The evolutionary significance of life history divergence among brook trout, Salvelinus fontinalis, populations

Hutchings, Jeffrey Alexander (1990) The evolutionary significance of life history divergence among brook trout, Salvelinus fontinalis, populations. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

I combined field studies, laboratory experiments, and empirically-based model simulations to study life history variation among seven populations of brook trout, Salveli nus fontinalis, on Cape Race, Newfoundland. My objectives were to describe population variation in life history on a small geographical scale, to identify the environmental factor(s) responsible for effecting inter- and intra-population life history differences, and to assess the evolutionary significance of the observed variation through empirical tests of life history theory. -- There were significant population differences in life history. Growth rate differed 2-fold between the most divergent populations. Among populations, the slowest growing females 1) matured at an early median age at the smallest mean size relative to the fastest growing females (3 yr, 99.8 mm versus 5 yr, 164.1 mm), 2) produced the largest eggs (4.43 mm vs. 4.01 mm), 3) had the highest size-specific fecundity, and 4) allocated proportionately the most body tissue to gonads. -- Estimates of offspring and maternal fitness depended on egg size, the trade-off between egg size and egg number, and juvenile food abundance. Egg diameter was positively correlated with embryonic and juvenile size but had no significant effect on embryonic survival or developmental time in laboratory reared fish. Juvenile survival was linearly related to egg size throughout the first 50 days of exogenous feeding at high and low food levels. Reduced food increased mortality significantly among the smallest eggs but not among the largest eggs. Empirically-based simulations 1) supported the hypothesis that environments with low food abundance select for increased offspring size, and 2) indicated that unpredictable, temporally heterogenous environments favour constant, rather than variable, investment in offspring size. -- There are costs of present reproduction to future survival in brook trout and these costs vary among populations. For a given reproductive effort, reductions in both size at maturity and resource abundance appear to increase reproductive costs. These costs appear to have a physiological basis: allocation of lipids to gonad production and their subsequent loss during autumnal reproduction decreases the fat stores available to meet energetic demands during winter. Reproductive costs and reproductive effort are related in a concave-shaped function that can account for the evolution of alternative expenditures of reproductive effort in brook trout. -- Population differences in brook trout life history can be explained as adaptive responses to differences in growth rate effected primarily by differences in food abundance. Reduced growth results in high reproductive investment near the physiologically minimum size at maturity. Individual life history traits appear to be consequences of the survival and fecundity costs associated with differential size- and age-specific reproductive investment.

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