The Northwest Atlantic has become a notorious example of fishing down
marine food webs. It is also an uncontrolled experiment testing the
performances of marine populations under extremes of abundance. In this
talk I draw upon field experiments and long-term monitoring of two of the
region’s iconic commercial species - sea urchins and lobsters - to
illustrate how an altered food web is testing species performance under
demographic extremes. On one hand, depleted sea urchin populations have
failed to recover likely because they need large spawning aggregations to
reproduce, and the dense kelp beds that have resurged in the absence of
these grazers further inhibit larval recruitment. In contrast, the collapse
of top predatory fish, such as cod, has allowed lobsters to increase to
historic highs, such that crowding is evident from changes in habitat use,
shelter competition and heightened intra-specific predation. As conservation
measures are implemented in the region, fishery scientists
must seize upon these opportunities to understand population dynamics at
the poorly studied extremes of abundance.