Fishery practices that go for the big ones may be counterproductive when mostly the small survive
Every year, people haul 1 to 3 trillion fish from the ocean. There’s little doubt that this is too many — fish populations have collapsed worldwide. But there’s another thing we are doing to fish: By killing them in such large numbers, we are exerting enormous evolutionary selection pressure. The adaptive changes that result could increase the chance that fish species will survive, but may pose a problem for the people who depend on them.....MUCH MORE
By preferentially catching the kind of fish we like — usually big ones — we are inevitably creating a disadvantageous situation for such fish. They’re less likely to survive and reproduce, which reduces the presence of their genes in the next generation. We are, in effect, creating evolutionary pressure for fish to be small. In that way, policies dictating that only large fish should be caught, and small ones left alone, could be counterproductive.
Mikko Heino, an evolutionary biologist at the University of Bergen in Norway, has spent more than 20 years trying to document and understand how humans are changing the way that fish evolve. Because it’s very hard to directly demonstrate genetic changes due to fishing, he has used a diversity of other approaches, from lab experiments to historical data analysis to computer modeling of long-term evolution.
As Heino and two coauthors explain in the Annual Review of Ecology, Evolution, and Systematics, there probably is no way for us to catch fish without evolutionary side effects. Yet he believes that taking evolution into account might put an end to well-intended but ill-advised policies and help develop more sustainable ways to manage fisheries.
This interview has been edited for length and clarity.
Overfishing is a global issue today, and it seems impossible to inventory all populations to know what exactly is going on. So how do you go about studying the impacts of fisheries?
First of all, we have learned a lot from studying fisheries where there is a tradition of data collection. In Norway, for example, these data go back almost 90 years, which reflects the very high importance of some of these populations to local economies. Because those catches were naturally fluctuating, there was a desire to better understand where these fluctuations were coming from, because they had such a huge impact on people’s livelihoods.
By using these kinds of data, we were able to show, for example, that a dramatic collapse of the cod population off the coast of Canada in the late 1980s and early 1990s was preceded by a decrease in the average size of individuals, and earlier reproductive maturation. Follow-up studies have found that similar patterns are affecting dozens of other commercially important fish.
To really demonstrate evolution, one has to show genetic change. Have you been able to?
Not yet. All our evidence so far is based on changes in the appearance of the fish, not their genes. I have been involved in a number of projects attempting to show genetic change, but it turns out to be challenging. This partly reflects the highly polygenic nature of traits such as body size and maturation. Almost every gene has an effect on body size, and so does the environment. The selection response is therefore quite spread out across the genome, and not very large at any single gene. Yet the fact that these changes are so common does suggest to me that adaptive evolution is the most likely explanation.
Another way we are approaching this is by doing experiments in the lab, often using smaller species such as guppies, to show what the evolutionary effects may be when we regularly remove the largest individuals. These experiments are necessarily a simplification of real life, but they may help to focus our attention on aspects we may not have thought about, such as the importance of cannibalism when different generations live in the same place, which is the case in certain populations of cod, for example....
