Identification of Anadromous and Nonanadromous Adult Brook Trout and Their Progeny in the Tabusintac River, New Brunswick, by Means of Multiple-Stable-Isotope Analysis

Multiple-stable-isotope analysis was used to infer anadromous and nonanadromous origins of adult brook trout Salvelinus fontinalis and maternal migration history of age-0 progeny in the Tabusintac River, New Brunswick. Forty-seven adults collected above head of tide displayed deviations (δ) from standard ratios of 13C/12C, 15N/14N, and 34S/32S of −30.3‰ to −16.0‰, 7.4‰ to 16.8‰, and 1.5‰ to 14.1‰, respectively; higher values (positive or less negative) denote relatively greater enrichment in the heavier isotope. Isotopically enriched brook trout exhibited isotope profiles typical of fish from marine environments, and those that were isotopically depleted were considered to be of freshwater origin. Age-0 brook trout from Home Camp Pool, the most downstream freshwater rearing site sampled, were more enriched (δ13C = −24.4 ± 2.7‰ (mean ± SD), δ15N = 12.5 ± 3.1‰, and δ34S = 7.4 ± 1.8‰) than those at two other sites, and were believed to be progeny of anadromous females. Age-0 brook trout from the Bathurst Highway site (δ13C = −27.9 ± 0.6‰, δ15N = 8.9 ± 0.8‰, and δ34S = 5.8 ± 0.8‰) and the Little Eskedelloc River (δ13C =−28.6 ± 0.5‰, δ15N = 8.1 ± 0.4‰, and δ34S = 2.9 ± 0.8‰), with less isotopic enrichment, were most likely from nonanadromous parents. Stable-isotope ratios varied with fork length; at Home Camp Pool, this relationship was thought to represent an “isotope dilution factor” as recently emerged juveniles assimilated new food from freshwater, grew, and masked the marine signatures of their maternal parents. This study suggests that stable-isotope ratios may be used to distinguish between sympatric anadromous and nonanadromous adult brook trout and their progeny as long as brook trout are collected before they dilute their maternal isotope signatures.