Autumn outmigrants in brown trout (Salmo trutta) are not a demographic dead‐end

Abstract

Genetic identity analysis and PIT (passive integrated transponder) tagging were used to examine the freshwater return rates and phenotypic characteristics of n = 1791 downstream migrating juvenile Salmo trutta in the Burrishoole catchment (northwest Ireland) across the period September 2017 to December 2020. In this system, juveniles out‐migrate (move from freshwater into brackish or marine habitats) in every month of the year, with distinct seasonal peaks in spring (March through June; mostly silvered smolts) and autumn (September through December; mostly younger, unsilvered fry or parr). Both types exhibited a sex‐bias towards females, which was stronger in spring (78% females) than in autumn outmigrants (67%). Sixty‐nine returning fish were matched back to previous juvenile outmigrants, and similar return rates were found for spring outmigrants (5.0%), autumn outmigrants (3.3%) and fish that out‐migrated outside of spring or autumn (2.8%). Spring and autumn outmigrants returned at similar dates (typically mid to late July), but autumn fish were away for longer periods (median = 612 days; spring outmigrants = 104 days). Autumn outmigrants were 25% smaller than spring outmigrants at outmigration and 6% smaller on their return, and within both groups smaller/younger outmigrants spent longer away than larger/older outmigrants. Autumn outmigrants were more likely to return unsilvered as “slob” trout (84%) than spring outmigrants (31%), suggesting they make greater use of brackish habitats that might be safer, but less productive, than fully marine habitats. Nonetheless, both types also produced silvered “sea trout” (≥1+ sea‐age), implying neither is locked into a single life‐history strategy. The findings emphasise that autumn outmigrants and the transitional habitats that support their persistence should not be overlooked in salmonid management and conservation.

Diverse migration patterns of anadromous white-spotted charr Salvelinus leucomaenis revealed from otolith microchemistry

“…Therefore, passively downstream-displaced whitespotted char may be able to survive in the sea even at the parr stage and return to rivers. Some salmonid species, like the brown trout (Limburg et al 2001;Wynne et al 2023), coho salmon Oncorhynchus kisutch (Koski 2009;Jones et al 2014Jones et al , 2021, and Chinook salmon Oncorhynchus tshawytscha (Chalde and Fernández 2017), utilize estuarine areas at the parr stage. Estuaries provide highly productive environments that enable juvenile salmon to grow larger than those in freshwater rivers (Quinn 2018).…”

“…Precocious emigrants, in which young parr shift their habitat to brackish water without smoltification, appear in short streams (Limburg et al 2001). Additionally, autumn outmigrants make use of safe but unproductive brackish waters (Wynne et al 2023). Such life-history diversity in portfolios may make the population more resilient to global changes (Price et al 2021).…”

Timing of reproduction underlies fitness tradeoffs for a salmonid fish

“…While YOY emigrating to the Snake River are likely in poor condition relative to residents (Elliott 1990, Kanno et al 2021), spring‐fed streams are small in size and limited in the number of resident fish they can support. Spring‐fed streams may therefore provide important larval subsidies to larger neighboring habitats, which offer contrasting and potentially complementary habitat conditions (Tsuboi et al 2022), suggesting that transitional habitats supporting phenotypic diversity should not be overlooked (Wynne et al 2023).…”