Climate change affects the species of fish found in Finnish waters
The warmer air temperature caused by climate change would also increase water temperatures. Increasing rainfall would increase the volume of surface runoff from land to water, which causes eutrophication in water. At least a few cyprinids would benefit from warmer, more eutrophic waters. Conditions for salmonids would worsen. Increasing rainfall and surface runoff would also decrease salinity levels in the Baltic Sea, which would make the survival of saltwater species more difficult in the sea areas near Finland.
At sea and in inland waters, as a pastime and a livelihood
More than 1.5 million Finns, which is approximately one third of the country's population, engage in recreational fishing. The majority of them fish in inland waters. Professional fishermen in Finland number just under 3,000, and the majority of them fish out at sea. The number of professional fishermen has been decreasing in recent decades. Based on the prices per kilogram paid to professional fishermen, the total value of fish caught by Finns is just under 80 million euros, of which more than half is attributable to recreational fishing. Baltic herring caught by professional fishermen out at sea accounts for approximately half of the total weight of the catch. The European sprat is the second most popular species fished by Finns. 
Most of the fish caught in inland waters in terms of both weight and value is caught by recreational fishermen . It is largely thanks to recreational fishing that Finland is one of the leading countries in freshwater fishing in Europe . In terms of weight, most of the freshwater fish caught in Finland are perch, pike, roach, vendace, and zander. The most important species in terms of monetary value are perch, pike, zander, vendace, and lavaret. 
Considerably more than half of the fish caught at sea are Baltic herring and European sprat caught by professional fishermen. Recreational deep sea fishermen focus on perch, pike, lavaret, and zander, which fetch a considerably higher price per kilogram than Baltic herring and European sprat. 
Recreational fishing is closely linked to fishing tourism, which has a monetary significance to society many times that of just the value of fish caught recreationally. In 2005, Finland had more than one thousand tourism entrepreneurs who made at least some of their living from fishing tourism. 
The deterioration of the quality of habitats and overfishing have caused some fish populations to decline in Finland as well. The worst affected include the Arctic char, the Atlantic salmon, the brown trout, and the Atlantic cod.  
Rising temperatures accelerate growth but complicate other aspects of fish life
Rising water temperatures accelerate the metabolic rate of fish and may therefore accelerate fish development and ontogeny and increase fish size. Faster growth in the summer may improve the possibilities of fry to survive over the winter.  However, the accelerated metabolic rate increases the need for nutrition, and fish cannot grow faster if food is not available: Rising temperatures may also accelerate and increase the growth of the species that fish eat, but the effect may not be as significant and it may not be contemporaneous with the effects on fish. Nutrition may therefore not be available when fish need it. For example, the young of species that predatory fish feed on may, due to changes in water temperatures, hatch at the wrong time from the predators' perspective.  Primary production, which increases with temperature, may also increase the size of fish populations: Greater primary production means more food for species on the lower levels of the food chain, which ultimately results in more food for fish on the top levels.
Fish use changes in their habitats as incitements for regular changes in their behaviour and physiology. For example, if lower water temperatures and lack of light are what trigger the spawn of autumn-spawning fish, spawning may not be triggered if temperature no longer drops at the familiar rate when the availability of light decreases. 
Many species of fish also have unique thermal optimums . For example, spawning may only be successful within a certain temperature range . Higher temperatures may also increase stress levels in fish, which makes them more vulnerable to pathogens . Many species of fish are at their most vulnerable to environmental changes before they reach maturity. Long-living and rapidly maturing species have the best ability to adapt to changing conditions. 
Rainfall affects fish through surface runoff
Rainfall and spring flooding cause nutrients and solids to leach from land to the water. Nutrients cause eutrophication, and solids cause turbidity in water. Rainfall and surface runoff, which may increase with climate change, can therefore indirectly cause aquatic habitats to become more favourable to species of fish that prefer eutrophic and murky waters .
Once dead, eutrophic plant matter sinks to the bottom of water bodies and consumes oxygen as it breaks down. This may cause hypoxia and therefore make the bottom layer of water bodies hostile to life .
Rainfall and surface runoff from land to the sea lowers the salinity of seawater. Salt levels are already relatively low in the Baltic Sea, and increasing surface runoff would make conditions less favourable to saltwater fish.  On the other hand, the rising sea level may increase the salinity of the Baltic Sea, which would offset some of the dilution effect of surface runoff . Increasing surface runoff is also likely to reduce the number of inflows of saltwater from the North Sea through the Danish straits. These inflows bring salty, high-oxygen water into the Baltic Sea. 
Surface runoff may also contain compounds that cause acidification in some areas . Acidification is also caused by atmospheric carbon dioxide dissolving directly into water . Acidification is harmful to most species of organisms .
Ice cover both benefits and harms fish
Ice cover on the sea, lakes, and rivers in the winter reduces variation in the conditions found in water bodies compared to the ice-free season. Ice cover keeps water stratified according to temperature, which ensures that the water column also includes water at temperatures above freezing in the winter .
On the other hand, winter stratification in lakes may cause hypoxia in the bottom layer, which is harmful to the entire aquatic ecosystem. Early onset of the spring melt may increase summer stratification in lakes and along the coastal deeps of the sea and therefore increase the possibility of hypoxia in the summer.   The ice cover season in Finland's inland waterways has become shorter with global warming .
Rising temperatures are disadvantageous for salmonids
Rising water temperatures are perhaps more of a threat to salmonids than to any other species found in Finnish waters, because many of them rely on cold water  . Problems already experienced by salmonids have been attributed to global warming at least in North America . For example, the Arctic char is known to be more vulnerable to pathogens in warmer temperatures . On the other hand, the overwintering success of species that winter in fluvial waters, such as the Atlantic salmon and the brown trout, may increase when less ice forms under the water's surface , if winters become milder.
Causal relationships may also be more complicated: In approximately a hundred years' time, water in Lake Päijänne is estimated to not be cold enough for vendace to spawn until December. At the moment, vendace spawn in Lake Päijänne between October and November. The eggs would nevertheless hatch around the same time in the spring. With higher mean temperatures, vendace fry would therefore be born into warmer water than they do now, and their food consumption would be higher that that of fry born in cooler water at the moment due to their accelerated metabolic rate. Predatory fish that feed on vendace fry could also be hungrier in warmer water.  Vendace would therefore have to adapt to at least three changes.
Many fish in the cyprinid family prefer relatively eutrophic water and can tolerate low oxygen levels. Many of them also favour warm water at least during spawning.  The combination of eutrophication and rising temperatures improves conditions for many cyprinids, at least the common roach and the common bream  , and for some Perciformes, such as zander and perch  , in both inland waterways and the sea . The numbers of roach have already increased in the Baltic Sea and in parts of Russia . However, the increasing numbers are likely to be attributable to just eutrophication at this point rather than to climate change, at least in the Baltic Sea. Species that are able to survive in many different kinds of habitats are also more adaptable to climate change. Highly adaptable fish that survive in all kinds of conditions include at least perch, pike, and roach. 
The effects of eutrophication and global warming are likely to be the greatest in shallow lakes that are vulnerable to eutrophication and less likely to stratify according to temperature . Fish that favour colder water have nowhere to escape in unstratified lakes .
Decreasing salinity affects species in the Baltic Sea
The decreasing salinity and increasing infrequency of saltwater inflows into the Baltic Sea are likely to hinder the breeding of at least the Atlantic cod and therefore lower their numbers. The fates of the Baltic herring and the European sprat depend on each other and on the future of cod: The Baltic herring and the European sprat compete with each other, and the Atlantic cod is one of the sprat's most important predators.  
The decreasing salinity of seawater may create new breeding grounds for the common roach along the coast of the Baltic Sea. The increasing infrequency of spring flooding may nevertheless prevent this: Dilution may no longer peak in the spring when the roach spawn.   It is uncertain whether other species of freshwater fish can offset the losses incurred by fishermen as a result of the falling numbers of marine species either .
On the move northwards
Even if water temperatures were to rise across the country, species that rely on warm water may not be able to disperse northwards in inland waterways very quickly. Escaping northwards will also be difficult for species that prefer colder water, such as lavaret, grayling, and vendace. Our inland waterways do not always provide direct routes for fish to disperse from one lake to another. The new habitats may also have characteristics that the old habitats did not, such as competing species, which may make conditions more difficult for new arrivals.  
Dispersal will be easier in the sea. One species that is already becoming increasingly common and well-established is the Prussian carp (Carassius gibelio), which has presumably dispersed from the Estonian coast to the shores of the Gulf of Finland. It has won territory from some fish species of greater economic importance in Central Europe. In some countries, however, it has become popular among anglers.  In the future, Finnish fishermen can expect to catch species such as swordfish (Xiphias gladius), the common ling (Molva molva), and haddock (Melanogrammus aeglefinus), which at the moment are still mostly only found to the west of the Danish straits.   This nevertheless depends on the ability of the new arrivals to survive in the low salinity levels of the Baltic Sea .
Climate change may also bring new pathogens and parasites from the south to Finnish waters , as well as species that compete with fish or are otherwise harmful  . On the other hand, some of the arrivals may be an excellent addition to the current diet of our fish, such as in the case of the fishhook water flea and the Baltic herring  .
What and how will we fish in the future?
Changes in the species of fish found in Finland affect the future of fishing. Maintaining current levels of sprat fishing may require more and more of our professional fishermen to head out towards saltier waters in the southern parts of the Baltic Sea.
The value of the catch, or at least the price per kilogram, is likely to decrease in the future as cyprinids become more numerous. On the other hand, the predicted increase in the numbers of the more valuable zander and perch, for example, could counteract this. 
The shorter ice cover season and thinner ice are likely to worsen conditions for catching vendace by means of seine fishing in the winter. Ice fishing in the spring will also become more dangerous. On the other hand, fishing methods developed for the ice-free season, such as trawling and lure fishing, can be used later in the autumn and earlier in the spring. In order for trawling, for example, to increase, however, the higher temperatures need to keep the surface of water completely free of ice rather than just increasing the volume of pack ice. Climate change may also increase the frequency of storms that prevent fishing.  
Climate change is just one factor among many
The future of our fish species also depends on fishing regulations to prevent overfishing and on fish stocking, for example . Even if climate change was to stop right now, the Atlantic salmon and the brown trout, for example, would have a bleak future in the Baltic Sea without regular fish stocking and the building of fish ladders. Eutrophication as a phenomenon is also not dependent on climate change.
Climate change alone does not determine what the future holds for fish and fishing in Finland. How much we fish, what we fish, and how we look after our fish populations affect our fish at least as much.