by mid‐century if its current rate of productivity were to persist. A second population model incorporated predation by grey seals. Model estimates of skate consumption by seals were consistent with historical and recent estimates of the contribution of skates to grey seal diets. The estimated consumption accounted for the increases in the natural mortality of adult skates. A Type III functional response for grey seals preying on winter skate emerged from the model results. This indicates that, when skate abundance is very low, grey seals are expected to switch to alternate prey, resulting in declines in the mortality of skates due to predation. Consequently, contrary to projections at current productivity, winter skate are expected to be trapped at very low abundance in a “predator pit” instead of declining to extinction. Nonetheless, extinction risk would remain very high at the very small population size in the predator pit. Our results emphasize the need for an ecosystem‐based approach to the management of living resources in this ecosystem.

Something strange in the neighborhood: Diverging signals in stock assessment data for Northeast U.S. fish stocks

“…To reproduce the patterns observed here, increases in M would have started in the mid‐1990s for most stocks when relative F started declining. This timing is coincident with the predator‐induced increases in M in neighboring Canadian ecosystems (e.g., Swain & Benoît, 2015; Swain et al, 2019b), and with the increases observed in the study region (Figure 6c), adding support to the notion of increased M via predation. It is also worth noting that the two stocks with the highest correlations between relative F and Z are species primarily found in the Mid‐Atlantic region where grey seals and harbor seals ( Phoca vitulina vitulina ) occur only seasonally, and at lower abundances than in New England waters (Hayes et al, 2018).…”

“…In nearby Canadian waters, there is compelling evidence that large increases in marine mammal abundance, most notably grey seals ( Halichoerus grypus atlantica ), resulted in large increases in natural mortality for a number of demersal fish stocks (Benoît et al, 2011; Chouinard et al, 2005; Neuenhoff et al, 2019; O’Boyle & Sinclair, 2012; Swain & Benoît, 2015; Swain et al, 2019b). Many of these stocks were severely depleted by overfishing, and increased consumption by grey seals or other predators could be resulting in a “predator pit,” preventing the recovery of these stocks despite management efforts (e.g., Swain & Benoît, 2015; Neuenhoff et al, 2019; Swain et al, 2019b; see Swain et al, 2011 for a detailed exploration into multiple hypotheses regarding the sustained high Z for cod in the southern Gulf of St Lawrence). In the mid‐1980s, grey seals from Canada began colonizing a few isolated locations in our study region (Hayes et al, 2018; Moxley et al, 2017).…”

Transient dynamics during kelp forest recovery from fishing across multiple trophic levels

“…However, recovering top predators, particularly marine mammals, appear to strongly affect their prey (Swain et al. 2019), with pinnipeds, for example, potentially removing enough fish biomass to cause conflicts with fisheries (Chasco et al. 2017).…”

“…Predator-first, prey-first, and synchronous fishery closures are approximately equally likely to be implemented for a given multi-trophic level fisheries management situation, though predator-first management is typically least observed historically (Samhouri et al 2017). However, recovering top predators, particularly marine mammals, appear to strongly affect their prey (Swain et al 2019), with pinnipeds, for example, potentially removing enough fish biomass to cause conflicts with fisheries (Chasco et al 2017). Managers have the difficult task of balancing competing interests when attempting to grow populations of interacting species, suggesting that an ecosystem-based management approach that explicitly accounts for biotic relationships could help to optimize ecosystem services in a multispecies context (Kellner et al 2011).…”

Distributions of threatened skates and commercial fisheries inform conservation hotspots

“…Al though harvest moratoria for groundfish on the eastern Scotian Shelf were introduced in 1993 (Bundy 2005), thorny skate and winter skate abundance remains low on Banquereau Bank. Several causes for a lack of recovery of skates on the eastern Scotian Shelf have been investigated, including increased predation by a recovering population of grey seals (Swain et al 2019). However, the ongoing surf clam dredge fishery reduces the forage base and greatly alters the affected seabed (Gilkinson et al 2003).…”