At the American Association for the Ad vancement of Science conference held last February in Washington DC, researchers pre sented data suggesting that selective fishing over decades has caused some Atlantic female fish to evolve in ways that could potentially threaten an entire stock\u2019s survival.<\/p>\n
In a session on evolutionary fisheries science, Miiko Heino of the Institute of Marine Re search in Norway reported that studies on northern Atlantic cod indicate that fishing has resulted in smaller body sizes of fish catch, with smaller females producing fewer eggs of lower quality and also having a shorter spawning period. As a result, \u201cThere is a disproportionate loss of reproductive capacity,\u201d he said.<\/p>\n
According to Jeff Hutchings of Canada\u2019s Dalhousie University, 75 percent of the world\u2019s fisheries are fully fished or over-fished and 232 marine fish populations have suffered sharp rates of decline. Off the coast of Nova Scotia in particular, the cod population has declined 92 percent since the 1960s, and in the southern bank, it\u2019s down 98 percent. Hutching says that in these areas, there has also been a 21 percent decline in age at 50 percent maturity (the age when half of the population can reproduce) and a 13 percent decline in length at 50 percent maturity in 20 and 21 populations, respectively.<\/p>\n
Similar decreases in the age and size at maturation of Northeast Arctic cod have oc curred off Georges Bank and in the Gulf of Maine, according to Ulf Dieckman of the International Institute for Applied Systems Analysis in Austria, who believes these changes are a sign of fisheries-induced evolution.<\/p>\n
\u201cEvolutionary change is ubiquitous in com mercially exploited stocks\u2026The age of matu ration drops in 40 years of exploitation and modeling shows it will take 250 years to recover. The change could be irreversible,\u201d he said.<\/p>\n
But some scientists are skeptical of whether these changes are evidence of evolution or simply a sign of the plasticity of fish stocks, which can exhibit physical changes in response to environmental pressures. So about six years ago, David Conover of the State University of New York at Stony Brook began experiment ing on the tiny Atlantic silverside (Menidia menidia) in search of evolutionary effects of selective fishing.<\/p>\n
In his presentation at the AAAS, Conover acknowledged that the fish were not as large, long-lived or com mercially exploited as cod or tuna, but chose the fish anyway because it shared enough of the same char acteristics as other commercially harvested species, with one signifi cant difference: it has an annual life cycle that allows him to assess gen erational changes.<\/p>\n
Conover collected six popula tions of 1,000 silverside each, and for two of those populations, he harvested all the fish above a mini mum size. For another two, he harvested randomly, and for the last two, he harvested the smaller fishes.<\/p>\n
He found that the size of the fish evolved rapidly by the fourth generation of harvesting. The populations where the larger fish were harvested evolved to be smaller fishes, the ones where smaller fishes were harvested evolved to be larger, and fish in the randomly harvested populations stayed roughly the same size, he found.<\/p>\n
The tanks where the smallest fish were harvested had the largest biomass (number of fish times their average weight), and after a few generations had much larger eggs, which led to an increase in larvae size and greater larvae survival, he said. He noted that these fish seemed more willing to forage despite the predation risk.<\/p>\n
After generation five, Conover began har vesting all populations randomly to test the reversibility of evolution. That experiment is ongoing, but evidence so far suggests that the fish will eventually return to their original condition.<\/p>\n
What do Conover\u2019s results mean for fishery management? Conover suggested that fisher men be more selective in the fish they take, or that increasing areas be set aside as no-take marine protected areas.<\/p>\n
In Hawai\u2018i, John Sibert of the University of Hawai\u2018i\u2019s Pelagic Fisheies Research Program is also concerned with the effects of fishing on local fish stocks. At the March Western Pacific Fishery Management Council meeting, he pre sented data that suggest the average weight and length of bigeye tuna has decreased over the decades.<\/p>\n
However, Sibert told Environment Hawai\u2018i, the decrease has nothing to do with evolution ary change, \u201cwhich takes place on veerrrryyy loooonnnnggg time scales. Tanks full of gup pies have little relevance to tunas in the ocean.\u201d<\/p>\n
Sibert says his data suggest rather that large fish have been removed from the population and that the fishery is hauling them in at a rate faster than they can grow back.<\/p>\n
\u201cIt is a common observed short-term conse quence of fishing,\u201d he wrote in an email to Environment Hawai\u2018i, adding that in Alaska, the size spectrum of fish has recovered with careful fishery regulation.<\/p>\n
Regarding the observed decreases in the age at maturity of Atlantic fishes, Sibert notes that such changes may be found among human females as well, but aren\u2019t necessarily attributed to natural selection.<\/p>\n
\u201cOf course, the situation in fish is a bit more difficult to understand, but it is not likely an evolutionary change,\u201d he said, while admitting he was not familiar with the literature on the Atlanic cod.<\/p>\n
To determine whether evolutionary changes are occurring in Pacific fisheries or anywhere else, Sibert wrote, \u201cI suspect you would have to approach the question on a species by species basis and look at the size-selectivity of the fishing gear, change in size spectrum with time, sex ratio of catch … It turns out that for yellowfin tuna, the sex ratio changes at about 110-120 cm so that the proportion of females in the population decreases sharply. Thus the large fish removed from the population are mostly males.\u201d<\/p>\n
— Teresa Dawson<\/p>\n
Volume 15, Number 10 April 2005<\/p>\n","protected":false},"excerpt":{"rendered":"
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