Brown trout (Salmo trutta), Atlantic salmon (Salmo salar) and Pink salmon (Oncorthynchus gorbuscha). Photo: NOAA Great Lakes Environmental Research Laboratory - BY-SA

Effects of climate change on salmonids

Salmonid fish are sensitive to increased water temperatures. Decreased precipitation, forest fires and ocean acidification are other threats that come with climate change.

Salmonid fish all originate in the Northern Hemisphere. Most species occur as freshwater, landlocked or anadromous populations, where the latter spawn and evolve as juveniles in fresh water and then swim downstream to the ocean where they grow until they are ready to migrate back to freshwater habitats like brooks, rivers or lakes to spawn and thus complete their life cycle.

This article deals with salmonid species of two genera only, viz. Oncorhynchus and Salmo, the species of which normally exist as anadromous populations, even if numerous populations occur as landlocked or stream dwelling.

All species described in this article have been and are of immense economic importance to the peoples and countries where they occur naturally, and in countries where some of them have been introduced. In the former group of countries the fish are caught both commercially and by sport fishermen and are very important in aquaculture. In the latter group of countries where these fish have been introduced and sometimes have formed self-sustaining populations, they are also of great economic importance, mainly to anglers. The latter group includes countries such as New Zealand, Chile and southern Argentina. Sea-run European brown trout have also established self-sustaining populations in Argentina.

Pacific salmon occur in coastal (marine) and/or river waters, from Alaska, western Canada and eastern Russia in the north to Taiwan and Mexico in the south. In this case we only considered the seven species of Oncorhynchus that are mainly anadromous, but this genus contains at least 12 recognized species. The Atlantic salmon and the indigenous anadromous brown trout occur on the high seas, and in coastal and river waters of the North Atlantic roughly from the White Sea in western Russia and in western Europe down to Portugal (salmon); both species also occur in Icelandic waters, but only salmon are found to the west of Iceland; in Greenland – in one river only; and in numerous lakes and rivers in eastern Canada and a few rivers in New England. The introduction of European brown trout has resulted in a few established anadromous trout populations in Newfoundland, Canada.

All anadromous salmon (of both genera) spawn in fresh water. The majority of species spawn in the fall and their fertilized eggs require some three to eight months to hatch and develop into juveniles. A few species spawn in spring and require only a few weeks before the eggs hatch. When they are young and remain in running water these salmonids forage on small crustaceans, insects and even plankton, but when they have returned to sea they become typical predators, often depending on schooling small fish such as herring, sand lance, capelin, and juveniles of many other species. In addition, Pacific salmon also prey on euphasiids, pteropods and squid.

After hatching, the juveniles still depend on their yolk sacs for several weeks, and eventually the juveniles begin their downstream migration, during which time they develop their tolerance to salt water. Depending on the species and habitat, juveniles remain in the river from a few months up to seven years.

Also, depending on the species, their growth period, maturation process and time of return to fresh water to spawn may vary between one and seven years.

In addition to their economic importance these fish have important ecological roles in their environments, particularly in the freshwater phase during which they are frequently the dominant species on which both the aquatic and terrestrial ecosystems surrounding their native streams depend. The latter fact applies in particular to the Pacific salmon.

Salmon are triggered by temperature increases. As water temperature increases some direct impacts on salmon biology can be predicted. These can include physiological stress, increased depletion of energy resources and increased susceptibility to disease. At extreme water temperatures in summer, massive fish kills may occur. Increasing water temperatures may cause rapidly developing juveniles to enter the sea before their planktonic food source is available in sufficient density. Increased summer temperatures may also present a thermal barrier which may delay or even prevent spawning.

In addition, loss of snowpack and less precipitation falling as snow will mean reduced stream flows in summer, and lower water flows in summer will also cause water temperatures to exceed the optimum temperature range for most salmonids. Reduced water flows will also impact the spawning migration of mature fish. Increased water flow in winter, due to most precipitation occurring as rain rather than snow, may also cause floods which will impact spawning habitats, by scouring river beds and causing physical damage to both salmon and trout eggs and young fish. It is reasonable to assume that increased input of fresh water will cause increasing sedimentation of river beds, which in turn will reduce the amount of gravel available for spawning redds.

Warmer and dryer conditions in summer have already substantially increased the number of forest fires, which may burn out root systems and contribute to increased erosion and silting in nearby rivers.

Predicting some of the specific effects of climate change on salmon in their marine environment is very difficult. This is due in part to our limited knowledge of where the salmon are in the high seas, but also uncertainties about how marine habitats at large will be affected. Warmer oceans will cause a northward shift in the range of salmon at sea, but probably more important are the effects of increased temperatures on the timing of new food webs, from planktonic species of importance to young fish, to higher levels of biological productivity. Disruptions of this timing may cause a scarcity of food in the salmon’s life cycle. Warmer ocean temperatures may also reduce the number of smaller fish, which could increase predation pressure on salmon.

Ocean acidification is a mounting problem impacting all levels of biological productivity in the oceans. The continued release of greenhouse gases, above all CO2, is rapidly acidifying the ocean. The most direct effect is that some zooplankton and some shelled phytoplankton are rapidly losing their ability to build shells, and so are most reef-building corals and mussels, clams, starfish and urchins, which have trouble building shells in more acidic water. The larger zooplankton are fundamental to the survival of young salmonids and of course to numerous other fish in the ocean.

Sea-level rise caused by warmer water and melting of the large areas covered by continental ice sheets will inundate estuaries, which is where most salmonids make their transition between the freshwater and marine phases of life.

Even at the present temperature level, which approximates a 2-degree temperature (C) increase at northern salmon latitudes, it is predicted that sockeye salmon on the Pacific northwest coast of North America may suffer a decline by 2050 as a result of climate warming. In 2015 it was estimated that exceedingly warm stream temperatures were to blame for the loss of an estimated 250,000 adult sockeye in the same area.

Although the study was carried out for Pacific salmon, there is no reason to believe that Atlantic salmon would not react in the same manner. Salmon migrating between freshwater and oceans are important vectors of energy in the two ecosystems they inhabit. The occurrence of short-term heatwaves will be most common in the southern areas of salmonid distribution, but a temperature increase will partly be compensated by new northern watersheds which are now too cold for most salmonids. Such an extension of northern distribution will probably require a long period of time to develop because of the strong homing behaviour of salmonids.

If the present increase in greenhouse gas emissions continues it can be estimated that the global ocean surface temperature may rise some 4 degrees (C) above the present level. This will change all aquatic ecosystems, freshwater and saline, at all biological levels – bottom fauna, aquatic flora, all the way up to fish, birds and marine mammals – and eventually also humans. Such changes are extremely complicated to predict in any detail but it is very likely that almost all anadromous salmonid populations will vanish, and even at higher latitudes few populations will have the ability to adapt within the short time span of just one century. The melting of permafrost areas in the north will release methane, an even more potent greenhouse gas than carbon dioxide, which will compound the negative effects of global warming both in terrestrial and aquatic ecosystems. It should also be pointed out that such a temperature increase will be augmented by a number of other human-induced threats to salmonids, such as overfishing, habitat destruction, pollution, and obstruction of migratory routes. Surely, this suite of threats will all jeopardise the chance of survival of all salmonid stocks.

Lennart Nyman



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