A kárókatona (Phalacrocorax carbo) halastavi kártétele és kárértékbecslése

Oláh, János and Oláh, János and Ecsedi, Zoltán (2003) A kárókatona (Phalacrocorax carbo) halastavi kártétele és kárértékbecslése. MAGYAR VÍZIVAD KÖZLEMÉNYEK, 10. pp. 337-379. ISSN 1416-1389

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Abstract

Waterbirds have a very complex role in the life of fishponds. The examined fishponds sustained altogether 238 breeding or migrating species. The various roles of waterbirds in fishpond management can be quantitatively studied and introduced by establishing metabolic guilds. Depending on the lakes, these guild groups can have a vital role in the fish management processes of fertilization, breathing, aeration. We have monitored the stocks of cormorants and other fish eating waterbirds in an area of 30,000 hectares for more than 9 years using modern methods. We have measured and analyzed the changes of the domestic breeding and migrating cormorant stocks. The number of breeding pairs was around 3000 and together with the migrating birds it can reach 26,000 individuals. We have analyzed the factors influencing nourishment with the help of secondary literature on the prey size and daily fish consumption of cormorants. In case of the cormorant, we accepted the average value of 500 g/day of fish consumption by taking into account these many factors. According to these inquiries, the average body weight of the consumed carp is 139 g, and the maximal body weight that the birds can swallow is 254 g. The estimation of the fish damage done to the fish producer and the calculation of the sum of compensation is based on the survey of the cormorant stock. The fish consumption of cormorant is 23 kg/ha, and the annual total fish consumption of all of the fish-eater birds is 43 kg/ha. It is more difficult to state the ratio of marketable fish and trash fish. According to theoretical estimation, the problematic and wide domain of trash fish biomass can range between 5-150 kg/ha/year. The production of garbage fish can oscillate in an even wider domain: 5-300 kg/ha/year depending on the ratio of predators from the sky (fisheating waterbird guild) and from the water (introduced or natural predator fish species). In our small pond experiments with inorganic and organic fertilization lasting for many years the average annual production of trash fish of 74 policultural experimental ponds with big populations was 36±14 kg/ha. In these small ponds there was neither predator fish nor fisheating bird activity. We counted with the value of 6,9 kg/ha for the trash fish consumption of cormorant in the whole growing period. The Hungarian fish-farms - particularly the ones in the Great Hungarian Plain - suffer severe losses because of the cormorant. It is time to compensate those farms because beside their fishing activity they sustain water habitats of vital importance. The biggest waterfowl and waders migration occurs in our fishponds. At the same time, they supply with food our heron colony of European significance. To preserve fishpond fishing, we have to ease the burdens of the fish-farms producing deficit. One justified method to do this is to compensate for the damage caused by the cormorant. To solve the problem, it is essential for nature conservation and fishery to cooperate. It is the task of nature conservation to establish special wetlands (cormorant-feeding ponds, increase of floodplain, restoration of grading system) in the future that diverts at least part of the cormorants from the waters of economic interest. After the measures the caused damage has to be verifiably estimated according to an individual compensation scheme, which has to be covered proportionately by nature conservation and tourist funds. Annual sum of compensation: water surface involved in fish production (ha) x [average annual fish consumption of cormorant (= 23 kg/ha) - average annual trash fish consumption of cormorant (6,9 kg/ha)]. Monthly sum of compensation: {[number of cormorants x value of daily fish consumption of cormorant (500 g/day)] - [number of cormorants xvalue of daily trash fish consumption of cormorant (g/day)] x number of days of the given month}.

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