Fish welfare evaluation index (fWEI) based on external morphological damage for rainbow trout (Oncorhynchus mykiss) in flow through systems

Abstract

Scientific studies concerning fish sentience and consumers requesting ethically produced food have raised the need to accurately determine fish welfare and to implement standardized protocols for comparable welfare assessments in aquaculture. This study aimed to evaluate reliable, practicable on-farm indicators and to establish a welfare evaluation index for rainbow trout (Oncorhynchus mykiss) during grow-out in flow through systems. Potential on-farm welfare indicators, assessed on nine different farms, included environmental- (water supply, −exchange, −quality) and management parameters (stocking density, feeding frequency), behavior and health observations (disease and low welfare indicating behaviors, social behavior, activity level), and external morphological damage (skin-, fin-, eye-, gill damage, skeletal deformities, emaciation). Principal component analysis followed by multiple linear regression showed that water supply, −supply per kg fish, duration of a total water exchange, nitrogen compounds, and activity level were the major predictors of external morphological damage. The direct and indirect effects of suboptimal rearing conditions resulted in external morphological damage, demonstrating external damage as highly relevant welfare indicators. External morphological damage, assessed using photographic images with different severity grades of impairment, forms a method that is easily and quickly applied at no material costs. Due to limited reliability and/or practicability of environmental, management and behavioral parameters the fish welfare evaluation index (fWEI) was solely based on external morphological damage. The fWEI summarizes the external morphological damage including prevalence and severity, resulting in categories of very good to poor welfare. The fWEI can be applied by fish farmers, veterinarians, certification and monitoring programs.