Population age structures confirm occurrence of the unconfined-resident, life-history form in a bull trout metapopulation

Juvenile bull trout ( Salvelinus confluentus Suckley, 1859) of migratory populations most often remain in natal creeks 2–3 years before moving downstream. Because migratory adult bull trout then begin to return to spawn as age-5 fish, abundant age-4 fish in the natal creek would indicate the nonmigratory life-history form. Population-specific, age-length keys, age-specific length frequencies, and Monte Carlo simulation were used to assign ages 0–5 to bull trout ( n = 14 553) electrofished annually (1998–2023) from eight proximate populations in the St. Mary River catchment, north-central Montana, USA. Resident (that is, nonmigratory) bull trout, characterized as fish < 330 mm total length that had coloration and external morphology indicative of mature fish, were obvious in four populations and regularly captured from distinctive, high-gradient reaches that included large boulders and deep plunge pools. These resident bull trout were not of the common isolated life-history form, which occur upstream from a physical barrier to upstream movement, but were consistent with the unconfined resident form. Age structures showed age-4 residents were abundant in three of the populations; the fourth population had inadequate age-length data. Size-based, assortative mating may preclude resident interbreeding with the larger migratory bull trout and thus foster sympatric speciation.