Assessment of fish health status in four Swiss rivers showing a decline of brown trout catches

Abstract

A pronounced decline in catch of brown trout (Salmo trutta L.) over the last 10-20 years has been reported for many rivers in Switzerland. Impaired health status of the fi shes has been suggested as one possible cause of the decline. The present study investigated the health status of juvenile brown trout from four Swiss rivers which experienced reductions of brown trout catches during the last two decades: Emme, Liechtensteiner Binnenkanal (LBK), Necker and Venoge. A gradient approach was applied, studying at each river a headwater (HW), midstream (D1) and downstream site (D2). Fish health was assessed by the following indices: hepatic 7-ethoxyresorufi n-O-deethylase (EROD) activity, organ (liver, kidney) histopathology, and gross biometric indices (condition factor, hepatosomatic index). Hepatic EROD activities were generally low without showing signifi cant within- or between-stream differences. His- topathological alterations of the liver displayed a moderate downstream increase in the Emme, LBK and Necker, but not in the Venoge. Between-stream differ- ences of liver pathology were small. Kidney histopathol- ogy was not different between upstream and downstream sites, except for Emme and Venoge, where fi shes at the downstream sites were infected with the parasite Tetra- capsuloides bryosalmonae, the cause of the proliferative kidney disease (PKD). The fi ndings from this study point to an association of within-stream gradients in water quality, PKD prevalence, fi sh health and brown trout bio- mass, whereas between-stream differences of actual fi sh health status refl ecting the different levels of catch de- clines observed in the four rivers during the last two de- cades are not evident.