Genetic impoverishment in fish?In proportion to their body size, large and mature fish invest relatively more energy into the production of eggs than small, young animals that have considerably less body mass and volume. Older fish thus provide a kind of reproductive insurance. As long as enough older fish are present, sufficient offspring will be produced. But in stocks that consist of few age groups, and primarily of younger age groups, the danger of offspring deficiency increases when the reproductive conditions intermittently worsen, such as times of food scarcity. Stocks in which older fish predominate can more efficiently withstand these kinds of fluctuations, because the mature ones will reli-ably produce offspring in the following season. Stocks comprising different age groups also exhibit greater resil-ience because the spawning season of the fish varies with their age. There are thus a sufficient number of spawning animals at any given time in a mixed stock. Periods of unfavourable environmental conditions therefore have a less severe impact.
- 1.16 > A fish stock before fishing, after fishing, and after reproduction. The changes in body size are a result of fisheries-induced evolution.
- Warnings are now being raised that fisheries can also cause genetic impoverishment, or “genetic erosion” in the species being fished. This phenomenon is also recognized in land animals. With the destruction of habitats like rain forests, the distribution areas of species become critically limited. Many individuals die before they can mate. In addition to the species-specific genetic material, every organism possesses a small share of individual genetic attributes. If the animal dies without producing offspring, these individual attributes are lost and the population is genetically impoverished. Extreme genetic erosion is referred to as a genetic bottleneck. In this case, a species is reduced to a small number of individuals. This could occur as the result of a natural catastrophe such as a vol-canic eruption or flooding. Intensive hunting of geographically restricted populations like the Siberian tiger can also lead to a genetic bottleneck. In extreme cases this leads to inbreeding. The animals produce offspring with genetic defects or that are susceptible to disease. Some scientists are concerned that genetic erosion leading to genetic bottlenecks occurs not only in land animals, but also in some fish species through overfishing. So far, however, this assumption is hypothetical and it is presumably not valid. For most of the commercially depleted fish stocks neither genetic erosion nor genetic bottlenecks can be statistically verified. Specialists believe that even fish stocks that have been commercially depleted still possess thousands of individuals capable of reproduction. The genetic variability thus probably remains great enough to preclude the erosion effects.
Slowing down fisheries-induced evolutionExperts recommend giving more attention to the ecogen-etic aspects of fishery management in the future. There is already a general consensus that fishery management should not consider a fish species independently of its habitat. Beyond this, however, ecogenetic models are necessary. These can be used to estimate which changes are caused by fisheries and to what degree genetic changes influence a stock, but also how these ultimately affect the future fishery harvests. Through responsible fishing, there is hope that fisheries-induced evolution can be reversed, or at least slowed down. It can probably not be completely stopped. Researchers also need to employ complex evolution models. Up to now, often only the age classes of a fish stock have been considered in detail for calculations of stock development. Fish sizes are entered into the calculation simply as the mean of an age class. This mean, in turn, has been calculated from long years of body-length measurements. An age class for a fish stock, therefore, always has a fixed, assigned average size. In fact, however, the mean size of an age class changes from year to year, depending mainly on the food supply. In years of scarce food supply immature animals grow more slowly. This variability has to be given greater consideration in the future. And, of course, there are always larger and smaller individuals within an age class. These fluctuations also have to be addressed. The mean value is not sufficient for an evolutionary model. Researchers therefore call for more intensive cooperation between fishery authorities, who have access to detailed data, and mathematicians and statisticians, who can develop powerful computer models.