EFFECTS OF REARING DENSITY UPON CUTTHROAT TROUT HEMATOLOGY, HATCHERY PERFORMANCE, FIN EROSION, AND GENERAL HEALTH AND CONDITION

Cutthroat trout of the Bear Lake Bonneville strain, Oncorhynchus clark i utah, were used in two separate density experiments. In the first, f ish were reared for 212 d in outdoor raceways at four densities; fish were allowed to grow into their final rearing density and were fed 7 d /week. Final rearing densities averaged 768, 1,597, 2,073, and 2,998 f ish/m(3), and corresponding density indices (DI = fish weight, lb/[fis h length, in x water volume, ft(3)]) were 0.40, 0.90, 1.10, and 1.46. In experiment 2, crowding screens were adjusted monthly, and fish were fed 5 d/week; final rearing densities were 338, 739, and 1,634 fish/m (3) (DIs of 0.19, 0.39, 0.75). Feed conversion and mortality did not s ignificantly differ among densities for either experiment. Final mean weights did not differ among the four densities of experiment 1, but m ean total length was significantly longer in fish reared at the lower densities. In experiment 2, final mean weight was significantly reduce d in the highest density and specific growth rates for all densities w ere lower than in experiment 1. Frequencies of agonistic behaviors did not differ among densities in experiment 2. Hemoglobin, total white b lood cell (WBC) counts, differential WBC counts, and hepatosomatic ind ices did not differ among densities in either experiment. Red blood ce ll (RBC) counts and the splenosomatic index (SI) did not differ among densities of experiment 1. However, in experiment 2, the RBC count was higher at the lowest density than at the highest, and the SI was sign ificantly higher at the highest density than the lowest. Condition fac tor, plasma protein, hematocrit, and relative dorsal fin length differ ences among densities were observed, but were inconsistent over time. Adverse effects of high density on mesenteric fat levels and pectoral fin condition were observed in experiment 2. Saltwater challenge tests resulted in greater mortality for fish from high densities. The data indicated that rearing cutthroat trout at a DI of 0.75 or higher (abou t 1,600 fish/m(3)) may compromise fish health when densities are adjus ted monthly and fish are fed 5 d/week; the data also indicated that ev en lower densities are needed for maximum growth.