engleski

The introduction of the fish from the Danube area into the Mediterranean Vransko lake, Croatia

The Vransko lake is a fresh-water lake that covers an area of 3000 ha. It is situated in southern Croatia only one kilometer away from the Adriatic Sea. This is an oligotrophic lake with high alkalinity and high quantities of magnesium and calcium, low chemical oxygen demand (COD) and low quantities of nitrogen and phosphorus. Mild climate allows long growing season because from May to September water temperatures exceed 200C. According to the data available at first the only fish species that inhabited the lake was eel (Anguilla anguilla), from the underwater karsts caves. After 1895, when the artificial channel was dug to connect the lake with the sea, marine euryhaline fish species sporadically migrated to the lake, namely mullets (Mugil cephalus, M. capito, M. saliens, M. chelo. M. auratus), sand smelts (Atherina sp.), sea bass (Dicentrarchus labrax), sea bream (Sparus aurata), common sole (Solea vulgaris), gobies (Gobius sp.), and blennies (Blennius sp.). Tropical mosquito fish Gambusia affinis was introduced against mosquitoes. In the late forties, after the Second World War, and food shortage, the idea came up of improving the fisheries in the lake by introducing the common carp (Cyprinus carpio). About 97.5% of these carps originated from fish-farms and the rest of them were wild carps from the river Drava. Both groups came from the north of Croatia where part of their native Danube basin is situated. Up to the mid-fifties, mostly accidentaly, the following species were also introduced: wells (Silurus glanis), tench (Tinca tinca), crucian carp (Carassius carassius), pike-perch (Stizostedion lucioperca), pumpkinseed (Lepomis gibbosus), Mediterranean roach (Rutilus rubilio), rudd (Scardinius erythrophthalmus). Up to 1980 the existence of Prussian carp (Carassius auratus gibelio) and pike (Esox lucius) were also registered. All these species adapted differently to the new environment. The existence of tench was not confirmed ever after, while the catches during fifties (average 24.5 kg.ha-1) consisted mainly of common carp (89.1%), mullets (8.3%) and eel (3%). Later the situation drastically changed. In 1985 the catch of four main species consisted of Prussian carp (61.2%), wells (31.3%), common carp (5.2%) and eel (2.2%). There are no reliable data on the present structure of the ichthyofauna. During more than 50 years of existence in the Vransko lake common carp adapted well to its conditions and developed into its wild, elongated and scaled form. The condition coefficients (CF) at the beginning of the sixties varied between 1.24 and 1.46 and were similar to those twenty years later (1.31&plusmn ; ; 0.07). At the end of the growing season 1999 (September and November) the CF was 1.62&plusmn ; ; 0.22, while the growth in length could be expressed by the following formula: Lt = 81.97 (1-e-0.122 (t+0.811)). The original idea of introducing common carp for commercial fisheries was never realized. Nevertheless, the angling of the carps nowadays attracts many tourists who visit the Adriatic coast or camp by the lake

Introduction; Common carp; Mediterranean; Croatia

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