Fish Moved by Warming Waters
13 May 2005
Climate change has fish populations on the move. In Europe’s intensively fished North Sea, the warming waters over the past quarter-century have driven fish populations northward and deeper, according to a study by conservation ecologist John D. Reynolds of the University of East Anglia in Norwich, U.K., and his colleagues.
Such warming could hamper the revival of overfished species and disrupt ecosystems, they assert. The warming is expected to continue in the North Sea, and although fish species living to the south will likely move north and replace departing ones, the forecast for the region’s fisheries will depend on whether the species that succeed are marketable.
“This is another clear indication that warming is playing a role” in ocean ecosystems, says physical oceanographer Ken Drinkwater of the Institute of Marine Research in Bergen, Norway. Although there have been many studies looking at the effects of climate change on marine species, “no one has looked in detail at changes in distributions of commercial and noncommercial species,” says fish biologist Paul Hart of the University of Leicester in the United Kingdom. Similar climate-induced shifts in fish populations, he adds, might happen in other temperate seas, including those around Europe and much of the United States.
The study, published online this week by Science (www.sciencemag.org/cgi/content/abstract/1111322), used extensive records of fishing catches made by research vessels between 1977 and 2001, a period during which the North Sea’s waters warmed by 1°C at the sea floor. Reynolds’s team cast a wide net, compiling data on the sea’s 36 most common bottom-dwelling fish. They found that two-thirds of the populations moved toward cooler waters–either going north or to deeper waters, or both. “We saw shifts in both commercial and noncommercial species, and across a broad set of species,” says conservation ecologist Allison Perry of the University of East Anglia. The fish species whose distribution have shifted tend to be smaller and mature earlier, she and her colleagues noted.
“Those fish that didn’t shift raise interesting questions,” adds Perry. Such species might be more closely tied to particular habitats or might not spread as quickly because of longer generation times. Because species are redistributing at different rates or not at all, the shifts could rend ties within ecosystems. Species are often adapted to each other and have developed mechanisms for avoiding certain predators or catching specific prey, Hart says: “If suddenly faced with new predators or prey, this could change the balance.” Moreover, if the timing of development shifts differently among various species, “this could affect the match or mismatch between the fishes’ food and predators,” Drinkwater says.
Heavy commercial fishing has already pushed some species in the North Sea to the edge of extinction, and some researchers worry that the changing climate will exacerbate those problems. “Fishing is undoubtedly the most important factor for all the commercial species,” says fisheries biologist Niels Daan of the Netherlands Institute for Fisheries Research in IJmuiden. “But it is possible the warming could prevent the recovery of stocks.”