Different fish species react differently to climate change. In addition to temperature levels, changes in precipitation, ice covers, water eutrophication levels, seawater salinity in the northern Baltic areas and near-bottom oxygen levels all have an impact on the numbers and distribution of fish through their ability to reproduce. Generally speaking, warmwater species will benefit from the changes, whereas coldwater fish will suffer.

The most significant changes will be seen in the behavior, distribution, numbers and growth of the fish which will, over time, be reflected in the number of fish caught. Studies have shown that, during warm summers, most fish grow better than in cold summers.

Rising temperatures are expected to extend the growing season of fish. Species that are likely to benefit from this include in particular fish that spawn in the spring and summer, such as perch, pikeperch, roach, silver bream, bream and tench. Rising water temperatures will be most harmful to species that prefer cold water, such as European whitefish, Arctic char, Atlantic salmon, brown trout, burbot and grayling. Rising temperatures might also increase the stress levels, interfere with reproduction and affect the juvenile stages of fish.

Salinity of the Baltic Sea might change, runoff water carries nutrients to sea

The increasing incidence of precipitation, flooding and runoff, as well as the change of their timing, will make it more difficult for migrating fish to run up the river branches to their spawning areas. As such, obstacles preventing fish from running should be removed and fish ladders should be built to ensure the continuity of in particular endangered fish species.  Changes in rain patterns have already increased and are expected to further increase eutrophication through nutrient runoff, which will benefit fish that live in water bodies with abundant aquatic vegetation, such as cyprinids. Together with higher winter sea temperatures and reduced ice cover, eutrophication will interfere with the reproduction of fish that spawn in autumn and winter.

The reproduction capacity of numerous brackish water fish species will be reduced, if the increased amount of rainfall and runoff reduces the salinity of the Baltic Sea below its current levels. The change can already be seen in the coastal European flounder populations as well in the scarcity of eastern Baltic cod and the increased abundance of its major prey fish, the European sprat. Vendace in the Bothnian Bay, on the other hand, might benefit from the change. However, it is also possible that strong storms will bring more saline pulses to the Baltic Sea, like during the 2014–2015 winter. Salinity has had a significant impact on the population and growth dynamics of Baltic herring.

Changing climate conditions are expected to introduce and increase the populations of alien species, such as Prussian carp and round goby, especially in sea areas. Some of the alien species are a threat to Finland’s native species, while others may be a food source for the current species in Finland. The changing climate may also aggravate the disease and parasite situations.

Not everyone manages to adapt in time – changes in fish populations are reflected in catches

Finnish fish species that have the least difficulty adapting to new habitats are perch, pike and roach. For pikeperch, higher water temperatures and eutrophication mean more ideal habitats as long as the oxygen levels in the body of water remain stable throughout the year. The European whitefish populations of southern Finland, on the other hand, might be pushed deeper into distress.

Changes in the fish populations affect the catch composition and consequently the value of the catches. Shorter periods of ice cover interferes with traditional winter fishing, and ice fishing seasons will also become shorter. Increased frequency and intensity of storms reduce the number of fishing days and soil and break fishing gears. The behavior of fish changes during warm winters with little to no ice cover, making them more difficult to catch. During winters like these, seals will interfere with coastal fishing nearly year round.

Luke studies the impact of climate change on fish and monitors the status of fish populations

Researchers of the Natural Resources Institute Finland have examined how climate change impacts fish and their spawning and nursery grounds as well as the related production conditions for example by measuring and modelling the temperatures and nursery grounds in archipelago areas.

Luke monitors the economically most valuable fish populations in the sea by e.g. monitoring the catches and contributing to the fish population estimates and recommendations regarding the fishes that are caught by several coastal countries (herring, salmon, sprat and cod) in the International Council for the Exploration of the Sea (ICES). Samples are collected from pikeperch, perch and whitefish caught by coastal fishermen, and the samples are used together with catch data to produce information about the status and development of fish populations.