Computer-assisted image analysis in the evaluation of fish wounding by cormorant [Phalacrocorax carbo sinensis (L.)] attacks

Fish that have escaped from a cormorant’s (Phalacrocorax carbo sinensis (L.)) grasp and/or which could not be swallowed due to their size suffer from various injuries resulting in consecutive infections and subsequent increased mortality. Computer-assisted image analysis was applied to describe the extent of such injuries. Two-year-old mirror carp, Cyprinus carpio L. (TL 200–300 mm, W 200–300 g) showed injuries ranging over approximately 10% of the total body surface. Immediately after wounding, the damaged epithelium (scars) cover 5–35%, and deeper subdermal wounds caused by the beak tip pervading into muscle tissue cover an area of 1–2% of the total body surface. On the side impacted by the cormorant’s lower mandible, extensive areas of epidermal contusions (scars) occur. As time progresses, these ratios change—deeper necroses represent up to 10% of the total body surface and healing epithelial scars comprise just 1–2%. In wounded silver carp, Hypophthalmichthys molitrix Val. (300–400 mm, W 500–700 g), the share of subdermal wounds usually does not exceed 0.5% due to their compact scaly cover. During pond draining due to fish harvesting, the size spectrum of wounded fish increases and may also often include bigger fish (e.g., European catfish, Silurus glanis L.) up to 2.2 kg.