CROATIAN FAUNA – FRESHWATER FISH, The common carp

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. dr. sc. Goran Jakšić, scientific assistant Aquatika public institution – Karlovac Freshwater Aquarium