DIFFERENCES IN RELATIVE SENSITIVITY OF NAIVE AND METALS-ACCLIMATED BROWN AND RAINBOW-TROUT EXPOSED TO METALS REPRESENTATIVE OF THE CLARK-FORK RIVER, MONTANA

Naive and metals-acclimated juvenile brown (Salmo trutta) and rainbow trout (Oncorhnchus mykiss) were exposed to a metals mixture containing zinc, copper, lead, and cadmium, to compare relative sensitivities to metals concentrations typical of the Clark Fork River, Montana. Diffe rences in the survival responses (tolerance versus resistance) measure d for naive fish indicated that the rainbow trout are more tolerant (h igher 96-h LC(50)) of the metals mixture, yet the naive rainbow and br own trout showed similarities in resistance (mean time to death). Diff erences for metals-acclimated fish indicated that brown trout are more resistant. Thus, the relative sensitivity of brown and rainbow trout varies with both the survival response measured and pre-exposure effec ts of metals on the physiological condition of the animal. Brown trout continued to acquire metals resistance for up to 5 weeks during accli mation to chronic concentrations of the metals mixture. Hepatic metall othionein and copper residue levels were positively correlated in both naive and metals-acclimated trout, and an energetic cost of metals ac climation was evidenced by reduced weight in brown trout acclimated to the metals. In contrast, rainbow trout demonstrated the least degree of acclimation and no significant growth inhibition was observed.