The collapse of cod stocks in the northwest Atlantic during the 1980s and 1990s has had a major impact on the entire ecosystem of the Scotian Shelf, off Nova Scotia in Canada.
The results of a new study, which analysed abundances of fish, seals, invertebrates and plankton from 1960 to the present day, show that removal of top predators such as cod has had knock-on effects through the whole marine food chain. The authors of the study, published in the journal Science on 10 June, believe that this pattern could be common to all Atlantic fisheries north of 44oN latitude.
Marine ecosystems in the northwest Atlantic were dominated by predatory benthic (bottom-dwelling) fish species – especially cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and pollock (Pollachius virens) – until their populations collapsed due to overfishing in the late 1980s and early 1990s. It’s well-known that removal of a top predator can have repercussions further down the food chain, although this ‘trophic cascade’ hasn’t been demonstrated in complex marine ecosystems before.
The new study shows that numbers of small pelagic fish and benthic macroinvertebrates – especially northern snow crab (Chionoecetes opilio) and northern shrimp (Pandalus borealis) – have increased following removal of larger predatory fish. They, in turn, form the main food source for grey seals (Halichoerus grypus), which may partly explain why seal populations are booming in the North Atlantic. Further down the food chain, the tiny organisms making up the zooplankton and phytoplankton have also been affected. With an increase in the number of their predators, zooplankton abundance has declined. And with fewer zooplankton around to munch them, phytoplankton numbers have increased.
Although these changes in the ecosystem are supporting a crab and shrimp fishery more valuable than the benthic fish fishery it replaced, it’s not known what the long-term repercussions may be, or whether the situation can be reversed. Stocks of cod and other benthic fish have failed to recover, despite fishing restrictions introduced in the late 1980s and early 1990s. And it may be that the pattern is repeated in similar Atlantic ecosystems, north of 44oN latitude, where cod stocks also collapsed in the early 1990s.