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

Abstract

There is growing interest in the phenology of juvenile Salmo trutta and evidence of significant downstream migration during the autumn in some anadromous populations. The present study used acoustic telemetry to examine the phenology of potamodromous trout parr across a region encompassing two large lake catchments. One hundred sixty‐seven trout parr were tagged in late summer across four lake tributaries between 2018 and 2020. In total, 75 tagged parr migrated into the lakes with 67 (89%) migrating between September and December and 8 (11%) migrating between March and June. Autumn migration was highly prevalent across all the tributaries, with 16%–66% of each tagged sample exhibiting autumn migration, and 0%–15% of each tagged sample exhibiting spring migration. Autumn migrants were significantly longer and heavier than spring migrants, but condition factor was similar. Autumn migrants were associated with higher river discharge levels and lower water temperatures than spring migrants. The management challenges posed by extensive autumn migration behaviour in migratory trout stocks are examined and discussed.

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

“…(Cunjak et al ., 1989; Jonsson & Jonsson, 2014) and brown trout ( Salmo trutta L.) (Aarestrup et al ., 2018; Birnie‐Gauvin et al ., 2021; Jonsson & Jonsson, 2009; Poole et al ., 1996, 2007; Winter et al ., 2016). Interestingly, so‐called “autumn parr” migrations are not restricted to anadromous stocks, as they also occur in Baltic Sea populations of Atlantic salmon and brown trout that migrate into brackish water (surface salinity 4.5‰–6.5‰) (Taal et al ., 2014), as well as in potamodromous brown trout populations (Kennedy et al ., 2022b; Leathe et al ., 2014).…”

“…Although predators in transitional or ultimate destination habitats might be less abundant in colder winter months or constrained by lower light levels, smaller migrants might still be vulnerable to gape‐limited predators. Indeed, autumn outmigrants are typically smaller at the time of migration than spring outmigrants (Birnie‐Gauvin et al ., 2021; Poole et al ., 2007; but see Kennedy, Rosell, & Allen, 2022), and therefore may suffer higher date‐independent mortality post‐outmigration for purely size‐or growth‐related reasons (Dermond et al ., 2019; Jonsson & Jonsson, 2009).…”