Co-organizer of this year’s AAAS Evolutionary Fisheries Science Symposium Mikko Heino from the Institute of Marine Research in Norway has been working on fisheries-induced evolution in the wild for a number of years. In total, he has now studied 12 populations of fish, including Atlantic cod (Gadus morhua), Atlantic herring (Clupea harengus), North Sea plaice (Pleuronectes platessa), American plaice (Hippoglossoides platessoides), and grayling (Thymallus thymallus). In all but one case, he finds that evolution plays an important role in trends toward smaller-sized fish. Fisheries-induced evolution, he says, can reduce productivity and reproductive potential. This means that current fishing practices are reducing the productivity of fisheries and may compromise the value of fish stocks as renewable resources.
Many life-history traits may be affected. Symposium co-organizer Ulf Dieckmann, a physicist at the International Institute for Applied Systems Analysis in Austria, says that, besides age and size at maturity, current harvesting also threatens reproductive effort, growth rate, and behavior in other species. The age of sexual maturation in several populations of cod has been reduced by one-fourth and for plaice by nearly one-third. Heino believes such examples are probably just the tip of the iceberg.
What is most worrisome about these evolutionary changes is that all the evidence thus far suggests that they can happen very quickly in fish populations—this is not the stuff of geologic time. Many studies suggest that the selection pressures are large enough for substantial change to occur within decades—even though the heritability of the traits is not large.
Jeffrey Hutchings of Dalhousie University in Canada also spelled out to the meeting the possible consequences of this kind of evolutionary change. By the early 1990s, the numbers of Atlantic cod had declined by 99.9 percent relative to their abundance in the early 1960s (4). New work suggests that evolutionary changes may have been a factor in this decline because, prior to their collapse, cod were rapidly shifting toward younger and smaller sizes. One study shows that, between 1980 and 1987, the age at which 50 percent of the females were mature had dropped from six to five years.
What the researchers are getting at is the idea that, when a population collapses due to overfishing, it is not simply the numbers of fish taken but also the type of fish taken that might be important. And just as evolutionary changes may be relevant to the collapse of populations, they may also be significant factors in the speed with which a population can recover. Hutchings says that, based on data from Newfoundland’s and Labrador’s northern cod, models suggest that “comparatively modest” reductions in age and size at maturity can slow recovery by 25-30 percent and more than double the likelihood of further population decline.
Although the evidence thus far suggests that complete cessation of fishing is likely to halt these worrying trends in fisheries, it is by no means clear whether this would allow them to recover to their historic sizes (5). This is because there is no equivalent-but-opposite, size-selective pressure forcing a population in the opposite direction. As a result, a population will regain its previous character only very slowly—if at all. Conover says there is now evidence to suggest that, when fish size is truncated by harvesting, the size structure of the population may take a long time to re-establish itself.
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So now scientists such as Conover are talking about the need for a new Darwinian fisheries science—a way of managing fisheries over time periods longer than the next few years. One management option is marine reserves. We know that marine reserves provide a source of eggs and fish free from evolutionary pressures of size-selective fishing. The trouble is, their effectiveness would depend on there being little gene flow back from areas in which size selection is still taking place. In other words, if fish inside and outside the reserve can mate, the reserve’s gene pool may still show a trend toward smaller sizes because it is being inundated with genes from smaller fish from outside.
There is no quick and easy way to integrate the complexity of fish population dynamics into management. But all scientists seem to agree on the need to preserve large, old fish and maintain the balance of age classes in the population. From a conservation perspective, there may be most traction to be gained by focusing on protecting the largest fish, which play both an evolutionary and an ecological role.
Not only is removing the largest fish harming the genetic status of a fishery, but the largest fish are disproportionately important in sustaining a population because older fish produce exponentially more larvae. A 50-cm-long Boccacio rockfish (Sebastes paucispinis), for example, will produce nearly 200,000 larvae, whereas one only 30 cm longer will produce ten times this number—nearly 2 million larvae.
Steven Berkeley of University of California Santa Cruz reported at this year’s AAAS meeting that the larvae of old fish have hugely improved chances of survival compared to larvae of younger fish. His team found that in Pacific black rockfish (Sebastes melanops), survival rates were nearly three times higher and growth rates were 3.5 times faster for larvae from older mothers. This is partly because older mothers produce larvae with larger oil globules.
The hope is that these developments may force a full about-face in fisheries-management strategies. Could it be time to rethink the minimum-size restriction as a basic management tool and instead to think of maximum-size restrictions? This could involve completely new net designs—ones that allow the very biggest fish to escape. Managing fisheries in such a way that large fish are unlikely to be caught might fulfill the short-term need of increasing the biomass of spawning stock and in the long term place a selective pressure for faster growth in the size range that is being harvested. More work, as scientists frequently say, needs to be done. However, some of these ideas are already encompassed in U.S. and Canadian regulations that protect egg-bearing female lobsters, very large lobsters, and lobsters marked as known breeding females.
Given the accumulating evidence of the obvious, it is difficult to be optimistic. Each harvest introduces tiny, hard-to-reverse genetic changes into a population. By ignoring these changes, we run the risk of reducing productivity in ways that will not be easily reversed in the future. It is a debt we are running up, a Darwinian debt that we owe to future generations of fishermen.
For decades it has been a struggle merely to bring the massive overexploitation of fisheries under control. Today’s struggle involves trying to rebuild fish stocks, and that is difficult enough to do. Changing to maximum-size restrictions would involve something of a sea change in attitudes in fisheries management. And yet this alone is probably not enough. Scientists such as Andy Rosenberg of the University of New Hampshire, a member of the U.S. Commission on Ocean Policy, believe it is necessary to work on multiple fronts if we are to recover the huge losses induced by the overexploitation of the past. We need to protect fish of all ages, we must safeguard genetic diversity, and we must conserve functioning marine ecosystems.
Whatever the science reveals, action is hard to achieve in a world of competing priorities. How on earth does one manage stocks for the day after tomorrow, when managing them for tomorrow is difficult enough? Politicians, who are ultimately faced with making the decisions about how we manage our resources, are caught between good long-term management and harsh short-term realities for their constituents. With politicians, as with fish, it is survival of the fittest. It is a shame that fish cannot vote.
Literature Cited
1. Palumbi, S.R. 2001. Humans as the world’s greatest evolutionary force. Science 293:1786-1790.
2. Conover, D.O. and S.B. Munch. 2002. Sustaining fisheries yields over evolutionary time scales. Science 297:94-96.
3. Law, R. 2002. Selective fishing and phenotypic evolution: Past, present and future. ICES Annual Science Conference 2002.
4. Hutchings, J.A. 2004. The cod that got away. Nature 428:899-900.
5. Olsen, E.M. et al. 2004. Maturation trends indicative of rapid evolution preceded the collapse of northern cod. Nature 428:932-935.
About the Author
Natasha Loder is a science and technology correspondent for The Economist and is based in London.
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