Consequences of shortterm evolution

Through harvesting, humans can impose impressive selective forces on populations. The North East Arctic cod Gadus morhua (Figure 8.5) has experienced a large reduction in age at maturity from 9 to 6 years in the last 50 years, a change that is typical of other exploited stocks of marine fish in the North Atlantic (Law 2000). One likely reason for this has been the change from fishing only at the spawning grounds off the coast of Norway to fishing largely in the Barents Sea, which is the feeding grounds of the fish. This change in fisheries practice was brought about by the introduction of motorized deep-sea trawlers in the 1920s. Currently almost 40% of the stock

Fig. 8.5 A cod, G. morhua, on the slab. Fishing activity has led to a decrease in the age of maturity and consequent reduction in yields. Photo courtesy of Gerd-Peter Zauke.

is removed annually from the Barents Sea. Over the course of several years, the cod have very low chance of maturing successfully. Genetic variants that escape the feeding ground to spawn earlier have a much better chance of reproducing. In contrast, when the fish were caught only at the spawning grounds, mortality would have been lower at the feeding grounds, creating selection for increased age at maturity.

Can these changes in fishing mortality exert effects on the fish population? There is some evidence that it can: for example, Conover and Munch (2002) studied the Atlantic silverside (see also Chapter 5). Removing smaller individuals gave an increased yield over selecting larger individuals. There were two reasons: first, when small fish were harvested the adults grew larger, giving greater reproductive potential. Second, removing smaller individuals selected for faster growth as the fish then spend less time in the more vulnerable stages. Such studies show the potential for harvesting to cause evolutionary changes that affect yield. They do not, however, easily suggest what the consequences have been in cod, where the regime is not purely a size selective one and the evolutionary changes have been in age at maturation. For this a specific cod model is needed.

The intuitive effects of reduction in age at maturity are that the yield of fish will decline: the fish mature earlier, hence are putting energy into reproduction instead of growth at an earlier age. Law and Grey (1989) developed the notion of the evolutionarily stable optimal harvesting strategy (ESOHS) to help quantify the effects on yield. At the ESOHS, fishers adopt a strategy that maximizes yield after evolution has reached its final state. They showed that the ESOHS for the North East Arctic Cod was the traditional practice of fishing only at the spawning grounds, and that fishing at both feeding and spawning grounds led to a reduction in long-term yield of up to two fold. One problem, for fishermen, of following the ESOHS is that they may only gain the benefits of maximizing yield after waiting for evolution to come to rest. In the meantime, it is possible that they may have to pursue a suboptimal strategy from the perspective of maximizing yield. Instead it would be sensible for fishers, at least in the short term, to pick the strategy that maximizes sustainable yield at each point in time. In response to this, the fish would experience a slightly different selective pressure, and evolve to a new state, after which the fishermen would perhaps need to change their optimal harvest strategy. This situation is thus, as in HIV infections, a co-evolutionary circuit, this time consisting of evolutionary changes in fish, followed by changes in harvesting strategy by the fishermen.

Heino (1998) has modelled these co-evolutionary dynamics for cod. He was interested to know what particular type of harvesting strategy maximized sustainable yield and whether the co-evolutionary dynamics also lead to an asymptotic fishing strategy that is the same as the ESOHS.

The technique he used to determine the evolution of the fish is known as adaptive dynamics (Dieckmann 1997; Waxman and Gavrilets 2005, see below).Like Law and Grey, he found that fishing in both the spawning and feeding grounds led to a reduction in yield relative to fishing in only the spawning grounds. However, the co-evolutionary dynamics frequently led to the ESOHS even if it was not pursued in the first place.

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