Investigating the phenology of juvenile potamodromous brown trout (Salmo trutta L.) in two large lake catchments

Does phenology influence predation rate on Salmo trutta parr during lake migration?

Acoustic tags fitted with predation sensors, which trigger following ingestion by piscivorous predators, were used to compare direct predation rates during downstream migration (out-migration) of potamodromous (freshwater) brown trout (Salmo trutta L.) parr from their natal river into a large freshwater lake system during spring and autumn. Thirty-eight spring migrants were tagged across two study years (2021 and 2022) of which 13 individuals (34%) were predated. By contrast 40 autumn migrants were tagged (2020 and 2021) of which three individuals (7.5%) experienced predation. The overall predation loss rate for spring migrants was 0.342% day-1 and was 0.075% day-1 for autumn migrants. Most predation events during spring (77%) occurred within the lower river before tagged fish entered the lake, whilst no predation events were recorded within the river in the autumn. Predation events were significantly linked to tagging season (spring or autumn), with the probability of tags remaining untriggered (as a proxy for survival) being higher 93% (95% confidence interval [CI] [87%, 100%]) in autumn than in spring 66% (95% CI [53%, 83%]). The spring migration periods showed significantly lower river discharge (0.321 m3 /s mean daily discharge, April 1 to May 31) to those measured during autumn (1.056 m3 /s mean daily discharge, October 1 to November 30) (Mann-Whitney U-test, U = 1149, p < 0.001). Lower flows, clearer water, and longer sojourn in the river may have contributed to greater predation losses in the spring relative to the autumn.

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 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.