Fish

 Crucian Carp (Carassius auratus gibelio)

The Crucian Carp is one of the two subtypes of the gold characin or goldfish (Carassius auratus). The body is covered with scales that easily detach. The Crucian Carp has a set of gullet teeth at the end of the mouth. The approximate body length is up to 30 cm, the body volume can be up to 1 kg. Exceptional catches of Crucian Carp have been recorded, being 50 cm long and weighing 2 kg. The lifespan of the Crucian Carp is about 8 years (average-aged fish).
The most suitable life conditions for the Crucian Carp are lower and middle flows of larger rivers, warmer reservoirs, and fishponds with warm water. It can bear organically polluted water, high temperatures, and large concentrations of oxygen in the water. The Crucian Carp lives in shoals. Since it has easily adapted to our waters, this has enabled it to take control over other habitats. It is considered an invasive species, because it suppresses other indigenous fish species.
The Crucian Carp is an omnivore. It feeds on zoo benthos (animals from the bottom of the water), insects, detritus (sediment), seeds and plant parts.
Female triploid (mutants) populations prevail in our waters. They reproduce gyno-genetically (incomplete fertilization where the spermatozoids only activate ovaries to divide, but there is no mergence or unification between the nuclei of ovary and semen cells). The milt of other fish (carp, characin, tench, bream, roach and European chub) activates their laid eggs. Once again, triploid fertile females are created, not bastards. The Crucian Carp spawns in May and June at the temperature of 16 to 18 oC. The young fish develop in the first or second year of life. Females lay their eggs once or twice in the spawning season within an interval of 10 to 20 days. This type of fish is a phytophagous species (lays eggs on plants) and can occasionally be an indifferent species too (capable of laying eggs on any surface). Absolute female fertility varies from 10.000 to 100.000 eggs.

Distribution

The Crucian Carp is used in trade as a consume fish in our areas, but at a low market price, compared to other species. This species is not bred in fish farming, but is brought to the market exclusively as a fish caught in the nature.
There is no legal protection of the Crucian Carp in the Republic of Croatia. It is an invasive species in our waters, which is why traditional fishing and sport angling catch quotas in Kopački rit do not restrict the amount of Crucian Carp catches.

 Pike (Esox lucius)

The pike's body is torpedo-shaped. The dorsal and anal fins are arranged characteristically in the back part of the body, close to the caudal fin. This kind of fin arrangement is important for the pike because it enables him to accelerate rapidly while attacking his prey. The pike has very sharp teeth arranged in several rows on the lower part of the jaw. The teeth on the upper jaw are smaller and are closely arranged. The pike also has teeth in the mouth and even on the bones of the gill arcs. All together, the pike has about 700 teeth. He also has large eyes and sharp sight. The body is covered with scales.
The body length can grow up to 150 cm, whilst the average body volume is about 24 kg. The biggest catch weighed 65 kg. The pike's lifespan is about 25 years (long-aged fish).
The pike lives in calm slow-flowing or stagnant waters rich in aquatic plants from where he prowls his prey. The pike gets his bearings with his sharp sight, and ambushes his prey as quick as lightning. He is most active in autumn, throughout the day, but in summer usually early in the morning and in the evening. Sometimes he even hunts in winter during sunny days. The pike belongs to the most adjustable species of freshwater fish because he can bear extreme water temperature variations, organically polluted water, and low concentrations of oxygen.
The pike is a carnivore and a typical predator. The young fish feed on zooplanktons and zoo benthos (animals from the bottom of the water) and start hunting other fish at the size of 5 cm. The pike hunts almost all types of fish above the bottom of the water. Cannibalism has been noticed among specimens of the same size. Having the status of a predator in the food chain, the pike regulates the natural water balance by controlling the quantity of certain species like the bleak, bream, characin and others.
The pike spawns in the first months of spring at the temperature of 7 to 9 oC. Eggs are laid in flooded and flowing waters and on weedy terrains. The spawning is single. The males in our area become sexually mature in the first year, whereas the females do so in the second or third year. Absolute female fertility varies from 8.000 to 120.000 eggs.

Distribution

The pike is a valuable and consumable fish. In our area, he has been traditionally bred as a secondary species in carp fishponds. His market price is high, thus he is welcome as a catch in traditional and commercial fishing. This type of fish is also very popular in sport angling. Since he is aggressive and greedy, he himself becomes easy prey for anglers.
There is no legal protection of the pike in the Republic of Croatia, but the catch quotas are restricted in traditional fishing and sport angling.

 European Common Carp (Cyprinus carpio)

The carp is economically the most important freshwater fish in Europe. The original wild species (from the Black Sea, Sea of Azov, Caspian Sea and Aral Sea) has a laterally compressed body, moderately elongated and covered with large cycloid scales. The snout is blunt; the mouth is large with two pairs of short barbells. The color is dark on back and golden on the sides. The belly, pectorals and pelvic are light yellow, the ventral fin is orange and the caudal fin is grey with orange shade. Coloration changes depend on habitat.
The carp is a strong fish up to one meter long or even more and can weigh 20 kg (sometimes over 30 kg). It grows very fast in the first year but later can weigh less than ordinary carps. This type of fish reaches sexual maturity at the age of 3 or 4 – males are mature one year earlier. They spawn in May and June in shallow waters with a lot of vegetation at the temperature of 15°C. Their spawning is very loud and they often jump high out of the water. The eggs are attached to the vegetation and the incubation period last for five days. Then the young ones hatch at the temperature of 20°C (approximately three). They are 5mm long and cannot stay attached to the vegetation. When old enough, they feed independently on small organisms and later on larger zooplanktons. By the time they are 2 cm long, they are eating animals from the bottom of the water. Feeding depends on the water temperature. Carps stop eating in autumn, move to deeper parts of the water and become inactive. The European Common Carp lives in deep but still waters; in rivers and stagnant waters; in armbands and basins. Their diet consists of large aquatic insect larvae, mollusks, scales, worms etc. Plants are also very important for their diet. The carp usually goes hunting in the evening or at night.

 Catfish (Silurus glanis)

The Catfish is one of the largest fish of European waters. It has a long body without scales; a large head and an enormous mouth surrounded with 3 pairs of pulpy barbells – two on the upper part of the mouth and 4 chin barbels. The back fin is much shorter than the anal fin. The eyes are generally small and the nostrils look like short tubes. The colors depend on the habitat; the back is usually black, sometimes green, blue or brown; the sides are cream-colored while the belly has dark spots. This type of catfish can grow up to 2m long and can weigh over 100 kg. It lives in large and deep rivers and especially likes deep damns or embankments. If there is enough food, the catfish will grow very fast and by the time its 9 years old, will weigh 12-16 kg. It spawns in May and June at the temperature of 18°C in waters with a lot of vegetation. The female lays her eggs in a very messy nest; the male impregnates and guards them until they hatch. At first, the young ones feed on crabs and worms etc. Adult catfish feed on other fish (smaller species) but also prowl on frogs, small mammals or aquatic birds. The catfish has a great economic significance around the Black Sea. In the last few years the catfish has been breaded in carp fishponds as a secondary predator.

 Zander (Stizostedion lucioperca)

The Zander can always be differed from the perch because of the large gap between the back fins and the elongated body that is never strictly bended as the perch’s. The Zander is a strong fish, up to 1m long weighing even 10 kg. It has a gray-brown or gray-green back; and light silver-green colors on the side of the body. During the spawning season, the male has a blue belly while the female has a white one. This type of fish usually lives in deep but still waters. It likes to hide during the daytime and hunt during the night or early in the morning. The Zander is extremely sensible when there is lack of oxygen in the water or if the water is polluted. It spawns in April and May in shallow waters at the temperature of 6°C. The male makes a rough nest at the bottom of the water so that the female can attach her eggs to the aquatic plants or roots. The male guards the nest, cools the impregnated eggs with his fins so that they receive enough oxygen and protects them from the sludge and other agents. The young zanders like to stay in shallow waters and first feed on planktons and small insects. When they are 3-4 cm, they feed on cyprinid fish eggs. The adult zanders feed exclusively on other fish. They never prowl on large fish, like the pike does, but they can stay in fishponds together with carps.