Evidence for non‐random spatial positioning of migrating smolts (Salmonidae) in a small lowland stream

Abstract

1. The ontogenetic development of anadromous salmonids includes downstream emigration of immature individuals from freshwater towards the marine environment. Although this migration of juvenile salmonids (smolts) may be associated with severe mortalities, only limited attention has been paid to the spatial positioning of smolts in small streams. 2. Using a novel approach, this study examined the vertical and horizontal positioning of brown trout and Atlantic salmon smolts while performing downstream migration in a small lowland stream. 3. Pre-smolts of indigenous and hatchery-reared (F1) brown trout (Salmo trutta), and two different populations of Atlantic salmon (S. salar), were tagged with passive integrated transponder (PIT) tags and subsequently released upstream of an antenna array consisting of five circular swim-through PIT antennas. Antennas were positioned in order to determine whether the migrating smolts were bottom or surface oriented, and if they were oriented towards the mid-channel or the stream bank. 4. During the smolt emigration period, data describing both the detection of the migrating fish and the amount of water passing through the antennas were collected. This was accomplished in order to determine if the fish were performing active positioning behaviour independently of the vertical and horizontal discharge distributions in the stream. 5. The results showed that the smolts migrated in a non-random spatial pattern independently of the stream discharge distributions. Vertically, the indigenous brown trout and the Atlantic salmon demonstrated a preference for the bottom orientated positions. In contrast, the distribution of the F1 brown trout was not different from the discharge distribution. The latter observation suggests random vertical positioning, which may be indicative of inferior migratory performance. Horizontally, all tested smolt populations strongly preferred the mid-channel positions. 6. The discharge-corrected preferences for certain spatial positions suggest that smolt emigration is not entirely a matter of passive displacement in lowland streams. 7. Anthropogenically altered channels may inhibit or delay downstream emigration of smolts resulting in increased mortalities. Given that the smolts in this study actively selected spatial positions in the mid-channel and near the bottom, it is suggested that deep, mid-channel furrows may be used to help guide migrating smolts past adverse habitats in lowland streams.

Linking individual behaviour and migration success inSalmo salarsmolts approaching a water withdrawal site: implications for management

“…Recent studies have demonstrated behavioural avoidance in smolts related to hydraulic conditions (Haro et al 1998;Enders et al 2009), in-stream structures (Kemp and Williams 2008), overhead cover and absence of visual clues (Kemp et al 2005;Kemp and Williams 2009). Moreover, fish guidance may be accomplished using strobe lights (Johnson et al 2005), bubble curtains and sounds (Welton et al 2002;Sonny et al 2006) and deep, mid-channel furrows (Svendsen et al 2007). These findings indicate that smolts can be guided rapidly past the water withdrawal zone and over the weir crest to ensure passage.…”

“…Smith 1982). More recent studies have however, demonstrated smolts responding behaviourally to in-stream structures (Kemp and Williams 2008) and visual clues (Kemp and Williams 2009), avoiding rapidly accelerating currents (Haro et al 1998;Enders et al 2009) and overhead cover (Kemp et al 2005), and preferring certain areas while migrating downstream (Davidsen et al 2005;Svendsen et al 2007). The present study corroborates the conclusion that smolt migration includes active components and depends on behavioural decisions.…”

“…After operative recovery, fish were released 200 m upstream of the hydrophone array following Svendsen et al (2010). Smolts are typically nocturnal migrants (Olsèn et al 2004;Svendsen et al 2007). Therefore, to reduce the risk of post release predation and facilitate nocturnal migration, fish were released in darkness (i.e.…”

Marine migrations in anadromous brown trout (Salmo trutta). Fjord residency as a possible alternative in the continuum of migration to the open sea

“…To detect any return of acoustic tagged (and PIT-tagged) individuals into the riverine environment, antennae system can be found in (Svendsen et al, 2007). The PIT-tags antennae were continuously in operation from the beginning of the experiment until September 2012 (approximately 3 years after the last fish were tagged).…”

Effects of passive integrated transponder tags on survival and growth of juvenile Atlantic salmon Salmo salar

“…Over the past decades, passive integrated transponder (PIT) tags have proven to be a powerful tool for monitoring migration, growth, survival, and spatio-temporal distribution of various fish species [1][2][3][4][5][6]. PIT tags are advantageous due to their longevity, small size, and ability to equip individuals with a unique identification code.…”