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

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
modelling 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.