Indirect manifestation of cormorant (Phalacrocorax carbo sinensis(L.)) predation on pond fish stock

Abstract

The damage to fisheries caused by cormorant predation pressure consists of losses due to direct predation and subsequent indirect losses elicited by cormorant feeding activities resulting in fish wounding and stress. Hea- led wounds reduce the commercial value of afflicted fish and stress may impact fish body and health condition. Fultons condition coefficient (FCC) was calculated for wounded and healthy two-year old carp originating from five South Moravian (Czech Republic) fishponds. Significant (P = 0.0011) differences in FCC (mean ± s.d.), were found between non-woun- ded (1.48 ± 0.11, n = 19) and wounded mirror common carp, Cyprinus car- pio (1.33 ± 0.14, n = 19). However no differences (P > 0.05) were recorded in scaly common carp between non-wounded (FCC 1.41 ± 0.25, n = 33) and wounded (FCC 1.46 ± 0.47, n = 33) fish of the same age and size ca- tegory. A computer assisted image analysis was applied to describe the extent of such injuries. In the case of two-year old mirror, scaly and bi- ghead carp (Aristichthys nobilis), signs of serious injuries (necroses) were recorded on 1.93, 0.89 and 1.61% of body surface, respectively. Fish with deep wounds and scars, often accompanied with progressive necroses, were subject to parasitological examination. The percentage of wounded fish from total fish harvested was evaluated as ranging between < 1 and 47.4% in five ponds under study. Key-words: piscivorous predators, fish wounding, fish condition, image analysis, carp pond culture