Catching Big Fish Gives Evolutionary Edge To Small Fry
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CATCHING ONLY BIG FISH LEADS TO SMALL FRY
By Jeff Hecht
July 4, 2002
Catching only big fish can make them evolve into small fish in just four
generations, scientists have discovered. The surprising result casts doubt
on the fishery practice of letting smaller fish go in the hope that they
will then be able to grow to maturity and sustain fishery productivity.
In a new experiment, David Conover of the State University of New York at
Stony Brook shows that when fishing kills the big fish, evolution selects
genes for slower growth.
Historical records confirm old fishermen's tales that the fish were bigger
in the past -- the average sizes of intensely fished species such as cod
have dropped significantly. Fishery managers have accepted this shrinkage
because they believed the faster growth of younger fish leads to higher
total production. But Conover warns that the selective pressure of
minimum-size rules can in fact "lead to the evolution of lower
Ralph Halliday, at the Bedford Institute of Oceanography in Nova Scotia,
says the research is a very interesting contribution to the debate but adds:
"The key question is whether these are permanent changes or whether if you
stop taking the largest, the population will go back to its previous
Conover and Stephan Munch looked for an evolutionary impact in laboratory
tanks using a small ocean fish common along the American coast called
Menidia menidia, or the Atlantic silverside. At 190 days after
fertilisation, they removed either the largest 90 per cent, the smallest 90
per cent, or a random selection of the fish in each tank. Then they
stimulated the remaining fish to spawn, and raised a new generation and
repeated the cycle.
After four generations of removing the largest fish, the average mass of
fish in the 10 per cent left to spawn dropped to 1.05 grams, compared to
3.17 grams for random selection. After four generations or removing the
smallest fish, the survivors averaged 6.47 grams.
Total productivity, measured as the mass of removed fish, also was much
higher after four generations of removing the smaller fish. All the fish
were the same age, so Conover attributes the change to selection of genes
for slower growth.
Slowing this genetic drift would require changes in fishery management.
Zones where fishing is banned would have to cover "an important fraction of
the range of a species" to preserve genetic diversity, Conover says.
It is also possible to limit the maximum size of catch but only "where each
individual fish is handled by a fisherman", he says. The state of Maine
already does that for lobsters, because the largest females produce the most
eggs. But it is hard to design gill or trawling nets to allow large fish to
escape while capturing small ones.
"We're not in a position to make specific recommendations. They have to be
tailored to the species and fishery," Conover told New Scientist.
Halliday notes that "adjustment of the size at first capture is a
refinement, not an alternative" to simply controlling of total amount of
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