Salmo trutta shows considerable downstream migration

Do Brown trout read books?

New telemetry study shows considerable downstream migration of juvenile trout outside the classic smolt migration period, providing important considerations for the management of sea trout stocks. The results emphasize, for example, the need for recording full-year fauna passage.

They also show that trout downstream migration pattern do not follow the classic description provided in books about trout biology. "Trout simply haven’t read them" says one of the authors of the study, Kim Aarestrup.

Sea trout is a key fish species in Northern European streams and also constitutes one of the most important fish for recreational fisheries in a range of countries. The fish reproduce and spawn in rivers, but migrate to sea where they undergo the majority of their growth. The classic description of anadromous brown trout populations states that juveniles remain in their native stream for one or several years, before adapting to life at sea and descending the river as smolts in spring. They then undergo extensive growth at sea, and subsequently return to the river to spawn. Read more about the sea trout life cycle.

Hence, in management, when considering anadromous salmonid population dynamics, the freshwater output of a river typically refers to the production of spring migrants. However, few published tracking or trapping studies have targeted juvenile trout outside spring.

A new study puts a critical light on this view and shows that juvenile trout also migrate downstream at other times of the year.

Using PIT telemetry, DTU-Aqua and Durham University investigated the downstream movement of young trout in the English River Deerness and the Danish River Villestrup. Both streams were electrofished during the summer and fall seasons, and young trout were PIT tagged. The downstream movement of tagged fish was recorded until the next summer using PIT antennas installed in the rivers. The results showed a significant downstream migration outside the so-called “normal” season in both rivers (defined conservatively as February 1^st to June 1^st in this study).

At least 46% of the fish had migrated downstream by February 1^st in River Deerness, while the corresponding figure for River Villestrup was 25%, rising to 30% if including downstream migration up to when the classic smolt season starts in Denmark, in mid-March.

There was no size difference between fish that migrated before and within the normal smolt migration period, but spring migrating trout in River Deerness were more fit and migrated faster. This may be due to the trout in the two periods having different motives for downstream movement perhaps because trout that migrate outside the regular smolt season are less adapted to life in marine waters.