Spatial-temporal variation in the vertical behaviour of sea trout kelts during their marine phase

Abstract

Sea trout, the anadromous form of brown trout (Salmo trutta), migrate from freshwater to coastal and offshore habitats for increased feeding and growth opportunities and ultimately, reproductive success. However, knowledge of habitat use and behaviour in the marine environment is limited, particularly for post-spawners (kelts). In this study, we used data collected from 63 sea trout kelts that were double tagged with acoustic and data storage tags, caught in three rivers feeding into the English Channel (rivers Tamar and Frome in southern England and Bresle in northern France) to describe depth use and diving behaviour at sea as a function of a suite of abiotic and biotic variables. Diving activity mostly occurred during daylight hours across all rivers, but we also identified population-specific behaviours with regards to diving frequency and depth as well as the relationship between size of fish and vertical behaviour. While we found distinct diel vertical behaviour consistent with existing literature on sea trout populations at higher latitudes, we showed kelts in this study to use relatively deeper water during the marine phase. This work contributes to limited knowledge of adult sea trout behaviour at sea and indicates necessary considerations for managing anthropogenic impacts at sea on this important life-stage.