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Fisheries

Be careful what you fish for: Using protected areas to save lobster claw size

Citation

Sørdalen, Tonje Knutsen, Kim Tallaksen Halvorsen, Leif Asbjørn Vøllestad, Even Moland, and Esben Moland Olsen. 2020. “Marine Protected Areas Rescue a Sexually Selected Trait in European Lobster.” Evolutionary Applications, May. https://doi.org/10.1111/eva.12992.

Fishing as a form of natural selection

Ever since Charles Darwin and Alfred Russell Wallace first formalized theories of natural selection, it’s been clear that one simple question— who reproduces?— has a major impact on the nature of a species.

In a nutshell, natural selection is the process by which species adapt over generations. In each generation, some individuals will have a collection of traits that increase their ability to survive and reproduce in their environment. As a result, they contribute more offspring to the next generation, and these heritable traits are passed along.  

These fish won’t reproduce again. What does that mean for the future generations? (Image Source: Fra Sildfiske, Wikimedia Commons)

When it comes to marine creatures, natural selection favors the traits that allow an individual to succeed against the challenges of the ocean environment— say, by surviving intense salinities, warding off waterborne disease, or avoiding predators in a 3D matrix. Individuals who lack the traits needed to survive in this watery world produce less offspring. Over time, species adapt to navigate these environmental challenges.

For creatures that humans like to eat, individuals face another environmental challenge: harvest. Much like disease or predation, harvesting fish removes them from the population and stops them from reproducing. Certain traits can confer a better ability to survive harvest, and these traits will be favored over generations. It’s just like natural selection— but because humans are so involved, this process is termed artificial selection.

Artificial selection is a long-recognized issue in a number of fisheries.  For many of these, there is a legal size limit to the fish that can be harvested. Fish that reproduce before they reach that limit are more likely to contribute offspring like them to the next generation, compared to fish that reach the size limit before reproducing. The result? The next generation is made up of more offspring from the small-reproducers than the large-reproducers. Over time, if this selective pressure of fishing is maintained, the entire species can be driven to smaller sizes.  Harvest tips the scales of selection.

Artificial selection changes lobster claws

Consider the lobster trap. The trap sits at the bottom of the ocean, baited with dead fish and waiting to ensnare lobsters to feed a hungry lobster-loving industry. Over and over again, trap after trap is baited, set, and retrieved across the world’s lobster fisheries.  For European lobster (Homarus gammarus), this harvest pressure is intense enough to act as a selective force on lobster traits. Evidence lies in the lobster claw. 

Fishers prepare lobster traps in Sweden. (Image Source: W. Carter, Wikimedia Commons)

For the European lobster, males with large claws are more likely to end up in traps than males with small claws. It’s not entirely clear why this is the case— it could be that large-clawed males are more aggressive, and outcompete small-clawed males for a chance at the trap’s bait— but the pattern is clear. Because large-clawed males are more likely to be trapped, they’re also more likely to be harvested. Small-clawed lobsters, in contrast, have a better chance of avoiding traps and surviving the fishing season. By dint of surviving and reproducing, small-clawed lobsters contribute more to the next generation than large-clawed lobsters. 

By removing large-clawed individuals from the wild lobster population, fishing shifts the makeup of future lobster generations. Over time, that results in a lobster population made up of small-clawed lobsters.

The dangers of artificial selection

There’s reason to be concerned about the small-clawed shift in the European lobster population. For one thing, this pattern runs counter to the characteristics of lobsters in the absence of fishing. Large claws help lobster. They improve a lobster’s ability to forage and defend against predators, as well as their odds of winning fights with other lobsters over scarce resources like food or shelter. Female lobsters tend to mate with large-clawed males, meaning larger claws can improve mating success as well. Natural selection sans-fishing drives the lobster population to have larger claws. 

Like their European cousins, American lobsters like the one pictured rely on their claws to survive their ocean world. (Image source: Dwayne Meadows, NOAA)

Beyond the individual benefits of large-clawed lobsters, there are benefits to the species as a whole in preserving overall variation in claw size. Trait variation provides more material to work with for future adaptation. That’s especially important in an ocean facing climate change. Preserving variation in the European lobster population might keep future lobster generations from falling into a different kind of trap, one where no individuals have the traits needed to survive in a new environment. The decline of large-clawed lobsters might be a worrying indicator of a broader loss of trait variation.

Since fishing can reduce variation by selecting for certain characteristics— like small claws— an important task in fisheries management is figuring out how to undo this selection to maintain variation across the species.

MPAs may counteract artificial selection

Marine protected areas (MPAs) may serve as a fisheries management tool that preserves large-clawed lobsters and the genetic variation they provide. 

While there are many different types of MPAs, many restrict commercial and recreational fishing. A number of MPAs protect European lobster from harvest. Large and small-clawed lobsters roam these protected areas. Because there is no fishing in these areas, the natural selective forces should be at play, favoring large-clawed lobsters and undoing some of the selection for small-clawed lobsters. 

A team of researchers set out to see if MPAs really serve this function for Norway’s European lobster population. They collected lobsters from within and outside MPAs at three sites in Norway, and compared claw size between their catches. 

Port Bergen, Norway, is close to valuable fishing waters and MPAs alike. (Image Source: Robert Bye, Unsplash)

Indeed, they found that lobsters captured within the MPA had claws that were, on average, 8% bigger than lobsters captured outside of the MPA. It seems like this hypothesis— that fishing protection can undo selection for small-clawed lobsters— might hold water.

Other factors may drive lobster claw patterns

But not so fast, authors caution. In other studies, lobster claw size has been shown to change over the course of a lobster’s life in response to fluctuating conditions like temperature or salinity. MPAs, due to their fishing restrictions, have higher overall lobster densities, and that alone may drive lobsters to grow larger claws. If that’s the case, MPAs are protecting lobsters, but aren’t necessarily getting to the root of counteracting fishing selection. 

Likely, MPAs are one technique in an array of fishery management tools that might help preserve trait variation. By carefully deploying these techniques, we might be able ensure the continuation of European lobster— and the lobster fishery— in a changing sea.

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