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Effects of rearing density on stress response in tank-reared juvenile steelhead trout, (Salmo Gairdneri) Loftus, Kevin Kenneth


The effects of rearing density on stress response in juvenile steelhead trout (2-60 grams) were examined in a series of experiments. Fish reared at high densities showed evidence of stress, however, both the magnitude and the nature of the response varied with size. In the first study, fry (initial weight 2.35 gm) were reared at 2 temperatures (12.5 and 15.0 °C) and 3 densities (0.6 to 3.0 times conventional densities). Fish reared at the high temperature showed a greater response to density than those reared at the low temperature. At 15 °C, growth rates were depressed by high densities, however the effect was of short duration and evident only during the first 2 of 8 weeks. Whole- body proximate composition was affected. At high densities, moisture and protein levels were elevated, and lipid levels were lowered. Condition factors were also low in fish reared at high densities. Both condition factors and growth rates followed a curvilinear pattern with time. At 12.5 °C, growth rates, proximate composition and condition factors followed trends close to those at 15 °C, however, tests were rarely significant. Plasma Cortisol concentrations were unaffected by either density or temperature. Activity was affected by density, but not by temperature. Fingerling trout (initial weight 15 gm), were differently affected over an 8-fold range in density (0.6 to 4.8 times conventional levels). At high densities, growth rates were suppressed, and the effect was of longer duration than observed in the first experiment. Whole body composition followed the same pattern with density as in the first experiment, but condition factors responded in the opposite direction, increasing at high densities. Plasma Cortisol concentrations were unaffected by density. Rapid increases in density induced a response in growth rates over and above that due to density alone, which suggested that fish become conditioned to rearing densities. Rapid reductions in density did not affect growth. Growth rates and plasma Cortisol concentrations of pre-smolts and smolts were unaffected by an 8-fold density range (0.4 to 3.2 times conventional levels). However, after an acclimation period, sudden increases in density caused significant reductions in growth (greater than that expected on the basis of density alone) and elevations in plasma Cortisol concentrations. Whole body composition followed a similar pattern with density as observed with smaller fish suggesting that the lack of growth responses to density, does not necessitate a lack of stress response. Condition factor data were inconclusive. Activity levels were unaffected by density, but did vary with time. After exposure to a salt water challenge test, fish reared at high densities regulated plasma sodium levels less efficiently than fish reared at low densities. Flow rates and container volumes did not significantly affect stress response. These results indicate that high density rearing induces physiological, and possibly behavioral changes, in steelhead trout. This suggests the fish are showing a stress response. There is evidence that, in some cases, fish may adjust to densities, and that changes in density, not density per se, may influence the stress response. Growth rates, when used alone, are inadequate indicators of stress.

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