Age structure, growth, and factors affecting relative abundance of life history forms of bull trout in the Clark Fork River drainage, Montana and Idaho

Abstract

Bull trout Salvelinus confluentus exhibit high variability in life history patterns. Better understanding of the underlying mechanisms is needed to assist conservation efforts. I assessed relationships among life history form, growth rates, age structures, and environmental variables, using pelvic fin rays and scales to estimate age and growth. First, I assessed the effects of pelvic fin ray excision on survival and growth of age-3 and age-4 bull trout. Survival and growth were similar between fin ray-excised and control fish within each age group, although a bacterial coldwater disease infection caused higher mortality in age-3 fish. Excised rays achieved a mean 42% regeneration by six months. Second, I developed methodology for using pelvic fin rays to estimate age and growth. Suitability of fin ray sections was based on overall morphology, appearance of early annuli, and presence of a conspicuous dash in the nucleus. Back-calculation for juvenile and non-migratory bull trout produced real error (mean ± SE) of 4.1 ± 2.0% and absolute error of 7.2 ± 1.2% of known lengths one year prior. Ageing precision and accuracy of growth estimates using fin rays compared favorably to those from scales.

However, obtaining adequate fin ray samples for large fish (>400 mm TL) proved difficult and additional work is needed to validate age and growth estimation procedures. Third, I analyzed age and growth of bull trout in relation to environmental conditions in study streams. Bull trout in predominantly migratory populations held lower proportions of individuals > age 3 during summer and generally displayed higher growth rates during ages 0 and 1 than those in resident populations. Migratory populations exhibited overall faster early growth, although not in all cases. Age-0 growth was positively associated with length of growing season, whereas age-1 growth was negatively associated with density of bull trout and positively related to presence of nonnative salmonids. Presence of the migratory life history was influenced by severity of barriers to migration and presence of nonnative brook and brown trout. These results suggest that the migratory life history may be encouraged by enhancing migratory corridors and juvenile rearing habitat in lower reaches of tributaries.