Catching Large Fish Leaves You With Only Small Fish To Catch

For decades, a principal idea of fisheries management has been to catch the large fish, and leave the small fish behind so they can reproduce and contribute to the next generation. To most of you, this probably sounds like a good idea for sustaining fish populations. But what if I told you that maybe it is not such a good idea after all…

In the early 1900’s, fishermen often caught very large fish. Today, truly big fish are very scarce. Did they all simply get caught? Or is something else also at work?

Scientists have found that over the last century growth rates and fish size have been declining in many heavily fished species. And fish have started to reproduce at an earlier age and smaller size. For instance, the age, length, and weight of first time reproducing cod in the Northeast Arctic has declined dramatically from the 1930’s to the 2000’s (depicted in the diagram below).

Reprinted by permission from Macmillan Publishers Ltd: Nature. Illustration by Wesley Fernandes/Nature; Source: Int. Inst. Appl. Sys. Anal., 2013.

These changes are bad for the health of fish populations, because smaller/younger fish produce fewer eggs compared to larger/older fish. So it makes fish populations less productive and ultimately less resilient to fishing. It also means reduced catches and profits for fishermen.

Many scientists originally thought that these changes in fish size and reproduction were the result of changing environmental conditions, such as increasing ocean temperatures or food availability.

But in the 1970’s and 1980’s, a few scientists began to argue that these changes were not simply the result of environmental factors, but rather evolutionary changes (or genetic changes), resulting from fisheries targeting only the largest fish in the population.  Scientists thought that because fisheries were targeting or selecting for large fish, this was creating an advantage for small fish. And causing fish to evolve towards smaller sizes (or slower growth) overtime. [The slower you grow, the longer it will be before someone catches you.] Similarly, they thought fish may have evolved towards earlier sexual maturation to ensure that they are able reproduce before they are caught.

Dr. David Conover, an expert in fisheries ecology and evolution [and my graduate thesis advisor], was among the first to study this idea of ‘fisheries-caused evolutionary changes’.  In the late 1990’s, he and his student Stephan Munch set up experiments to simulate the size-selective removal of large fish that occurs in fishing. The experiments showed that after removing the largest fish over several generations, that fish growth rates and weights did indeed decrease. And because environmental conditions, like temperature, were held constant in the experiments, the changes had to be genetic. Therefore these experiments provided compelling evidence that fishing could cause evolutionary changes in fish populations!

Since then, many other scientists have found that fishing-caused evolutionary changes have likely occurred in cod, plaice, and many other heavily fished species. In one large analysis of 37 fish populations, scientists found that in almost all the populations the size and age of first-time reproducers had declined over time, and the changes were highly correlated with the level of fishing.

But despite the evidence for fisheries-caused evolutionary changes, some still remain skeptical of the idea. And the issue has largely been ignored by fisheries managers. This is concerning, because evolutionary changes caused by fishing can have severe impacts, and are likely hard to reverse. For example, some scientists contend that evolutionary changes in the age/size of first-time reproducing Atlantic cod have lowered the productivity of cod populations. And, that this has prevented cod from recovering in some areas.

So today, scientists like Mikko Heino at the University of Bergen, Norway are still trying to prove that evolutionary changes from fishing occur. He is looking at DNA samples from cod and other species to try to identify genetic changes in fish growth.  As well, he is conducting his own size-selective fishing experiments. He is trying to improve upon those conducted by Conover in the 1990’s by making them more naturally realistic.

And Dr. Heino and others continue to call for the consideration of evolutionary changes in fisheries management. Dr. Heino says that to prevent evolutionary changes, we should spread fishing over a wide range of size classes and ages—rather than just targeting the largest/oldest fish. He says we should get rid of most minimum size limits [a common fisheries management strategy]. Many fisheries scientists and managers are likely to call this idea crazy and un-sustainable. I have to admit that wrapping my head around the idea is a little difficult.

But, on the other hand, some scientists have found that this type of management strategy could have several benefits, in addition to preventing evolutionary changes [assuming of course that fishing levels are kept at sustainable levels]. It was found that catching both small and large fish is better for maintaining healthy food-webs. And that it could even improve fishery catches. So perhaps this idea deserves further consideration.

Dr. Conover says though, that the best solution, in some cases, is actually to impose a maximum size limit [in addition to a minimum size limit], so that fishing is concentrated on intermediate sizes. This would promote the evolution of fast growth [rather than slow growth]—Because now the faster the fish grows, the faster it will reach the maximum size limit and gain protection from fishing. The problem is that in some fisheries, such as those that use trawl gear, it is often hard to avoid catching large fish.

Scientists also say we should create marine protected areas for a wide-range of fish habitats, not just for reproductive or spawning habitats [another common management strategy]. Protecting only spawning habitats could give another advantage to early-maturing fish—since the sooner fish sexually matured, the sooner they could move to the protected spawning habitat.

There is no single, perfect solution for preventing fisheries-caused evolutionary changes. But it is time for fisheries managers to stop ignoring the problem.  Conventional fisheries management has not worked that well so far. So why not try a new, non-conventional approach to fisheries management—one that includes evolutionary considerations and more long-term thinking!


Elizabeth Brown is a research research scientist at Blue Ocean Institute.

4 Comments on “Catching Large Fish Leaves You With Only Small Fish To Catch

  1. Even without fishery selection/evolution, it has been well known for many years that selectively taking largest critters has non-linear population effects, and not just via no.eggs/female. Example is breeding effectiveness in male red king crabs (big crab dudes service more females more effectively).

    Good post!

  2. I’m a believer in getting rid of bottom dwellers that really take a toll on other fishs eggs. You mentioned that If there was more management on spawning areas or more protection them.. It might be a helpful way for fisheries. These junk fish kill so many other new fry almost ready to be born. Not to mention some of these junk fish were introduced through humans. I don’t know.. I could chat on this info all day long. Great ideals here.

  3. Thank you for your comment. Spawning areas do need protection. But we need to protect habitats other than just spawning areas too. So glad you are interested in this topic! I encourage you to explore more ocean issues on our website.

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