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Last updated 3/07

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Flyfishing in Saltwaters

 

WHY WE CATCH ONLY SMALL FISH

The evolutionary effects of managing fish though minimum size limits

By Capt. John McMurray

 Unless you failed out of school by junior high, you are familiar with “evolution” and Darwin’s Theory of Natural Selection.  If you are not, it can be summarized quite simply: “the fit survive.”  In the natural world predators pick off the weak, the young and the infirm, leaving the fastest-growing, strongest, and biggest of the species to contribute to the gene pool.  But, today’s methods of fishery management, particularly those used in many recreational fisheries, may be turning the natural selection process 180-degrees.

Instead of weeding out the weak from the herd, well-intentioned fisheries managers trying to manage fish through size limits have directed a great majority of the fishing pressure and subsequent mortality on the biggest, oldest fish that are most fit for survival.  Call it “unnatural selection” if you will, and the top predators (us fishermen) are the ones doing the selecting. 

small bass 4Dr. David Conover, head of Marine Sciences Research Center at Stony Brook University, NY published some interesting findings in that regard.  Through his work with Atlantic silversides (a small bait fish in the mid-Atlantic and northeast) he was able to provide the first evidence that the growth rates of fish and the productivity of populations can evolve rapidly in response to size-selective harvest.

Conover’s work was based on three populations of 1000 Atlantic silversides raised in identical, but separate environments.  Ninety percent of the fish from each group were harvested according to size.  In one population he removed the smallest 900, in the second he removed the largest 900, in the third, he removed a random 900.  The remaining fish in each controlled setting were allowed to reproduce. 

After four generations, the average weight of the largest fish in the population from which the largest fish were removed was 1.05 grams, compared to 3.17 grams in the random-selection tank and 6.47 grams in the tank from which only the smallest fish were removed.  The survivors of the group from which only the largest fish were taken show very obvious reductions in average size and total weight.  Furthermore, the number of survivors in the group from which only small fish were taken was double that of the population from which only large fish were harvested.

Thus, it appears taking only the biggest fish from a population may cause that population to produce smaller and fewer fish in the future.   It may also concentrate genetic growth traits.  Because traditional minimum size limits concentrate harvest on larger individuals, fish have a low probability of making it to spawning age, and of surviving once they get there.  Thus, evolutionary adaptation may favor fish that spawn younger. 

If that is true, anglers will be looking at future sportfish populations comprised almost entirely of “runts,” containing very few if any “trophies.”  We may already be seeing this trend.  In my neck of the woods, there seems to be fewer large stripers, yet an abundance of small fish.  In 1940 the average cod brought to the dock in New England was 36 inches long.  Today they average about 24 inches, a decline of one-third.  The big cod are for practical purposes, gone.  There is also data showing that cod are now maturing at much smaller sizes than was previously the case.  Pelagic species are showing the same trend.  There is some evidence suggesting that that eastern-stock bluefin tuna mature far earlier, and at smaller sizes, than western-stock bluefin, perhaps as a result of thousands of years of natural selection in the face of traditional Mediterranean fisheries. 

A lack of large fish, and the loss of a natural distribution of age and size classes within a fish population, has important implications for long-term stock productivity and stability. Dr. Conover’s studies revealed that removal of the large individual silversides from the population resulted in the production of smaller egges and larvae.  Such decline in larvae size was accompanied by a decline in swimming strength and feeding rates.  The smaller larvae were also less able to convert food to body mass, and less willing to forage for food.  It took only five generations for such changes to occur. 

Smaller fish produce fewer offspring, which are less able to survive.  A lack of variability in the small-fish gene-pool also renders such fish less able to withstand environmental challenges, such as periodic spawning failure or, disease. A spawning stock comprised of many age classes is more stable, and the better able to withstand the extraordinary events in the ecosystem which occur from time to time.

If minimum size limits are not the answer, how should we to manage sportfish?  The obvious answer is to implement slot limits, allowing folks to keep a fish that falls within certain size parameters.  This is already done with a number of species in a number of states.  But it does not work with every species. 

A slot limit can have an undesirable effect by focusing the pressure on a weak year that can’t tolerate the concentrated fishing effort.  In popular fisheries such as striped bass there is a risk of severely depleting a weak year class that happens to fall within a slot limit’s bounds.  The right way to use a slot limit is to reset it annually, tailoring the limits to the needs of the stock, by concentrating harvest on those year classes most able to tolerate the pressure and directing it away from the most vulnerable year classes.  For instance, one year the slot limit might be 28 to 36”.  The next year it might be 24” to 28,” to take advantage of a younger, very abundant cohort.  However no such system exists today. 

In species with long adult lives and low susceptibility to release mortality, the key factor may be “escapement” -  setting a maximum size to assure that sufficient large fish survive to constitute a healthy spawning stock.  Generally, the best course of action may also be to keep fishing mortality low enough to allow a more natural age and size structure in the population.  Emphasis should be placed on a size limit that permits the majority--ideally 100%--of the fish in the population to spawn prior to being harvested.  That would require fishing at a rate well below Maximum Sustainable Yield (the most fish that can be taken out of the population while maintaining long-term sustainability) and at higher minimum size limits. 

Unfortunately, the politics of fisheries management have historically prevented such a management approach. Many fishing industry spokesmen argue that we have to kill as many fish as possible.  Any time that knowledgeable and responsible folks even suggest reducing mortality and raising size limits, they are immediately labeled “elitists” that “don’t care about the common man being able to take a fish home for his family” and subsequently get trashed by the angling press. Unfortunately there may not be any fish worth taking home unless managers stand up and take notice of Conover’s work as it relates to the declining size trends in many local fisheries.