Cod stock collapse
Aug 13, 2009 7:15:22 GMT -1
Post by SEAJUNKY on Aug 13, 2009 7:15:22 GMT -1
www.fishnewseu.com/late…..antic.html
Cod stock collapse and ecosystem change in the Northwest Atlantic
Wednesday, 12 August 2009 10:49
A NEW study, involving Sheila Heymans from the Scottish Association for Marine Sciences may help to explain why cod fisheries in the Northwest Atlantic have failed to recover despite the cessation of heavy fishing pressure in the four ecosystems studied.
Overfishing in the late 1980s greatly reduced the abundance of large piscivorous fish, which 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 researchers used mass balance models, empirical data and a suite of ecosystem indicators, to explore how and why the systems changed over time.
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, 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 vice: 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…..amme.shtml
Cod stock collapse and ecosystem change in the Northwest Atlantic
Wednesday, 12 August 2009 10:49
A NEW study, involving Sheila Heymans from the Scottish Association for Marine Sciences may help to explain why cod fisheries in the Northwest Atlantic have failed to recover despite the cessation of heavy fishing pressure in the four ecosystems studied.
Overfishing in the late 1980s greatly reduced the abundance of large piscivorous fish, which 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 researchers used mass balance models, empirical data and a suite of ecosystem indicators, to explore how and why the systems changed over time.
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, 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 vice: 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…..amme.shtml
