Behavioural transition during the estuarine migration of wild Atlantic salmon (Salmo salar L.) smolt

Abstract

– Ultrasonic telemetry and hydrodynamic modelling were used to study the migratory behaviour of 54 wild Atlantic salmon (Salmo salar) smolt captured in freshwater during their downstream migration and tracked in 2 years through a shallow estuary system. A high‐density, fixed array of receivers provided detailed spatial and temporal resolution of behaviour in the second year of study. Smolt migration in the river occurred mostly at night and downstream migration was slower during the day. In the estuary, smolt moved seaward on ebbing tides and landward on flooding tides. The effect of current velocity was greater during the night than during the day. We documented for the first time that current velocity and diurnal period only accounted for approximately one‐third of the variation in smolt ground speeds in the estuary, indicating that smolt movements were far less passive than previously reported. Smolt energetic status had no effect on smolt swimming behaviour or migratory performance. With an increase in salinity, smolt seaward movements during flooding tides were more frequent, and overall seaward ground velocity increased. The increase in salinity experienced by the smolt during their migration through the leading edge of saltwater intrusion thus induced a behavioural transition from a more passive, fluvial migration to a more active‐ and seaward‐oriented migration.

In situ measurement of salinity during seaward migration of Atlantic salmon post-smolts using acoustic transmitters with data-storage capabilities and conventional acoustic transmitters

“…However, other studies have demonstrated that water currents are not systematically used as an orientation cue, because smolts often show random movements compared to the water current direction [27,31]. It has also been suggested that positive salinity gradients may be one of the key factors for seaward movements [6,26]. Our results support that post-smolts may exploit variation in salinity cues for navigation purposes.…”

“…A preference for low salinities in the first phase of the marine migration may furthermore be related to acclimatization to sea water, because the transfer from freshwater to sea water involves considerable osmoregulatory stress [24]. Although several studies have shown that post-smolts migrate in the upper few metres of the water column [6,25,26], little is known about the salinity they actually experience in situ, and how they respond to variation in salinity.…”

Linking Behavior, Physiology, and Survival of Atlantic Salmon Smolts During Estuary Migration

“…Other laboratory experiments have found no period of acclimation occurred for smolts upon reaching salt water (Moore et al 1995). It is, however, possible that fish use tidal movements to minimize energetic costs, especially because this is the period of migration during which smolts are thought to transition from passive to active migration (Hedger et al 2008;Martin et al 2009). Further investigation of diurnal and tidal factors that influence behavior and survival on localized spatial and temporal scales could provide an improved understanding of those mechanisms and might have implications for strategic stocking of hatchery-reared smolts.…”

“…The reduction in movement rates through estuaries might also result from the reversal of migratory direction during the passage of tidal estuaries and bays (Kocik et al 2009;Dempson et al 2011;Halfyard et al 2013). These behaviors appear to be related to tidal cycles in both estuarine (McCleave 1978;Martin et al 2009) and coastal systems (Lacroix et al 2005). This behavior previously has been hypothesized to relate to saltwater acclimation (Gudjonsson et al 2005;Dempson et al 2011;Halfyard et al 2013).…”

Reduced migratory performance of farmed Atlantic salmon post-smolts from a simulated escape during autumn

“…The smoltification process is the physiological and morphological transition that enables these fish to switch from a freshwater-adapted metabolism to a marine pelagic lifestyle, and that also marks a behavioural shift from resident to migratory behaviour (Hoar 1976, Gribson 1983, Martin et al 2009). Cultured salmon are transferred from the hatchery to cages in the sea at the smolt stage.…”