Cod disappearance has rippling effect
Q & A:
Kenneth Frank, Bedford Institute of Oceanography
Interview by Lori Valigra
KENNETH FRANK'S seminal work on the disappearance of cod and the rippling effects or “trophic cascades” caused by removing the top predators in a community, began in a seemingly unrelated place: a pond in Ohio, where he studied the impact of small fish on their food source as part of his Ph.D. research at the University of Toledo. But throughout his career he has always been interested in food chain effects.
Now a research scientist at the Department of Fisheries and Oceans, Bedford Institute of Oceanography, in Dartmouth, Nova Scotia, where he has worked since 1983, Frank has broadened his sights. In a paper published June 10 in the journal Science, Frank and his colleagues provided some of the first conclusive evidence that trophic cascades, or the changes in position an organism occupies in a food chain, exist not only on land and in freshwater systems, but also in the open ocean.
The scientists examined various levels of nutrients and discovered that the virtual elimination of cod in the early 1990s in the eastern Scotian Shelf area of Nova Scotia restructured the food web, causing population explosions of previous prey fish like herring and shrimp.
In an interview with the Gulf of Maine Times, Frank talked about those research findings and how that knowledge might relate to his recent studies of the western Scotian Shelf in the Gulf of Maine.
Q: How did you become involved in your current work?
A: About three years ago I was given the challenge to begin to study the relationship between changes in the abundance of different species in the food chain. So rather than just looking at fish, I was looking at plankton and nutrients to develop an assessment of a particular geographic area's ecosystem. The first area we studied in detail was the eastern Scotian Shelf, an area a couple of times larger than Georges Bank that is upstream from the Gulf of Maine.
Q: Why did the cod disappear? A: Overfishing played a major role. Some environmental changes also took place, such as very cold water temperatures in the region. It looked like the growth rates of cod were very low due to the temperature effects. One source of the cold water was a greater outflow of the Labrador Current from the mid 1980s to the mid 1990s.
Q: What are you finding in the Gulf of Maine?
A: About 10 months ago we started studying the western Scotian Shelf, which includes Browns Bank and the eastern half of the Bay of Fundy in the Gulf of Maine. We should have some results before Christmas. We're seeing a lot of similarities between the eastern and western Scotian Shelf. It looks like there are strong predation effects that are causing the species at one trophic level, or position that an organism occupies in a food chain, to decrease and those at the trophic level below to increase.
But what is a little different in the western Scotian Shelf is that some fish species at the higher levels are filling in for the depleted groundfish. So when cod, haddock and pollock and other groundfish have been heavily fished, the dogfish shark is filling the empty space, which seems to be keeping the next trophic level in check. We're not seeing the explosion of herring and other forage fish as we are on the eastern Scotian Shelf because the dogfish in the Bay of Fundy feed on herring and other forage species.
Q: Are there other differences between the two areas?
A. The eastern Scotian Shelf isn't very strongly tidally mixed. The little mixing that does exist seems to be causing a limitation of nutrients. In a more tidally mixed area like the Bay of Fundy or Georges Bank you don't normally get nutrient limitation because the bottom and surface waters are always being mixed. So the food chain may be working a little differently. There may be a bit more of a bottom-up effect in which the nutrients may be giving rise to an increase in phytoplankton, which are causing zooplankton to increase. So everything goes up and down simultaneously in abundance. With the top-down effect, which may be more common in areas that are not strongly tidally mixed, when one level goes up, the next level goes down in a seesaw manner among adjacent levels in the food chain.
Q: Why haven't cod reestablished themselves?
A: One hypothesis is that former cod prey like herring, mackerel and sandlance are now so abundant that they're eating cod eggs and larvae. The other possibility is that there may be so few cod that it is difficult to find a mate. Cod and many commercially important species are accustomed to living in large groups, so when you make them artificially rare through fishing you upset their whole social structure.
Q: Is it necessarily bad when a dominant predator is removed from the food chain?
A: In the eastern Scotian Shelf there's a lot of mixed opinion about whether people want to see cod return. The landed value of the shrimp and snow crab fisheries is much greater than it ever was when fishing only groundfish.
Q: Isn't there a risk of those new species becoming overfished?
A: Yes. The point is where do you go next if you don't learn the lesson from the overexploitation of cod and do the same for the crustaceans.
Q: Are there efforts to protect or reestablish cod?
There's a lot of discussion about marine protected areas or MPAs. We have a stock of cod on the south coast of Newfoundland that did collapse, but has managed to recover to some extent and is now fairly good sized. It was never depleted to the same extent as the surrounding stocks in other areas, so the recovery is exciting. Maybe some cod from this stock will spill over to some adjacent stocks.
Q: Where does your work go from here, especially regarding the Gulf of Maine?
A: We're testing our trophic cascade model in Browns Bank/eastern Bay of Fundy area. Maybe one thing we'll find is that if the system is bottom-up controlled and/or more tidally mixed it may be more resilient to the effects of fishing. That would be a wonderful thing to demonstrate making it easier to rehabilitate the depleted groundfish stocks on Georges Bank or the Gulf of Maine.
Kenneth Frank can be contacted at: frankk@mar.dfo-mpo.gc.ca© 2005 The Gulf of Maine Times