engleski

Health assessment of northern pike (Esox lucius) from the Mrežnica River, Croatia

Northern pike (family Esocidae) is an important fish for aquaculture and the fishing industry in Croatia. Despite its commercial significance, there is only limited information on the health status of this species in natural waters. Here we report the results of an initial evaluation of the health status of wild northern pike collected from 3 locations along the Mrežnica River. In the spring of 2020, a total of 9 specimens (30 – 67 cm in length, an average body weight of 925 g) were captured by electrofishing and examined as part of a fish health and ecotoxicological study, conducted under the Croatian Science Foundation project, “Metal-binding biomolecules and health disturbances of freshwater organisms exposed to industrial wastes” (IP-2019-04-2636). For each pike, the presence of gross external and internal lesions was documented, and samples were processed for virological, bacteriological and parasitological examination. Additionally, liver and kidney samples were fixed in 10% neutral- buffered formalin and processed for histology. The 5-µm thick sections were stained with haematoxylin and eosin (H&E), periodic acid-Schiff (PAS) and Mallory's trichrome. No viruses or pathogenic bacteria were detected. However, a number of protozoan and metazoan parasites were observed, including ciliates (Trichodina sp.), myxosporeans (Myxidium lieberkuehni), monogeneans, intestinal nematodes, leeches (Hemiclepsis marginata) and crustaceans (Argulus foliaceus). Histologically, different changes were evident in the liver and kidney of several fish. Histopathological changes in the kidney were associated with Myxidium infection. In the liver, changes of unknown aetiology were mostly characterized by hydropic degeneration, vascular congestion, granuloma formation, lytic necrosis and necrosis of single hepatocytes. These results suggest that special attention should be given to the health status of wild northern pike.

Northern pike ; health ; Mrežnica river ; parasites ; histology ; liver

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