A validated macroscopic key to assess fin damage in farmed rainbow trout (Oncorhynchus mykiss)

Fin damage, commonly referred to as fin erosion, has been highlighted as a welfare issue in farmed fish. An objective means for describing fin damage is required by both fish welfare researchers and the industry for quality assurance. For rapid quantitative assessments of fin size, 6-point photographic scales have been designed for small (< 50 g) and large (> 50 g) fish, covering the range of fin damage observed on rainbow trout on UK farms. Inter- and intra-operator tests show that scoring is consistent both within and between operators. An experienced operator can assess all 7 rayed fins on a fish within 2 min. Additional qualitative descriptors (splitting, exposed rays, haemorrhage, inflammation, scaring/thickening, kinking/folding) have also been developed to categorise resultant malformations to the fins.