Length-weight relationship and condition factor of sea trout from the Słupia River (Poland), during the spawning migration

Abstract Ulcerative dermal necrosis (UDN) has been observed in sea trout ( Salmo trutta L.) entering the Słupia River to spawn. During this time, various biotic and abiotic factors can disrupt homeostatic balance, leading to severe pathological changes, including those on the skin surface. The aim of the study was to determine whether the symptoms of ulcerative dermal necrosis affect the growth and condition of sea trout migrating to spawning areas in the Słupia River. Between 2021 and 2024, 63 fish ascending the fish ladder in the Słupia River were tested. The length-weight relationship (LWR) and Fulton’s condition factor ( Kc ) were estimated. A comparison of the slopes and intercepts of the LWR regressions for both healthy and diseased males and females indicated no statistically significant differences among the groups examined. Consequently, this suggests that UDN infection does not have a relevant impact on linear regression. Additionally, a comparison of Kc values between the studied groups did not reveal a statistically significant effect of UDN in sea trout. The current study provides basic biometric data on spawning stocks of sea trout in the Słupia River, where UDN is observed, thereby expanding knowledge on this subject.