Sustainability Issue #2 July 2009

Biological variation at genetic level is essential for the survival of species over a longer time scale. This variation is the basis of the ability to adapt to changes in the environment. Especially in conjunction with the rapid climate changes, the issue of genetic variation will therefore have key importance among wild animals and plants.

Fishing and other exploitation of wild populations (such as hunting and forestry) can give rise to a reduction in genetic variation. The reasons are that the sizes of the populations are often reduced, and also that the selective harvesting may result in certain genetic variants decreasing or vanishing from the population. One more factor is that fishing that is concentrated to certain age or size classes may result in loss of genetic variation. Research into exactly how this occurs is in progress at present within the framework of projects financed by Formas.

Salmon are different in different waters

Salmon and brown trout are species which have been known for a long time to have a highly developed genetic population structure – there are thus large genetic differences between populations in different rivers. A large proportion of this genetic variation therefore disappears if individual populations are obliterated. As far as salmon are concerned, it is mainly the expansion of hydroelectric generation that has resulted in large losses of natural populations. Today, viable and natural salmon populations remain in only about 15% of the rivers round the Baltic Sea.

In order to compensate for the loss caused by hydroelectric schemes, large numbers of salmon are released every year. These salmon are bred in farms and are genetically different from the natural populations. Studies now show that such releases are genetically changing the few remaining wild populations. We dot not know today what this change and impoverishment of the genome of salmon will lead to in the long run. Nor do we know whether, and if so how, it may affect the capacity of the salmon to adapt to climate changes.

Owing to its highly developed genetic population structure, salmon should be managed river by river. Genetically sustainable fishing of salmon demands that the population to which the fish caught belong can be identified. Otherwise there is a risk that the few remaining natural populations will be overfished.  The same situation prevails for brown trout in e.g. the Stockholm region. Few natural populations remain, but large numbers of farmed brown trout are released so that recreational fishing may continue to be profitable. This is a threat to the remaining natural populations. In Stockholm County there is no genetic surveillance that monitors any effects, but this is done to a certain extent on Gotland. There, there is a risk that unique Gotland brown trout strains will be affected by the inflow of genes from the farmed brown trout that spread out from Stockholm.

Baltic herring are one population

Herring have a population structure quite different from that of salmon and brown trout. Few and very small genetic differences are detected between populations in the Baltic Sea. A recent study also suggests that the fishing of herring over the past 20 years has had no appreciable genetic effect. On the other hand, there are indications that herring in the Baltic Sea are exposed to selective pressure that is different from that of herring further to the west. Genetic variants that are uncommon in herring from then Atlantic occur in high frequencies in the Baltic. This may reflect genetic adaptation to the unique environment of the Baltic. Signs of such adaptation are found in many species. From a genetic perspective, Baltic herring can be managed as one single population.

Pike genetics

The population structure of pike in the Baltic Sea represents an intermediate stage between that of salmon and brown trout. There is a genetic population structure, but it is not so pronounced as in salmon. Pike exhibit a continuous genetic change over their habitats in the coastal zones of the Baltic. For such species, it is difficult at present to identify appropriate management groupings. Studies carried out so far suggest that pike are relatively local as regards the dispersion of their genes. Pike in areas of about 100-150 km in extent appear to constitute genetic units. More research is however needed into the problem of genetic management of species which have continual genetic change, and Formas is funding projects with such focus.

In order that fisheries management and other management of the use of wild populations may be sustainable as regards diversity at genetic level, there is a need for surveillance programmes that  monitor how genetic variation changes over time, so that any threats to diversity at gene level may be detected in time. Several proposals as to how such surveillance can be carried out have been presented, but none has been into practice so far. Salmon, herring and pike are all species that are highly interesting from the standpoint of genetic monitoring.

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