Brook Trout (Salvelinus fontinalis) Population Structure in Acidified Lac Tantaré, Quebec

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Abstract
To present evidence of acid-induced stress, the population structure and distribution of brook trout (S. fontinalis) in acidified Lac Tantare was studied. Trout > 21 cm represented only 16.2 and 10% of the population sampled in 1979 and 1980, respectively. Survival rates of trout from 1979-1980 indicated higher survival among individuals > 25 cm, mortality occurring primarily among fish of 13-25 cm. The marked decline in numbers of fish occurring at 21 cm coincided with the size at which 50% of the female trout population reached sexual maturity, suggesting that postspawning mortality contributed to the high mortality observed among these fish. Comparisons of population and individual growth rates revealed that smaller fish of age-classes 1-5 suffered higher mortality than larger individuals of these age-classes, implicating mortality agents other than those related to spawning. Evidence was provided to support the hypothesis that the most probable cause of mortality was size-related differential exposure to toxic stress whereby small brook trout (< 25 cm) were found in lake areas (brooks) exhibiting the most potentially toxic combination of pH and Al, enhancing the exposure of the most susceptible part of the population to toxic stress. Large brook trout (.gtoreq. 25 cm) were found in the safest lake areas (springs), diminishing the exposure of the least susceptible part of the population to toxic stress and enhancing survival. Female trout exhibited delayed sexual maturity relative to other Quebec populations, attaining 50% maturity at age 3 and a length of 21-24 cm such that the major part of the population''s reproductive effort was provided by those fish representing only 15% of the population in 1979 and 1980. During the Oct. spawning season, 20% of potential spawners exhibited retarded oogenesis. These reductions in reproduction effort and changes in spawning site selection provided evidence in support of the hypothesis that recruitment failure may have occurred in the Tantare brook trout population.