Cod disappearance has rippling effect
Q & A:
Kenneth Frank, Bedford Institute of Oceanography
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Interview by Lori Valigra
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?
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.
© 2005 The Gulf of Maine Times
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.
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.
Kenneth Frank can be contacted at:
frankk@mar.dfo-mpo.gc.ca