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Catching Big Fish Gives Evolutionary Edge To Small Fry

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    NHNE News List Current Members: 677 Subscribe/unsubscribe/archive info at the bottom of this message. ... CATCHING ONLY BIG FISH LEADS TO SMALL FRY By Jeff
    Message 1 of 1 , Jul 6 12:19 AM
      NHNE News List
      Current Members: 677
      Subscribe/unsubscribe/archive info at the bottom of this message.


      By Jeff Hecht
      New Scientist
      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

      Evolutionary impact

      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.

      Genetic diversity

      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
      fish caught.


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