Introduction
Global warming will gradually increase the temperature of Norwegian and European coastal waters and have significant effects on the aquaculture industry. Thus, optimal farming conditions for the species common in European and Norwegian aquaculture today will gradually move northwards in a parallel shift. Unlike wild fish stocks, farmed organisms are quite literally locked into specific localities, and conditions may be less suitable for the species biological In many places, fish farms will have to be relocated and farming technology modified in order to reduce the undesirable effects of higher temperatures. Current sites will in turn be suitable for other species than those farmed today, and cultured species that are presently farmed in Southern Europe could gradually enjoy better conditions in Norway than further south. Examples of species that may be more important in the future are turbot, sea bass, sea bream, oysters and scallops.
Aquaculture is a relatively new industry in Norway and Northern Europe, and in contrast to the fisheries, it lacks the historical reference to, and experience with, natural changes in climatic conditions. The fisheries have adapted to natural climate cycles for centuries. In contrast, the cold winters along the Norwegian coast during the early part of the 20th century would have caused severe problems for the salmon farming technology of today, if it had been present. Furthermore, despite the dramatic global context, most models of global warming point out that the expected changes will be relatively slow in the context of industrial life-cycles. Although there is every reason to expect, and plan for, a gradual movement of the aquaculture industry northwards, there is no reason to quit salmon farming in Southern Norway tomorrow.
Which changes are important?
Studies of the future climate show that air temperatures will rise by 2-4°C during the 21st century. In the seas off the coast of Norway, the temperature will raise by 1.5-2.0°C (Stenevik and Sundby 2007). This implies that the temperatures presently found in Southern Norwegian coastal waters will be common along the coast of Northern Norway in a 50-100y perspective. The change in the mean temperature is however not a major obstacle for farming of the presently most important species in Southern Norway, Atlantic salmon, rainbow trout and Atlantic cod. On the contrary, culture of cold-blooded animals would in principle benefit from higher temperatures up to the point where they exceed or compromise the biological limits of the organisms.
However, in a biological context extreme temperatures may be more important, especially considering the impact of high temperatures on the immune system and the proliferation of pathogenic agents. Periods with high temperatures suboptimal to the fish will be longer, and events with temperatures higher than the biologically safe limits of the farmed species will be more common.
Other major effects of the expected climatic changes along the Norwegian and North European coast the next century will be increased intensity and frequency of storms and increased rain, plus a moderate increase of high tidal level. Such effects will cause a need for improved technological solutions, and significant investments may be needed. However, they do not constitute "impossible" biological or technological problems. Intense storms may, however cause damages to fish farms, and increase the with high numbers of escaped fish. However, as escapees are considered a major environmental problem, it should be anticipated that improved technological solutions are developed and implemented.
Diseases
Many diseases may occur more frequently in warmer weather, particularly bacterial infections with bacteria or parasites adapted to relatively high temperatures. Just as important as the temperature range of bacteria and parasites, is the temperature range of the cultured fish. Rearing a species at too high or low temperatures inevitably may compromise the immune system, leading to increased disease problems. For instance, important diseases such as francisellosis, vibriosis, furunculosis, as well as several parasites are typically associated with high water temperatures. On the other hand, winter ulcers and cold water vibriosis are typical examples of diseases occurring in cold waters. In this context, the extreme temperatures are much more important than the middle temperatures.
Reproduction of parasites like salmon lice is temperature dependent, thus shorter generation times, and thereby increased infestation rates may be expected with higher average temperatures.
In addition, spreading of salmon lice may be altered by increased freshwater along the coast, particularly in the fjords. Improved modelling of water movements in fjords and coastal environment in various climatic scenarios may be a useful tool for predicting changes, as is the case for predicting salmon lice migration today Asplin et al. 2014, Johnsen et al. 2016). It should be noted however, that hydroelectric power plants may have a greater impact on hydrography than the anticipated climatic changes in this century.
However, not only the pathogenic agents are subject to a changing environment. Also, the immune system of the fish will be, and typically extreme temperature events may be anticipated to increase susceptibility of the fish to diseases. Exposure to temperatures outside the ecological niche will normally affect the immune system.
References
L.A. Asplin, I.A. Johnsen, A.D. Sandvik, J. Albretsen, V. Sundfjord, J. Aure and K.K. Boxaspen. 2014. Dispersion of salmon lice in the Hardangerfjord. Marine Biology Research 10(3):216-225.
I.A. Johnsen, L.C. Asplin, A.D. Sandvik, R.M. Serra-Llinares. 2016. Salmon lice dispersion in a northern Norwegian fjord system and the impact of vertical movements. Aquaculture Environment interactions. 8:99-116.
E.K. Stenevik, and S. Sundby 2007. Impact of climate change on commercial fish stocks in Norwegian waters. Marine Policy 31:19-31