The perch, i.e. Perca fluviatilis, is a highly adaptable fish species. It is a freshwater fish that also lives in the low salinity coastal waters of the Baltic Sea. Its occurrence outside the zones of the archipelago is mainly limited by the cold. It can even withstand temperatures as high as 30°C for a short time. Also, it can compete successfully for its food and habitat with almost all indigenous species of fish.
Juvenile perch feed on zooplankton, such as rotifers, and at one or two years of age they change to benthic animals, such as crustaceans and insect larvae. While the proportion of fish as the main food item increases as perches grow, there may be major differences in diet between individuals. Some specialise early as piscivores, while others continue to feed throughout their entire lives on benthic animals. Fish eaters grow faster than those which feed on benthic fauna. Fish eaters will readily devour all fish species of a suitable size, even taking their fellow species as prey.
Perch are the target of fishermen and predators alike
Both commercial and recreational fishermen fish for perch on the coast. The perch caught in nets are almost exclusively females because they grow faster and larger than most males. By contrast, during the spawning season fish traps produce mainly male perch.
The catch of perch in recreational fisheries is many times higher than in commercial catches. Annual fluctuations in catches are large since the abundance of a perch’s year class depends on warm summer temperatures. The juveniles must reach a sufficient size to survive their first winter.
According to research findings, the abundance of pike-perch, i.e. Sander lucioperca, is a limiting factor for perch due to predation and food competition. Juvenile and adult perch are also caught by pike and burbot, as well as by larger specimens of perch. On the coast, perch is also an important prey species for cormorants.
Perch benefit from eutrophication by exhibiting site fidelity
Although perch usually wander within a radius of a few kilometres in coastal waters, individuals have been observed to travel distances over 100 kilometres. Early development takes place in waters protected by the archipelago or in rivers and streams connecting small lakes to the sea. Flad lakes are also important breeding sites due to their higher water temperatures. The breeding area must have a suitable substrate over which to lay their spawn, which looks like sticky white ribbons. This includes aquatic vegetation, submerged trees and sticks, and sometimes rocks.
In order to reproduce, perch require a suitable substrate, e.g. aquatic plants, on which to deposit their spawn, which resembles sticky white ribbons.
Although perch benefit from eutrophication, in very nutrient-rich, dark and turbid waters, its competitiveness is impaired because it hunts by sight. Due to global warming, perch have also increased in the northern parts of the Finnish coast. However, in southern coastal waters, perch numbers may decline because competing species such as pike-perch and roach are better adapted to eutrophic and turbid waters.
Developing perch juveniles can be seen within the spawning ribbon.