How does fish farming affect wild fish, and why do fishermen experience the quality of wild fish near aquaculture facilities differently from what scientists have documented?

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Lidunn Mosaker Boge  

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Scientists have reviewed documented knowledge of how wild fish are affected by aquaculture facilities in their vicinity, and have also carried out their own investigations to find answers. What fishermen experience, and what research indicates, is not always the same.

“Relatively little research on how fish farming affects wild fish has been carried out. Some fishermen and fish buyers find that their experience is not the same as findings from research.  We are not comfortable with this disagreement, and want to understand what it is that makes the perception of quality so different from what has been documented,” says Bjørn-Steinar Sæther at Nofima.

A research project where the effects of fish farming on wild fish was investigated, is now complete. The project was a collaboration among Nofima, the Norwegian Institute for Nature Research (NINA), the Norwegian Institute of Marine Research, and the University of Alicante, and was financed by the Norwegian Seafood Research Fund (FHF). It started as a result of inquiries from fishermen and fish landing facilities, with respect to the poor quality of wild fish that had eaten salmon fodder near farming facilities.  The first mention of such effects came from Ryfylke, and over recent years, we have heard of similar situations along several parts of the coast.

“For fisheries and aquaculture to be able to live side by side, it is important to acquire knowledge about issues when disagreements arise. There is increasing competition about the use of coastal space, and we ought to have as a goal that prioritising of how it is used is based on knowledge,” says Sæther.

Small difference in quality

The project has carried out an extensive literature review of research worldwide, and has summarised this.  In addition, scientists carried out field studies where they looked more closely at individual conditions. Initially they investigated the quality of saithe that are caught near fish farms, and compared this with saithe in areas without fish farming facilities. Scientists found a difference, but it was not big.

“The saithe that had eaten fish farm fodder had somewhat softer and more fissured muscle than other saithe, but it was still within the good quality category. Such a change in quality, however, is usual for fish that have good access to food; for example cod, when it preys on capelin,” says Sæther.

The biggest difference scientists found had to do with “pellet saithe” (wild fish that fed on salmon fodder) that had been caught in nets and had died while in the net.  These fish spoil more rapidly than other fish.

“We have found some answers; still, there is more that we wonder about. How could it be that there is such a big gap between our results and what fishermen actually experience? We would like to cooperate with them to understand why we experience the fish so differently,” says Sæther.

Next generation wild fish

But what of the offspring of wild fish that eat salmon fodder? Will the parent’s diet affect the next generation of wild fish? The common diet of wild fish contains marine fat, but there is not enough of this. A large portion of plant fat is therefore used in salmon fodder. The project group wishes to determine whether the wild fish roe could be adversely affected.

The scientists caught saithe and cod, held them captive, alive in cages, and gave them salmon fodder over two spawning seasons.

The cod roe and fry were then followed. Both the fish and the roe had an increased content of vegetable fatty acids, but scientists found no essential negative effects on the quality of the roe or fry, which developed normally.

Knowledge of the impact from fish farming:

  • Nutrients in the sea: 
    Emissions of nutrients from aquaculture facilities along the Norwegian coast are relatively small in relation to what is supplied naturally. In general, there is little risk for significant global and regional over-fertilization, perhaps with the exception of local effects in places where there is especially poor water exchange/circulation, or where there is intensive fish farming.
  • Effects on the seabed:
    Current conditions and depth below the facility determine if the seabed is affected by the descending salmon fodder and faeces. If conditions are shallow with little current under the facility, or if it is located furthest inland in a fjord, the seabed under the facility will be affected more than if conditions are deeper with more current, often in outer fjord areas. It is considered very unlikely that the seabed will be permanently affected in open coastal areas and large, deep fjords, but there may nonetheless be a significant overall local impact in areas with a lot of fish farming.
  • The quality of wild fish that eat salmon fodder:
    Fishermen and fish landing facilities have experienced that wild fish that have fed on salmon fodder outside the cages are of poor quality and cannot be traded. Research shows a small average quality difference between wild fish that has fed on salmon fodder and other wild fish, but that the quality is clearly reduced in some individual fish.
  • Impact on wild fish offspring:
    Wild fish that have more access to food can also produce more roe; but there is a question as to whether fodder ingredients might be unfortunate for the offspring. Experiments where wild fish are fed salmon fodder showed natural development in the next generation.
  • Less fish for fjord fishermen:
    Since fishing nearer than a hundred metres from an aquaculture facility is not allowed, it is thought that access to wild fish is limited in fish farming areas, but it may also be that the amount of wild fish will increase. The figures available are not good enough to draw a conclusion about this. If large-scale fishing is organised for wild fish that are attracted to aquaculture facilities, there should be a plan to avoid over-fishing the local area.
  • The behaviour of wild fish that are outside the cages:
    Wild fish that feed on fodder that falls out of the cages can help to reduce the environmental impact on the seabed below. Simultaneously, it cannot be ruled out that wild fish outside the cages may help spread pathogenic bacteria or viruses among the facilities, or to wild populations.  Wild fish outside cages may also feed on escaped farmed fish, and there is some speculation that predators may bite holes in the cages and thus cause escape, but this has not been documented.
  • Sexual maturation and spawning conditions:
    Wild fish that feed on salmon fodder achieve better growth and fitness, which can lead to earlier sexual maturation. It is possible that early sexual maturation may affect spawning time and journeys to spawning grounds, but this has not been investigated.
  • Behaviour of wild fish on spawning journeys:
    Wild fish are attracted to the access to food, and there is a worry that journeys to spawn will be delayed because journeying wild fish will stop to eat outside fish farm cages. But it is also claimed that wild fish avoid fjords with fish farms.  It is very difficult to carry out controlled investigations of changes in the behaviour of wild populations. Investigations must be made of the behaviour and distribution of fish in the area before, during and after the establishment of a fish farm. So far, such surveys have not been conducted.

Balancing act

The literature review of the effects shows that aquaculture can affect the biology of a number of marine organisms. But the effects can vary among different species, life stages and other ecological factors. “Different interest groups may experience the effects differently; both in a positive and negative direction, depending on the perspective of the various groups.

“For those making decisions about how the coastal environment will be used, it will be a balancing act between the ecosystem and various stakeholder concerns.  It is important that those who consider this take the entirety of ecological and social concerns into account,” concludes Sæther.


Toledo-Guedes K., Ulvan E.M. and Uglem I. (2016). Commercial gillnetting is more stressful for saithe (Pollachius virens L.) than jigging: but is fillet quality affected? Aquatic Living Resources. DOI: 10.1051/alr/2016013

Skjæraasen JE, Devine JA, Godiksen JA, Fonn M, Otterå H Kjesbu OS, Norberg B, Langangen Ø, Karlsen Ø. (Accepted for publishing). Timecourse of oocyte development in saithe Pollachius virens. Journal of Fish Biology. DOI: 10.1111/jfb.13161

Fourdain L., Arechavala-Lopez P, Uglem I, Sæther B-S, Sanchez-Jerez P (Submitted). Variations of trace elements composition on wild saithe (Pollachius virens) populations under the influence of coastal Norwegian salmon farms

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