A tale of two fishes: depth preference of migrating Atlantic salmon smolt and predatory brown trout in a Norwegian lake

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Abstract
To understand the predator–prey interactions during this transitional migration, we tracked 22 Atlantic salmon ( Salmo salar) smolts and their most prevalent predator, brown trout ( Salmo trutta) ( N = 21), and recorded their depth use in a basin of Lake Evanger, Norway, with acoustic telemetry during May 2020. Both salmon smolts (mean ± SD: 3.8 ± 3.3 m) and trout (2.9 ± 1.7 m) were distributed relatively shallow in the lake despite depths in the area largely exceeding 30 m. Both species were deeper at midday and smolts tended to be deeper in the water earlier in the migration, overlapping less with trout early in May, but as daily daylight increased and water temperature warmed vertical distribution of smolts and trout increasingly overlapped. Based on depth traces from the tags, only seven were detected at the end of the lake and confirmed to make it through. Despite the behaviour of the salmon smolts mostly matching with predictions of the risk allocation hypothesis, it seems a large share of the tagged smolts succumbed to predation.

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