Effects and Interaction of Phenotype and Rearing Density on Growth and Fin Erosion in Rainbow Trout

Abstract

AbstractIntensively reared rainbow trout Oncorhynchus mykiss that receive formulated feed frequently exhibit torn, ragged, and eroded fins. To better understand the factors affecting fin erosion, two rainbow trout phenotypes (albino and pigmented) were reared to verify reports that albino rainbow trout suffer less dorsal fin erosion than do pigmented fish. A 12‐month study was conducted to evaluate the effects of phenotype, rearing density, and the phenotype × density interaction on fin erosion. Extremes in rearing density were tested using group rearing (maximum density = 200 fish/tank) and individual rearing as the treatments. To evaluate interactive effects of phenotype, rainbow trout were reared either in mixed populations (MPs) containing equal proportions of albino and pigmented fish or in separate populations (SPs) containing only albino or pigmented individuals. Group rearing treatments were each applied to three tanks at 200 first‐feeding fish/tank. Eight albino and eight pigmented fish were reared in individual rearing chambers. At 3‐month intervals, fish were measured for weight, total length, condition factor, and dorsal and pectoral fin lengths. Survival exceeded 90% in all treatments, but albino fish had lower survival than did pigmented fish in both MPs and SPs. Albino fish from MPs were significantly smaller (37 g) than MP‐reared pigmented fish (77.5 g) at the end of the study. Dorsal fin index scores were greater (i.e., indicated less fin erosion) for MP‐reared albino fish (7.5%) than for MP‐reared pigmented fish (5.6%), but considerable erosion occurred in both phenotypes reared in MPs and SPs. Rainbow trout that were reared individually had complete dorsal fins and pectoral fins during the entire study (i.e., dorsal fin and pectoral fin indices [fin lengths expressed as a percentage of total length] > 11%). This study demonstrates that albinism does not directly affect the occurrence or rate of dorsal fin erosion, but less fin erosion is observed in albino fish than in pigmented fish when reared in MPs.