Something fishy
POSTED 5 JULY 2002
 

RELATED WHY FILES:
Overfishing

Darwin's right!

Fast evolution

Giant animals die first!

Fishing disputes in the Galapagos

 

Results after four generations: Top: Bigger fish remained when the smallest 90 percent were removed. Bottom: When the largest 90 percent were removed, the remainders were small fry -- er, fish. Middle: The control group was randomly culled. Courtesy David Conover

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Today's regulations for commercial fisheries may backfire.

 

Want to preserve the biggest fish in a valuable fishery? Then, logically, you'd want to allow bitty fishes to mature and have young 'uns before they risk the frying pan.

Three groups of silver fish, range up to a few inches long.  The strategy of protecting the young from the net and hook is widely used to protect slow-growing fish like bass, salmon, flounder and trout in fresh and salt water alike (small fish like sardines are regulated differently).

Now comes a study indicating that the tactic may backfire. Instead of ensuring a large, healthy catch, harvesting the large fish may cause genetic change favoring slow growth.

The research was done by David Conover, professor of marine science at the State University of New York at Stony Brook, and his graduate student, Stephan Munch. It is published in the current issue of Science.

The researchers grew a bunch of Atlantic silversides, a fast-growing little fish that seldom ends up on a dinner plate. Once a year, the researchers removed 90 percent of the individual fish and allowed the surviving 10 percent to reproduce.

The culling tactic was different in each of three tanks:

 The largest 90 percent of fish were removed from the "large-harvest" tank. The removal exaggerated the normal regulation of slow-growing fish.

 The smallest 90 percent were culled from the "small-harvest" tank -- the opposite of typical regulations.

 Fish were culled at random in the control tank.

Among the first generation of offspring, the large-harvest technique did result in a greater harvest of fish in terms of mass and average size. But things quickly began to fall apart. By the fourth generation, only half as much biomass was being taken from the large-harvest tank as the small-harvest tank. The average large-harvest fish weighed only half as much.

Graph shows large-harvested fish sharply declining in size over time. Small-harvest populations eventually produced larger fish.
Average weight of harvested fish. By the fourth generation, removing the largest fish caused genetic change: the fish were half as big as in the tank which lost the smallest fish. Data from David Conover, Science.

Getting it bass-ackwards...
In other words, in the long run, sparing the big fish makes more sense than sparing the shrimpy ones. Excuse the pun....

Because the fish lived in identical tanks, Conover is confident in attributing the size changes to genetic change. "Given that they are in a common environment," he says, "the difference in phenotype [physical form] is almost certainly caused by a difference in genotype."

The situation reminds us of artificial evolution -- the changes in farm animals that result when farmers breed the best animals. Artificial selection was a foundation for Charles Darwin as he developed his ideas about evolution through natural selection.

Fishing boat in ocean
Peruvian purse seiner fishing for small deep-water fish. Courtesy Teobaldo Dioses, NOAA.

Just a fish story?
To determine whether Conover's results should be used to change fishing regulation, we must consider some key differences between his experiment and real life:

 The Atlantic silverside grows quickly. But fast-growing fish are generally regulated by total catch weight, not minimum size.

 Conover and Munch culled large fish, but since they were all the same age, the scientists actually culled the fastest-growing. In the real world, fish of any age are eligible for the grill once they reach the minimum catch size.

 The researchers also applied extreme selection pressure. In the real world, it's rare to remove 90 percent of a fish stock at once.

Although the extreme tactics do not reflect reality in the ocean, Conover explains that they do reflect financial and academic realities: "We chose the experimental design to make sure we could get effects that were measurable in the lifetime of the grant."

While he's provided proof of the principle that taking the largest fish can cause genetic change, in the ocean, he concedes "it's more complicated."

Spare the rod and spoil the fish?
If Conover's results are indeed widely applicable, today's techniques for managing commercial fisheries may backfire. Instead of removing the largest fish, he suggests protecting at least some of them. In the process, genes for slow and rapid growth would be preserved, maintaining genetic diversity.

A variation of that tactic has been used in certain areas, says Jack Mattice, director of the New York Sea Grant program, which funded Conover and Munch's research.

The new "slot limits" protect fish of reproductive length. "There is a slot in the middle where you must put the fish back," Mattice says. Fish unfortunate enough to be longer or shorter are eligible for marinating.

Man holds large striped bass.
What a catch! This striped bass is large enough to keep and tasty enough for dinner. Minimum size limits on Atlantic striped bass have been reduced, as the fish has become more common -- an example of current fishing regulations actually working! Courtesy National Ocean Service Photo Gallery, NOAA.

A second suggestion, protected marine reserves, is intended to protect ecosystems and fish biodiversity rather than commercial fisheries. But if they can sustain more of the gene pool, reserves might slow the genetic effects of removing larger fish. President Bill Clinton set up a large reserve in the western Hawaiian Islands, and California is in the process of setting up marine reserves.

However, Mattice points out that because the primary goal is saving fish habitat, it's unclear whether marine reserves will influence the genetics of fish outside their borders.

One line of prediction is safe: Before the scales of fishery management can be tipped in a new direction, the arguments will get tangled. Scientifically speaking, researchers may have some alluring arguments, but the net result is their notions may sink unless they hook commercial and recreational fishers.

-- David Tenenbaumlittle fish inspecting 'David Tenenbaum'

 

 

     

BIBLIOGRAPHY
Sustaining Fisheries Yields Over Evolutionary Time Scales, David Conover and Stephan Munch, Science, 5 July 2002, pp. 94-6.

 
home
 


Credits | Feedback | Search

©2002, University of Wisconsin, Board of Regents.