Throwing the Little Ones Back
Throwing the little fish back, a common practice, causes fish to shrink over time since there is selection against the survival of larger fish:
Survival of the Smallest, Ecology, Scientific American: Any commercial fisher or weekend angler knows to “throw the little ones back.” The idea is to give small fish time to grow up... But that strategy may actually be harming fish stocks. Ongoing experiments on captive fish reveal that harvesting only the largest individuals can actually force a species to evolve undesirable characteristics that diminish an overfished stock’s ability to recover, says David O. Conover, director of the Marine Sciences Research Center at Stony Brook University. The results may explain why many of the world’s most depleted stocks do not rebound as quickly as expected.
The genetic effects appeared in Atlantic silversides, a small, usually fast growing fish. Conover brought a batch of wild silversides to his laboratory in 1998. He and his students then reared six generations, each time removing the largest 90 percent from one group, the smallest 90 percent from another group, and a random 90 percent from a third.
By 2002 it was plain to see that killing off the largest fish had a dramatic effect. Individuals in that group were only about 70 percent the average weight of their randomly harvested counterparts... Because the compared fish were the same age, the scientists could attribute the shrinkage to selection of genes for slower growth.
Even more alarming, the slower growth came with a suite of deficiencies. Detailed examinations of the fifth- and sixth-generation fish ... revealed that members of the large-harvested group were less willing to forage for food and less able to outwit predators. They also produced smaller and fewer eggs, and a lesser portion of those eggs grew into healthy offspring. ... Historical records confirm that cod and other popular food fishes were bigger in the past...
Whether weaker fish are making it to the dinner table now is not at all clear, Conover admits. But letting some big fish go along with the little ones is probably a smart strategy nonetheless. Meanwhile Conover’s team has halted all size-selective harvesting and is waiting to see whether subsequent generations will recuperate—and how long it takes to do so.
Throwing the little fish back, a common practice, causes fish to shrink over time since there is selection against the survival of larger fish:
Survival of the Smallest, Ecology, Scientific American: Any commercial fisher or weekend angler knows to “throw the little ones back.” The idea is to give small fish time to grow up... But that strategy may actually be harming fish stocks. Ongoing experiments on captive fish reveal that harvesting only the largest individuals can actually force a species to evolve undesirable characteristics that diminish an overfished stock’s ability to recover, says David O. Conover, director of the Marine Sciences Research Center at Stony Brook University. The results may explain why many of the world’s most depleted stocks do not rebound as quickly as expected.
The genetic effects appeared in Atlantic silversides, a small, usually fast growing fish. Conover brought a batch of wild silversides to his laboratory in 1998. He and his students then reared six generations, each time removing the largest 90 percent from one group, the smallest 90 percent from another group, and a random 90 percent from a third.
By 2002 it was plain to see that killing off the largest fish had a dramatic effect. Individuals in that group were only about 70 percent the average weight of their randomly harvested counterparts... Because the compared fish were the same age, the scientists could attribute the shrinkage to selection of genes for slower growth.
Even more alarming, the slower growth came with a suite of deficiencies. Detailed examinations of the fifth- and sixth-generation fish ... revealed that members of the large-harvested group were less willing to forage for food and less able to outwit predators. They also produced smaller and fewer eggs, and a lesser portion of those eggs grew into healthy offspring. ... Historical records confirm that cod and other popular food fishes were bigger in the past...
Whether weaker fish are making it to the dinner table now is not at all clear, Conover admits. But letting some big fish go along with the little ones is probably a smart strategy nonetheless. Meanwhile Conover’s team has halted all size-selective harvesting and is waiting to see whether subsequent generations will recuperate—and how long it takes to do so.
Seriously a 90% removal rate is pretty darn high in my opinion. I doubt in many of the streams I fish that 90% of any population is being removed in a year.
