PHYSIOLOGICAL-RESPONSES OF LAKE TROUT TO STRESS - EFFECTS OF WATER HARDNESS AND GENOTYPE

We evaluated stress due to net confinement among three genetically iso lated strains of juvenile lake trout Salvelinus namaycush in relation to water hardness. Changes in plasma cortisol, glucose, sodium, and ch loride were used as the stress indicators. In lake trout acclimated an d then confined for 8 h in hard water, plasma glucose and cortisol inc reased significantly (by three- and sixfold, respectively), reaching a plateau by 4 h. while plasma Na+ and Cl- significantly decreased by a bout 7%. Fish transferred from hard water to soft water and then confi ned exhibited significantly greater changes in blood chemistry than di d fish confined in hard water. This soft-water effect on the stress re sponse disappeared if fish were first acclimated to soft water, but fu ll softwater acclimation may take at least 2 months. Comparisons of bl ood chemistry among three different strains of lake trout (Lake Manito u, Killala Lake, and Slate Island) revealed significant differences in their responses to confinement stress. These findings have important implications for two fish culture issues: the choice of Whether to rea r stocks in soft-water versus hard-water hatcheries, and the selection of stocks for stress resistance.