Overfishing in the late 1980s greatly reduced the abundance of large piscivorous fish, which have not recovered 20 years after the cessation of heavy fishing in the four ecosystems studied. This decline has left marine mammals such as seals as top predators of many species during the mid- 1990s and had profound effects over all trophic levels (top-down effects) in Newfoundland-Labrador, the northern Gulf and the southern Gulf of St Lawrence. This, coupled with the re-opening of fisheries before stocks had recovered, may explain why cod biomass is still at extremely low levels in these ecosystems.

The facts:

- four Northwest Atlantic ecosystems
- three cod stock collapses 15 years ago (plus one severely depleted)
- seals now top predator in all ecosystems
- all had cod as a top predator before collapse
- groundfish declines in all areas
- forage base increased in most systems
- No recovery in any system.

Have these ecosystems fundamentally changed? Why?

The challenge: compare and contrast these four ecosystems.

The answer: using mass balance models, empirical data and a suite of ecosystem indicators, we explore
how and why these systems have changed over time. At the ecosystem and community level, we see
broad similarities between ecosystems. However, structurally and functionally these systems have
shifted to an alternate state, with changes in predator structure, trophic structure and flow.

In detail:

On the eastern Scotian Shelf, top-down predation by seals does not appear to be a significant energy flow or cause of mortality of cod, nor has there been a fishery since 1993. However, the high abundance of forage fish may be out-competing small cod for food (small zooplankton), and larval cod may be consumed by forage fish. This is a variant of the cultivation-depensation hypothesis suggested by Bundy and Fanning (2005), where cod are caught in a trophic vise: with the exponential increase of grey seals, and the large reduction of cod due to fishing, cod were squeezed, and as the small pelagics increased, competition from small pelagics with young cod causing the loss of the cultivation effect.

There is no evidence for this effect in Newfoundland-Labrador or the northern Gulf since the forage fish biomass did not increase, although there is scope for further investigation in the southern Gulf. All systems show evidence of a potential trophic cascade, a result of the removal of the top fish predators by fishing. Thus, the changes in top-predator abundance driven by human exploitation of selected species resulted in a major perturbation of the structure and functioning of the four Northwest Atlantic ecosystems. Each represents a case of fishery-induced regime shift, to alternate states that may not be reversible in the short term.

The research was funded as part of the CDEENA project: www.osl.gc.ca/cdeena/en/programme.shtml

Bundy A, Heymans JJ, Morisette L, Savenkoff C. 2009. Seals, cod and forage fish: A comparative exploration of variations in the theme of stock collapse and ecosystem change in Northwest Atlantic ecosystems. Progress in Oceanography, 88: 188-206 doi:10.1016/j.pocean.2009.04.010

Contact Dr Sheila Heymans for more information.