Ecological effects of escaped farmed salmon

Tid lakseyngelen gjemmer seg etter eksponering for en modell av en predator. (etter Einum & Fleming, 1997)

Publish date: November 26, 2008

Escaped farmed fish affects populations of wild fish. In addition to increased competition for food and habitat the farmed salmon can dig up and destroy the wild salmons spawning beds.

Impacts in the ocean

Numerous studies have shown that the percentage of farmed fish in the ocean can be up to 50% several places (e.g. Fiske et al., 2001).This can have implications for wild stocks in two areas: transmission of disease and competition for food. However, problems with designing studies that can demonstrate this quantitatively mean that to Bellona’s knowledge there are currently no data on this.


Later migration

Farmed salmon migrate up rivers later than wild salmon (Lura & Sægrov, 1991; Fiske et al., 2001).The reason for this is partly environmental and partly genetic: being raised in hatcheries and sea cages make the fish less imprinted by location.They often migrate back to the area that they escaped from, and from there up adjacent rivers. This delays farmed salmon compared with wild stocks. In addition, selection for late sexual maturity (see Table 1) will have a reinforcing effect outside the fish farm, which affects wild salmon populations in many ways. Furthermore, as a rule, the angling season tends be concentrated around the time before most of the farmed salmon migrate (Fiske et al., 2001).This means that it is primarily wild salmon that are subject to being caught by anglers. Another consequence is that the farmed fish ruin spawning beds where wild stocks have already spawned. Fleming et al. (1996; 2000) showed that farmed fish largely display spawning behaviour that yields highly unsuccessful attempts at spawning and spawning beds.


Growth in the river

Einum and Fleming (1997) conducted laboratory experiments as well as stocking tests with parr (salmon upto two years old).They compared farmed salmon from the Aqua Gen breeding station in Sunndalsøra with firstgeneration descendants of wild stocks from the rivers Imsa and Lone as well as hybrids from crossings between farmed fish and wild salmon. In sum, the laboratory experiments concluded that the farmed parr behaved differently compared with the wild salmon, and that the hybrids were somewhere in between.

In the experiments with parr, the farmed fish were shown to dominate wild stocks from Imsa. Again, the hybrids were somewhere in between. For the Lone fish, the hybrids were dominant.This shows that all the dominant fish had at least half of their genes from farmed fish. An estimate of aggression yielded a corresponding result. 

It may seem paradoxical that a breeding programme has selected for aggressive fish. Unintentional selection of behavioural traits is known from a number of animals; for instance, the breeding of animals such as lions and tigers in zoos has selected for less aggressive individuals (Gilligan & Frankham, 2003).That the opposite has been the case for salmon makes sense, since rapid growth in an environment with a nearly unlimited supply of food and the absence of predators (discussed in Fleming & Einum, 1997). Fleming & Einum (1997) conclude that farmed fish have altered many traits that make them poorly adapted to a natural environment compared with wild stocks. 


In another experiment (Einum & Fleming, 1997) fish were exposed one at a time to a model that was supposed to resemble a potential predator fish.The fish were subjected to a simulated attack from a predator in a tank with only one hiding place. The time before they emerged from hiding, as well as the time they stayed for longer periods (more than a minute) outside the hiding place, was recorded (see Fig. 1).The experiment yielded significant differences between farmed fish vs. hybrid fish and hybrid vs. wild fish. Similar findings have been made for rainbow trout Onchorhynchus mykiss (Johnsson & Abrahams, 1991; Brejikian, 1995).This may indicate that this type of behaviour in salmonids has a strongly heritable component and that several genes are involved. 

Fleming & Einum (1997) used the same farmed line from Sunndalsøra, while they took wild fish from the river Namsen, which has provided most of the genetic basis for the original parent fish for this farmed line (Gjøen & Bentsen, 1997). The results of Fleming & Einum (1997) are also supported in the literature (Johnsson & Abrahams, 1991; Brejikian, 1995; Einum & Fleming, 1997); farmed fish are significantly more aggressive than wild fish.  

Ever since 1975, growth speed has been selected for in farmed fish (see Table 1). All studies Bellona is familiar with show that farmed fish and their progeny grow faster than wild fish. Fleming et al. (2000) showed that that progeny of farmed fish and wild stocks compete for habitats and food in the river. Domineering and aggressive progeny of farmed fish will therefore give wild stocks keen competition. 

From experiments in the wild (McGinnity et al., 1997; Fleming et al. 2000) we know that wild salmon’s production of emigrating smolt is sharply reduced when they grow up in a river together with the descendants of farmed salmon and hybrids. In Fleming et al., (2000) the reduction was greatest for the progeny of wild female salmon; more than 30% fewer than expected. In this study, salmon fry and smolt grew up without competition from older generations, and the result may therefore be an underestimation of the problem. Nevertheless, this shows that the productivity in a salmon stock drops when farmed salmon infest the river during the previous generation. The apparently paradoxical result shows that wild salmon populations are expected to shrink in the following generation when individuals are added that originate from fish farms.


Reduced success in spawning in escaped farmed fish

Fleming et al. (2000) released wild and farmed salmon in a restricted area in the river Imsa. Parallel with this, farmed and wild fish were kept in an artificially created spawning area inside a laboratory. From this experiment they discovered that farmed salmon had poorer success in spawning than wild salmon (the farmed males had just over 20% of the success in spawning of the wild males. For the females, success in spawning was over 30% of that of wild fish.); the farmed fish displayed unsuitable spawning behaviours; made fewer spawning beds; the farmed females did not use up the milt (most remained, even after spawning); and the roe was both smaller and had lower survival rates than those of the wild salmon. In addition they saw that the descendants of farmed fish migrated earlier out to sea and were smaller in size. 

In the study of Fleming et al. (2000) the wild fish, farmed fish and hybrid fish were marked and then allowed to migrate to the sea.The fish were then recaptured when they returned to the river as sexually mature. On the basis of this, the lifelong fitness of the farmed salmon was calculated at 16% of that of wild stocks. 

The percentage of sexually mature parr is lower in the farmed salmon (about 20%) compared with the wild salmon (about 45%) (Fleming & Einum, 1997). Although this is a natural change in the life history resulting from selecting for late sexual maturity (see Table 1), at the same time this will change the balance between these two life histories that are both evolutionarily stable strategies (ESS). How this will change the population sizes in the wild has, as far as we know, not been investigated, and what the impact will be is therefore uncertain. 

Einum & Fleming (1997) conclude that farmed fish outcompete wild fish at certain life stages and thus displace the locally adapted wild stocks. At other stages of the lifecycle, however, farmed fish will be less well adapted than wild stocks, and the population will thus decline. The results of this study may indicate that the risk of population reduction is greatest in populations with a lot of predation and slow growth.