Development of a fish welfare evaluation index for rainbow trout (Oncorhynchus mykiss) in aquaculture

Scientific studies concerning fish being sentient raised the need to accurately assess fish welfare and to implement standardized protocols for comparable welfare assessments in aquaculture. Aim of the thesis was to define welfare indicators that can be reliably and practicably assessed directly on farm and to set up a welfare evaluation index for rainbow trout (Oncorhynchus mykiss) in flow-through systems. Assessed on-farm welfare indicators included among others environmental/management parameters, behavior, and external morphological damage. To gain additional in-depth insights into the effects of different rearing conditions various health and stress parameters were analysed. Water supply, water supply per kg fish, the duration of a total water exchange in the tank, nitrogen compounds, and the estimated activity level were identified as the major predictors of external damage. Analysed health and stress parameters were as well reflected in the external damage. The findings indicate direct and indirect effects of suboptimal rearing conditions to result in a visibly degraded external appearance of fish, thereby demonstrating external damage as being highly relevant welfare indicators. The reliability and/or practicability of using farms’ environmental/management conditions, and behavior as likewise essential welfare indicators were limited, while external damage could be reliably assessed using photographic images, matched to a scale of different severity grades of impairment. Such a method is easily and quickly acquired and applied at no material costs. The fish welfare evaluation index summarizes the external damage including prevalence and severity, resulting in categories of very good to poor welfare. The index can be applied by fish farmers, veterinarians, certification and monitoring programs.


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