Human-induced fish evolution

Anne Maria Eikeset Some important findings will be submitted this spring by doctoral research fellow Anne Maria Eikeset of the Centre for Ecological and Evolutionary Synthesis (CEES). Her findings are notable not only for basic research, but for Norway's economy as well. She and an interdisciplinary team of Norwegian and international biologists, economists, statisticians and mathematicians are tracking down the reasons why Norwegian fishermen are seeing fewer and fewer large cod in their catches.

In her pending doctoral thesis, Ms Eikeset concludes that an evolutionary change is likely occurring in the cod population. Ever since the 1930s, the beginning of the fisheries industry boom, cod have become smaller and reach sexual maturity earlier.

Fish evolution: Large cod are becoming a less common sight for Norwegian fishermen. (Photo: Berit Roald/Scanpix)

Darwinian principles at work

"Basic evolutionary principles are involved here, just as Charles Darwin described them," says Ms Eikeset. "Commercial exploitation of the fish stock has greatly intensified," she continues, "and the largest fish command the highest price. Large, mature fish spawn larger and more offspring. But when the large fish are caught, they cannot reproduce and so cannot pass on their genetic material to future generations. It is the smaller individuals that slip through the nets that live to reproduce."

Under such intense selective pressure, evolution would reward fish that reached sexual maturity earlier - favouring those that reproduce before getting so big they get caught in a net. Researchers believe this is one of several reasons that the fish have changed since trawlers began large-scale fishing of cod in the Barents Sea in the 1930s. Back then, cod reached sexual maturity at the age of 9-10 years. Today, the cod are reproducing as 6- and 7-year-olds.

Ecology in conjunction with evolution

The hypothesis that fishing can function as an evolutionary force affecting cod stocks first appeared in a 2004 article in the journal Nature. The authoring researchers, from the Institute of Marine Research in Bergen among others, were investigating the causes of Canada's cod stock collapse of the 1990s.

Evolution, however, is not the only explanation put forth. Another hypothesis postulates that cod are reaching sexual maturity at a younger age because they have better access to food. When the large cod are caught, there is less competition for food among the remaining individuals.

"The one explanation does not preclude the other; they act simultaneously," believes Ms Eikeset. According to her assessment, it is likely that evolution and ecology interact to make cod smaller.

"Less genetic variation means that the fish stock may lose some of its ability to adapt to new environmental conditions. The population is laying smaller eggs and producing fewer offspring, making it less hardy and more vulnerable to forces such as climate change."

Climatic models indicate that climatic changes will be far more extreme in Arctic regions than in other areas. Furthermore, it is well known that marine ecosystems react much more quickly to climatic changes than land-based ecosystems.

Impact on the economy

In their numerical models, researchers have combined principles and mechanisms of evolution, ecology and economics into a bio-economic model. They employ this model to study the ecological and evolutionary changes resulting from various catch strategies. By including economics, they can study how the financial yields from fisheries may change, and how this in turn would affect the determination of a financially optimal catch. This has never been done before. Their models have proven to be accurate, matching the observed events in the Barents Sea well.

"In addition to being pure basic research, this research also has short-term social benefit," asserts Ms Eikeset. The interaction between evolutionary and ecological changes can cause major repercussions for the Norwegian fisheries industry and Norway's economy. Although Norway has rights to just 0.6 per cent of the globe's ocean area, the nation is nevertheless the world's third largest producer of fish and seafood in terms of value. The Barents Sea is home to the world's largest stocks of cod and capelin.

Breeding consequences

"Our models can provide insight as to where it pays to fish and how it pays to fish. It seems likely that continued, intense selective pressure in the fisheries could have economic ramifications. Simply put," adds Ms Eikeset, "we can earn more money by fishing less."

"Ask any farmer working with animal production if he or she would slaughter the very best individuals year after year. Of course the answer would be 'no'. Every farmer knows that in professional selective breeding, one obviously must not destroy the genetic variation of the stock."