Escaped farmed fish is one of the Norwegian aquaculture industry’s biggest environmental challenges. Farmed salmon that migrate up rivers to breed may undermine the population’s genetic material and resulting in a weakened population of wild salmon.
Simulations carried out by NINA with 20% escaped farmed salmon at spawning (close to the average in Norwegian rivers the last fifteen years) shows that there will be significant changes in the wild salmon stocks during ten salmon generations (about 40 years). It does not appear that farmed salmon establishes itself in rivers with a low number of escaped farmed salmon in the spawning population. But in rivers with a high number of escaped farmed salmon it appears that the population is gradually dominated by the offspring of farmed and hybrids of salmon. Even after many decades without new escapes, it is possible that these populations will be dominated by descendants of escaped farmed fish. NINA suggests that the average mix of escaped farmed in the spawning stocks should be less then 5%. An alternative value can that the gene flow from escaped farmed to wild salmon should be less than what is typically found between two wild salmon populations. Regardless, it is required that measures to sharply reduces the number of escaped farmed in nature must be implemented as possible.
There has long been concern that escaped farmed salmon may harm the various wild fish populations through hybridisation and altering the gene pools of wild populations (Hansen et al. 1991).There are several problems that can arise in this connection. If the farmed salmon have different characteristics and adaptations from wild salmon populations, gene flow may cause the wild salmon populations to lose characteristics that are crucial in a natural environment, while they adopt more of the farmed salmon’s characteristics. On the other hand, if the escaped farmed salmon have less genetic variation than wild stocks, gene flow to the wild population will cause individual populations to lose variation (Tufto & Hindar). Variation is essential for two reasons (Hedrick, 2000), evaluated from both a short-term and a long-term perspective. A population that loses variation and thus becomes genetically uniform will be less resistant to disease and parasites. Or put another way: it is easier for a parasite to adapt to a population of genetically similar individuals (few polymorphic loci in the population and low heterozygosity) and where the individuals themselves have little variation (the individuals have few heterozygous loci). Additionally, in theory some of the harmful, recessive alleles will increase in frequency and produce less viable individuals (inbreeding depression). Studies just out (Reed & Frankham, 2003) empirically show that there is a good connection between fitness and heterozygosity, population size and quantitative genetic variation. Heterozygosity explains about 20% of the variation in fitness. In the long term, a population with little polymorphism will not have as great an evolutionary potential as a population with a lot of genetic variation.
All escaped farmed fish will come from a small number of farmed populations, which will lead to different populations becoming more like one another. It has also been claimed that coadapted gene complexes may dissolve. The following is an attempt to clarify relevant concepts and summarise empirical studies.
Evolutionary forces: mutation, selection, migration and drift
From modern evolutionary theory we know that there are four key forces that induce a population to change over time. These forces are selection, mutation, drift and migration. Even if all alleles originate in mutations, such events are all too rare to be an important force in ordinary evolution. It is therefore disregarded as a cause for fixing an allele or trait in a population. Drift is the random selection of gametes with different sets of alleles. The potential for evolution caused by drift is therefore inversely proportional to population size. Selection is probably the best-known evolutionary force and is caused either by survival and reproduction ("natural selection"), breeding ("artificial selection") or sexual selection. Migration (gene flow) means that individuals from a donor population reproduce in a recipient population. Migration results in the recipient population becoming more like the donor population. Although the donor and recipient populations will at the outset normally differ from each other in several characteristics and genes, unlike drift, migration will necessarily impact all these characteristics simultaneously. Like selection, migration is both deterministic and directional.1 For its part, drift is deterministic, but not directional. Selection can counteract migration, drift and mutations; if drift or migration has increased the frequency of alleles that produce low rates of survival and reproduction in earlier generations, natural selection can reduce them. Nevertheless, modern evolutionary theory says that migration is a very important evolutionary force, which can override selection and lead to less adapted populations (Graur & Li, 2000;Tufto, 2001; Lenormand, 2002).
Breeding and evolution
Traditional livestock breeding and evolution in the wild have several similarities. The biggest differences are that natural and sexual selection are more important in the wild, whereas artificial selection is more important in breeding programmes. In addition one can say that unlike natural selection, artificial selection has a goal. The fact that these mechanisms have certain similarities does not mean, as we try to elucidate in this chapter, that bred organisms are necessarily "natural" or harmless if introduced into the wild. Genetic drift is an important evolutionary force in small populations. A general rule of thumb2 says that populations of fewer than 500 individuals will lose genetic variation. After many generations, the genetic variation will no longer allow adaptations to the environment. A population of fewer than 50 individuals will after a few generations suffer from inbreeding depression.
Practically all traits have a genetic component. In addition, many traits have an environmental component, which is not inherited in the same manner. In an evolutionary perspective, only the genetic component is interesting. A trait may be physiological, behavioural, anatomical etc, and one or more genes may be controlling the trait. Most genes have a specific location in the genome. This place is called a locus. When a mutation occurs, a new variant of the gene arises. Such gene variants on the same locus are called alleles. All alleles once arose by a mutation, but mutations are so rare that they cannot be an evolutionary force. Many of the alleles can be found in a population’s overall "genetic library", its gene pool.
Gene flow from farmed salmon to wild salmon
With a higher than 95% probability, wild salmon will return to the river they grew up in. The probability of migrating to the wrong river is greatest for geographically close rivers (Bentsen, 2000).This means that the salmon populations along the Norwegian coast are structured essentially according to what in ecology is called a "steppingstone model" (Kimura & Weiss, 1964.).
unless otherwise indicated.)
abiotic – of a non-biological nature, e.g. salinity, temperature etc.
allele – gene variant
anadromous – life history in which reproduction takes place in fresh water while portions of the years of growth take place in the sea
coadapted gene complexes – (= coadapted gene pools) a population or set of populations in which the genotypes are composed of alleles on two or more loci which in combination provide higher fitness compared with hybrid individuals and/or their progeny
fitness – (w) a measurement of individuals’ potential for survival and reproduction (Graur and Lie 2000, p. 41)
gamete haploid – germ cells (with n alleles)
gene pool – total quantity of genes in a population
genetic drift – evolutionary force in which alleles are lost as a result of chance. Increases as the effective population size shrinks
heterozygosity – percentage of heterozygous loci in a population
inbreeding depression – condition of a population caused by inbreeding in which fitness is reduced as a consequence of recessive, negative genes are found in an abnormally high percentage of heterozygotes
locus (pl. loci) – area on a chromosome where a gene is located
maternal effects – inheritance that is not directly coded for in genes, transmitted from mother to child but not necessarily to subsequent generations, e.g. ovum size/nutritional value and disease (Futuyma, 1998)
Ne – effective population size
neutral locus – locus with more than one allele, where the various alleles have identical fitness. Selection cannot affect such.Thus they can provide important information on the other evolutionary forces: genetic drift and migration
phenotype – characteristic(s) of an organism caused by one or more alleles