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

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 outmigrated 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 (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. However, both types also produced silvered 'sea trout' (≥1+ sea-age), implying neither is locked into a single life-history strategy. Our findings emphasise that autumn outmigrants and the transitional habitats that support their persistence should not be overlooked in salmonid management and conservation. This article is protected by copyright. All rights reserved.