Scientific Method / Science & Exploration

A catch-22: commercial fishing and the evolution of fish

Evolving catches can help or hurt bottom line, depending on how fishing is done.

Atlantic cod

Few things illustrate the concept of sustainability as clearly as overfishing. When fish are harvested from an area more quickly than they reproduce, the population necessarily shrinks, and the catch shrinks along with it.

But fish may actually be evolving in response to the pressure of fishing, reaching reproductive maturity more quickly. That adaptation comes at a cost, however. Fish that spawn earlier might be less fertile, grow less after they’ve reached sexual maturity, and could become more susceptible to other non-human causes of death. Previous studies have shown that those trade-offs could exacerbate the economic and ecological problem of collapsing fisheries.

A new study this week suggests that these genetic changes could actually be economically beneficial to fisheries—but only if they’re managed sustainably.

The researchers built a computer model that simulated both the fish population and the economics of the fishing industry, using the Atlantic cod fishery of Russia and Norway as an example. While it used to be much healthier, this fishery still yields a billion US dollars or so of fish each year.

The team first explored simulations where the catch was managed at a sustainable level. In one version, the model allowed the simulated fish to evolve in response to fishing pressures. In the other, genes were frozen. In the evolutionary simulation, the catch actually became a little greater over time as the genetics of the population changed. In this scenario, adaptation is an economic boon.

Unfortunately, the fish are not being harvested sustainably. So the researchers ran the same experiment using historic harvest figures. Here the result was different—the evolutionary simulation showed a slightly smaller catch over time compared with the “frozen genes” version.

How could this genetic adaptation be beneficial in one scenario but detrimental in another? It has to do with the minimum size of fish that are caught. If you only catch large fish, the population won’t evolve much because fish can easily reproduce before being caught. If you catch even the small fish, the advantage of genetic adaptation is quite large. Very few late-maturing fish will reproduce before being caught, but in the middle, things get a little weird.

If the fish grow faster, that means they also reach catchable size at a younger age. At this intermediate minimum catch size, too many fish are caught before reproducing. Pressure to reach a reproductive size earlier makes fish bigger, and being bigger gets fish caught—it’s a fish catch-22. Greater evolutionary pressure (in the form of catching even the smaller fish) can push maturity early enough to move into the win column, but in the intermediate case it’s a net loss.

If fish are harvested sustainably, evolution doesn’t need to be taken into account. But where overfishing is still taking place, the wrong minimum catch size can exacerbate the problem. “Admittedly, those evolutionary costs are small,” the researchers write, “but they may just be enough to push a fish stock from the state of overexploitation into collapse.”

The study also notes another ramification: the genetic changes could skew the data used to monitor fisheries. The number of sexually mature fish is commonly used as an indicator of the total population. In an evolving population, a greater proportion is sexually mature—so counts of mature fish could hold steady even as the total population drops.

PNAS, 2013. DOI: 10.1073/pnas.1212593110  (About DOIs).

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