Interannual shifts in brook trout anadromy inferred by stable isotopes and otolith microchemistry

The marine parental contribution to brook trout (Salvelinus fontinalis) populations was examined in coastal watersheds in Atlantic Canada. It was hypothesized that coastal populations would be influenced by anadromous spawners, with influence diminishing as distance from seawater increases, and that anadromous brook trout produce larger eggs, resulting in larger juveniles. Eggs were collected during spawning in November 2021–2022, and juveniles were collected from late September to October of 2021–2023. Stable isotope analysis (13C, 15N, 34S) of eggs and primordia (core) otolith microchemistry (Sr:Ca, Ba:Ca) from juveniles were used as tracers of maternal life history. Pb:Ca was revealed as a third freshwater tracer for maternal influence. Both techniques independently identified three maternal contingents: resident, anadromous, and estuarine/anadromous. The interannual variability in the contribution of marine mothers to juveniles ranged from 0 to 100% between years. Egg diameter and juvenile weight showed no significant differences between anadromous and resident contingents. Anadromous and freshwater strategies may impart complementary survival advantages, adding to the resilience of the species to environmental and anthropogenic stress.