Variation in intrinsic individual growth rate among populations of leatherside chub (Snyderichthys copei Jordan & Gilbert):: adaptation to temperature or length of growing season?

Leatherside chub (Snyderichthyes copei Jordan & Gilbert) comprises two lineages (northern and southern) whose centres of geographical range differ by about 3° latitude corresponding to about 30% shorter growing season and about 2 ° C lower mean temperature during the growing season. We document patterns of variation in size-at-age among populations of leatherside chub in nature, and we test for variation in intrinsic growth rate of juvenile chub in a common-environment experiment to determine if lineages exhibit different intrinsic growth rates. Northern leatherside chubs at ages 1-3 were about 15% shorter in length compared with southern populations. Variation in hatching date or age at maturity could not account for differences in growth and body size, suggesting that temperature-specific intrinsic growth rates differed among populations. Based on a common environment experiment, we found that reaction norms for temperature-specific growth in length and mass were crossed. At the lower temperature, individuals from the north exhibited higher intrinsic growth in length, and at the higher temperature individuals from the south exhibited higher intrinsic growth in mass. Crossing reaction norms for temperature-specific intrinsic growth rates support a model of local adaptation to thermal regime.