Further development of the “Fin Index” method for quantifying fin erosion in rainbow trout

Fin erosion is common in farmed salmonids and has recently been highlighted as a fish welfare issue. Methodology is needed to quantify the severity of fin erosion to support research, and this paper examines Kindschi's [Kindschi, G.A. 1987. Method for quantifying degree of fin erosion. Prog. Fish-Cult. 49, 314–315.] method which expresses fin length measurements relative to fish length. Naturally spawned wild rainbow trout Oncorhynchus mykiss (n = 26; 92–335 mm standard length (LS)) were sampled from a UK river to establish reference values for intact fin length. The relationships between intact fin size and LS were isometric for the anal, pectoral and pelvic fins, but negatively allometric for the dorsal and caudal fins, indicating that relative fin size decreases with increasing fish size for the latter two fins. Predictive relationships were derived between fin length and LS for the five rayed fins. These relationships showed good correspondence to maximum fin lengths observed in samples of the UK farmed rainbow trout metapopulation, and a UK feral rainbow trout population. The potential errors associated with assuming a constant relative fin size and not allowing for allometric fin growth, are illustrated. The equations predicting intact fin length from LS were formatted to enable estimation of the proportion of fin length lost from measures of observed fin length and LS in farmed fish. Such equations provide an intuitive fish welfare indicator (higher values representing greater fin loss), and represent an advance on Kindschi's “Fin Index” method for assessing the severity of fin damage in farmed rainbow trout.