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Fishermen know they're supposed to keep the big fish and let the little ones go. But that might not be such a good idea after all. As this ScienCentral News video explains, scientists have shown that leaving only the runts to reproduce makes fish smaller and weaker year after year.
Negative Evolution
If you've ever spent a nice summer day on a boat, patiently waiting to reel in your dinner, you're probably familiar with your local government's fishing rules ... and the disappointment of having to toss back your hard-won catch. You can keep the big ones, but you have to throw the little ones back. Now a new study suggests that we've got it all wrong – this old-school rule is actually causing genetic changes in fish stocks, making them evolve to be smaller in size and have other negative traits.
Fisheries scientist David Conover, dean and director of the Marine Sciences Research Center at SUNY Stony Brook, is leading the most extensive laboratory study to date on the effects of size-selective harvesting in fish stocks. As described in Scientific American magazine, he and his team mimicked fishing by removing 90 percent of the largest fish from a lab population of Atlantic Siversides (Menidia menidia), a small marine fish. In another experimental group they removed 90 percent of the smallest fish, and in a control group they removed the same amount of fish, but without regard to size.
David Conover and Matthew Walsh
The lead author on the most recent study, Matthew Walsh, a PhD student at University of California Riverside, says that the results were striking – in five generations of this kind of selection, the different categories of fish greatly diverged in characteristics.
In the group where only the smallest fish were taken out, the fish grew larger, ate more, produced more eggs, had a higher rate of survival, were better at foraging for food, and tired less in swim tests. But in the group that mimics our current fishing regulations, where the largest fish were removed, showed opposite, negative characteristics.
"Where we selectively removed the largest individuals, these fish produce less eggs, they survive less than the other populations, they consume less food, even grow less efficiently," says Walsh.
The control group with random harvesting had intermediate traits located between these two extremes. This shows that the little fish aren't always just the younger ones, as our regulations implicitly assume – they're also slower-growing and genetically disadvantaged in other ways.
Conover and Walsh say that they weren't surprised that selection of this sort was able to cause change in general, but they were surprised at the number of other traits that were influenced and the speed with which the changes occurred. They think that these genetic changes may explain why just limiting fishing often doesn't lead to quick recovery for depleted fish populations.
"Fish are far lower in abundance than they have been historically, but a second observation is that fish are much smaller than they were historically," says Conover. Both Conover and Walsh cite Atlantic Cod as a good example of where this problem has actually been observed in nature. After more than 150 years of intense fishing, even cutting down on harvests has failed to help the Cod population recover.
Conover notes that the percentage of fish harvested in this experiment was high: "No doubt we probably caused evolutionary change more rapidly in our experimental populations than would occur in the wild." But says they simply fast-forwarded what would naturally occur over time. He also adds that these principles would apply to every kind of size-based harvest, which includes shellfish like crab and lobster. "We're talking about basic principles of evolution that apply to all kinds of life, not just fish, or small fish versus large fish," he says.
Conover and Walsh both say that it's no longer a safe assumption that it doesn't matter which portion of the population you remove when fishing. "We have to think about the genetic effects of size-selective harvests," says Conover. But he says that changing the fishing rules could reverse the problem over time.
"There are different sorts of regulations that we could use that would minimize or even reverse these changes; for instance, if we had a minimum and a maximum size rule. The other way to deal with the problem is to designate preserves where harvest is not allowed," Conover explains.
That would keep more of the big fish parenting the next generation and keep fish from evolving "resistance" to large size.
Conover has stopped harvesting the big and small fish in his experimental groups and is now trying to find out how long it will take for the fish to return to their former characteristics. "The thing that you want to avoid the most is causing irreversible harm," he explains. So while we wait to see how quickly these genetic changes can be reversed, it might be a good idea to start keeping some of the little guys and letting some of the bigger fish go free.
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