CROATIAN FAUNA – FRESHWATER FISH, The brown trout

The brown trout (Salmo trutta) (Linnaeus, 1758) lives in rivers and streams of the Danube and Adriatic basins, as well as in lakes in its lake-dwelling form. The lake trout is considered a form that developed under the influence of a specific type of habitat and is regarded as a single species. The body is elongated and spindle-shaped, covered with small, round scales. The mouth is large, and the teeth are well-developed. Its back has black spots with white edges, while its sides feature red spots outlined in white or light blue. The body coloration is variable and depends on environmental conditions. It most commonly inhabits fast, cold and shallow mountain streams with temperatures ranging from 2 to 16°C. It typically grows up to 50 centimeters and rarely reaches 70 cm, while the lake-dwelling form can grow to an impressive 140 centimeters and weigh over 20 kilograms. The maximum recorded age is 38 years. It feeds on various animals: aquatic invertebrates, small fish, insects that fall on the water, tadpoles and frogs. Larger individuals may also prey on smaller rodents (such as mice swimming in the stream). It spawns by migrating upstream from October to January, with the female selecting a suitable spawning site. She uses her tail to create a depression in the rocky and gravelly bottom, where she lays her eggs, which are then covered with small gravel after fertilization. The damming of watercourses represents the main threat to brown trout, as dams divide the river into segments, altering the water regime and preventing migration to upstream areas. Pollution of watercourses and increased eutrophication also negatively affect trout, which live in clear, cold, oxygen-rich waters. This directly threatens their survival through suffocation or physiological toxicity and indirectly by degrading the food web on which they depend. Climate change can lead to hydrological changes that affect thermal regimes in rivers, which serve as an ecological stressor for the temperature-sensitive brown trout. A significant period of high temperatures is the most critical, as it can deplete stored energy and impact the capacity for growth, reproduction and survival. Additionally, deforestation along the edges of streams and rivers creates problems by altering microclimatic conditions, especially during the summer months. Due to its distribution, beauty and high-quality firm flesh with low fat content, the brown trout is one of the most important species for sport fishing. Conservation efforts can be based on several solutions: an effective response to climate change related to the removal of non-functional dams and obstacles that reduce river flow speed and dissolved oxygen concentration in the water, as well as a ban on deforestation along rivers and streams that serve as natural shade, and fishing restrictions with the establishment of reserves. dr. sc. Goran Jakšić, scientific assistant Aquatika public institution – Karlovac Freshwater Aquarium