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’s survival.
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, “There is a disproportionate loss of reproductive capacity,” he said.
According to Jeff Hutchings of Canada’s Dalhousie University, 75 percent of the world’s 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’s 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.
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.
“Evolutionary change is ubiquitous in com mercially exploited stocks…The 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,” he said.
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.
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.
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.
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.
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.
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.
What do Conover’s 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.
In Hawai‘i, John Sibert of the University of Hawai‘i’s 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.
However, Sibert told Environment Hawai‘i, the decrease has nothing to do with evolution ary change, “which takes place on veerrrryyy loooonnnnggg time scales. Tanks full of gup pies have little relevance to tunas in the ocean.”
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.
“It is a common observed short-term conse quence of fishing,” he wrote in an email to Environment Hawai‘i, adding that in Alaska, the size spectrum of fish has recovered with careful fishery regulation.
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’t necessarily attributed to natural selection.
“Of course, the situation in fish is a bit more difficult to understand, but it is not likely an evolutionary change,” he said, while admitting he was not familiar with the literature on the Atlanic cod.
To determine whether evolutionary changes are occurring in Pacific fisheries or anywhere else, Sibert wrote, “I 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.”
— Teresa Dawson
Volume 15, Number 10 April 2005
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