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.
The common carp (Cyprinus carpio) (Linnaeus, 1758), in its original form, is today one of the most endangered fish species in Europe. In the Republic of Croatia, it is endangered in the Danube basin, while in the Adriatic basin, it has been introduced and is not considered endangered (it also lives in brackish waters). It inhabits rivers and standing waters with sandy and muddy bottoms, as well as dense aquatic vegetation, with water temperatures ranging from 10 to 30°C being most suitable for it. It has a robust body, with horizontal mouth openings and two pairs of barbels, with the barbels at the corners of the mouth being longer. The dorsal fin has a long base, and both it and the anal fin contain a hard, serrated spine. The color of the carp is highly variable and, despite its genetic predisposition, depends on factors such as diet, condition, age and other environmental conditions. The back is usually dark green, gray-blue or gray, while the sides are yellow-green to golden, and the belly is yellow-white. Due to the coverage of the body with round scales, changes have occurred during cultivation, ranging from the original form fully covered in scales to carp with a reduced number of scales (large-scaled and small-scaled), as well as scaleless carp (naked). These changes in its cultivation are explained by genetic mutation and selection. It can grow up to 120 centimeters and 50 kilograms, and it can live up to 50 years. It is omnivorous and feeds on mollusks, shrimp, insect larvae, adult insects, algae, aquatic plants and small fish. Unlike the brown trout, it is quite resistant to low oxygen concentrations and temperature changes, and it is also not bothered by water turbidity and is tolerant of organic pollution. It spawns from the end of April to mid-June in the shallow parts of the floodplain zone among aquatic plants, as well as in the calm areas of rivers, at a water temperature of 18 to 20°C. The main cause of endangerment is the mixing of wild and cultivated forms by releasing the latter into open waters, as this affects the genetic diversity of wild populations. Equally important is the reduction and disappearance of natural spawning habitats, such as floodplain areas. Furthermore, the carp has migratory tendencies, especially during spawning, and its migration is significantly threatened by the regulation of watercourses. The introduction of foreign invasive fish species, such as the Prussian carp (Carassius gibelio), has a negative impact on the carp, as it competes for habitat, food and leads to hybridization (the hybrids resemble both species, are fertile and produce offspring).
Interestingly, China has a tradition of carp farming that spans over 2,000 years, while Croatia has a tradition of over 100 years. The traditional interest in carp meat as a lean food is the reason for its introduction into many countries. To preserve the wild forms of carp, it is essential to protect the remaining natural spawning habitats, establish reserves and prevent the stocking of open waters with farmed carp.
The European catfish (Silurus glanis) (Linnaeus, 1758) inhabits the Danube basin and has also been introduced into the Adriatic basin, preferring calm areas with slower water flow, while not requiring high levels of dissolved oxygen in the water. During the day, it mostly rests in deeper areas or shelters made of aquatic plants and branches, as well as under roots along steep banks, while being active at night. Its eyes are very small, which indicates its nocturnal activity, although it also moves during the day when the water is stirred up and becomes murky. It is known that the catfish will search for food during summer storms, heavy rain and thunderstorms, moving even in the strongest water currents, and it will also venture into shallower areas. Its skin is thick and without scales, with a wide head and a mouth whose lower jaw extends beyond the upper jaw. Teeth are present on the jaws and palatine bone. It has well-developed sensory cells on its lips and barbels and is highly sensitive to sound. It has one pair of barbels on the upper jaw and two pairs on the lower jaw. The dorsal fin is very small, while the anal fin is extremely long and almost reaches the tail fin. The body color is olive green, the back is dark, the sides are marbled, and the belly is white or light gray. It is one of our largest freshwater fish, with recorded sizes of up to 5 meters and more than 300 kilograms, and it can live up to 80 years (a catfish weighing 306 kilograms was caught in the Dnieper, and according to written records, catfish up to 340 kilograms were caught in the Danube, while in our natural conditions, they grow up to 3 meters and more than 100 kilograms). Smaller individuals feed on zooplankton and aquatic invertebrates living on the bottom, while larger individuals hunt fish, frogs, ducks and smaller mammals. It spawns in the second half of May and in June, at water temperatures up to 20°C, and during this time, it is extremely aggressive. The female lays highly sticky eggs on aquatic plants. With movements of its tail above the eggs, it enriches the water with oxygen and clears it of sediment, while protecting the eggs from predators during incubation. Locally, river regulation destroys its shallow spawning areas. It has excellent meat with few bones, making it interesting for sport fishermen, as well as economically valuable.
Interestingly, catfish and carp, unlike trout, possess what is known as the Weberian apparatus, a series of modified anterior vertebrae that connect the inner ear to the swim bladder. This adaptation allows for improved hearing (as the swim bladder acts as a sound resonator) and hydrostatic control through the regulation of gas volume in the swim bladder. Additionally, the catfish's head has an interesting system of sensory canals that allows it to track hydrodynamic and chemical traces left by prey, which can be up to 10 seconds old and at a distance up to 55 times the length of the prey.
dr. sc. Goran Jakšić, scientific assistant
Aquatika public institution – Karlovac Freshwater Aquarium
