Active migration of wild Atlantic salmon Salmo salar smolt through a coastal embayment

Abstract

Migration patterns of wild Atlantic salmon Salmo salar smolt were examined in a coastal embayment in the Gaspé peninsula of Québec, Canada. Twenty-four smolt in 2005 and 30 in 2006 were tagged with coded ultrasonic transmitters, and their migration throughout the bay was monitored using an array of fixed VR2 hydrophone receivers. Migration patterns were complex, with some smolt taking a direct route through the coastal embayment and others repeatedly changing direction over short spatial and temporal scales. Migration was mainly an active process with an overall outward (seaward) migration in the face of an inward residual circulation. Swimming direction was mainly outward during nocturnal inflowing currents but was more dispersed during daytime and nocturnal outflowing currents; swimming speed was greater during daytime than during nighttime. This pattern was consistent with smolt migrating offshore nocturnally and using daytime for prey detection and predator avoidance. Salinity had a strong effect: exposure to more saline waters caused increased swimming speeds. These observations are consistent with the hypothesis that smolt exploit an innate compass to maintain a preferred bearing and that the speed and direction of swimming is controlled by salinity and the diurnal cycle.

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.…”

“…The smolts normally show rapid and directed seaward movements in a fjord system, and during the seaward movements little is known about visits to freshwaters and brackish waters. Swimming speeds normally increase with the distance from the natal river [6,12]. However, in the present study, the swimming speeds became slower as the post-smolts moved further away from their home river.…”

“…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.…”

“…The tagged animals may be tracked either manually or by using stationary receivers that record presence when within the detection range of a receiver [5][6][7]. When multiple receivers are deployed in an array, either at locations of particular interest, or across a river or a narrow bay or fjord, the obtained…”

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.…”