CROATIAN FAUNA – FRESHWATER FISH, The European catfish

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