Genetic and Environmental Factors Involved in Increased Resistance of Brook Trout to Sulfuric Acid Solutions and Mine Acid Polluted Waters

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Abstract
Several strains of hatchery‐reared brook trout, Salvelinus fontinalis (Mitchill), were exposed to low pH in the laboratory (sulfuric acid solutions) and in the field (mine acid polluted waters). Wild brook trout were also used in some field tests. Tests were both acute and chronic (up to 2 mo). Pronounced strain differences in survival ability were detected among embryonic, juvenile, and adult brook trout in laboratory tests, and among juvenile brook trout in field tests. However, in one case a strain difference in resistance times detected in field tests was not evident in several laboratory studies. A single selection of NYSV strain brook trout for high resistance to sulfuric acid solutions did not yield F1 progeny of greater resistance. Prior exposure to nonlethal acidic conditions also did not enhance resistance of brook trout. In some cases, the fish actually became more sensitive to the low test pH after such a prior exposure. Fish had longer resistance times in sulfuric acid solutions and in mine acid polluted water if they were held previously in, respectively, pH 8 laboratory water, or nonacidic field environments. Wild brook trout survived longer at lethal field pH levels than hatchery fish which were tested immediately upon transport from the hatchery; there was no difference if the hatchery fish were held, prior to testing, in nonacidic field environments. Development of embryonic brook trout was delayed in sulfuric acid solutions of low pH. The time to loss of equilibrium at low pH was well correlated with total survival time. Fish removed at the time of equilibrium loss (in pH 2.75 and 3.25) did not recover when placed in pH 8 water. The rate of net sodium loss (in μmol/100 g˙h or μmol/h) at low pH was inversely correlated with the resistance times of brook trout. Fish had shorter resistance times in mine acid waters than in laboratory sulfuric acid solutions of comparable pH. Larger and older fish tended to survive longer although size was not strongly correlated with resistance times within narrow size categories of equal age fish. There was no difference between the sexes in survival times at low pH. The most important factors in enhancement of acid resistance in hatchery brook trout were the hereditary resistance of a given strain and the acclimation of fish to nonacidic stream or laboratory conditions prior to acid exposure. The establishment by selection within strains of one superior in acid tolerance will probably require many generations; screening for a tolerant strain from among extant strains may accomplish the goal more quickly.