Investigating the behaviour of Atlantic salmon (Salmo salar L.) post‐smolts during their early marine migration through the Clyde Marine Region

Abstract

It is thought that survival during migration is particularly poor for Atlantic salmon postsmolts immediately after entry into sea and particularly in the estuarine environment.Nonetheless, there is currently a lack of information on Atlantic salmon post-smolt movement behaviour in estuaries in the UK. This study used acoustic tagging to estimate loss rates and compare the behaviour of Atlantic salmon post-smolts migrating from two distinctly different rivers draining into the Clyde Estuary, the River Endrick (n = 145) and the Gryffe (n = 102). Contrary to most literature, post-smolts undertook rapid migrations through the estuary, potentially decreasing their exposure to predators/anthropogenic stressors and reducing their estimated loss rates (river: 1%-3% km -1 ; estuary: 0.20%-0.60% km -1 ). The low loss rates in the estuary occurred despite post-smolts engaging in passive reversal movements with the tide upon entering the estuary, possibly allowing them more time to adapt to the increased salinity. Atlantic salmon post-smolts from both the rivers used similar migration pathways exiting into the coastal marine zone during ebbing tide. This study provides novel information on the timing and migratory routes of Atlantic salmon post-smolts in the Clyde Estuary that can ultimately be used to inform management decisions on how to assess and reduce the potential impacts of current natural and anthropogenic stressors. Temporal repeatability of this study over multiple years is required to determine if there is variation in the factors driving the migratory patterns and loss rates of smolts in this system.

A novel automatic release cage increases survival of Atlantic salmon (Salmo salar) smolts released at night

“…However, given the greater distances between subsequent receivers (range 1.3–3.1 km), it is considered that any detection beyond the first receiver reflects active downstream movements by smolts. Overall loss rates for day‐ and night‐release groups are within the range reported in other studies (e.g., Jepsen et al, 1998; Lilly et al, 2022; Thorstad et al, 2012), but, importantly, are approximately halved for the night‐release group (1.9% km −1 ) compared to the day‐release group (4.0% km −1 ).…”

“…Diurnal predation on smolts is high, and nocturnal migration is a recognized evolutionary strategy to increase survival by avoiding visual predators (Ibbotson et al, 2006; Thorstad et al, 2012). Behavioral impacts on smolts resulting from handling can also increase predation risks for several hours following release (Iversen et al, 1998; Lilly et al, 2022).…”

Migration patterns and navigation cues of Atlantic salmon post‐smolts migrating from 12 rivers through the coastal zones around the Irish Sea

“…Detection data was filtered for false detections using the short‐interval criterion in the R package Glatos and by removing consecutive detections at a single receiver that occurred within the period that was less than the minimum nominal delay of that tag (Table 1; Holbrook et al, 2018; Pincock, 2012 (for a detailed description of false detection filtering see Lilly et al (2022)). Post‐smolts that were detected at monitoring lines A and B for multiple days beyond the date when 75% of post‐smolts had left the Irish Sea were removed from the analysis, as such detections were likely either a fish mortality, or a tag that was consumed by a predator.…”

“…Sunset and sunrise times were calculated using the getSunlightTimes function in the R package suncalc (Thieurmel & Elmarhraoui, 2019). The initial detection of a post‐smolt on monitoring line B during each hour was then converted to degrees (and to a circular object) using the R package circular and visualized using circular rose diagrams (Lund & Agostinelli, 2018), where 0° and 180° represented midnight and noon, respectively (Lilly et al, 2022).…”

“…Pincock, 2012 (for a detailed description of false detection filtering seeLilly et al (2022)). Post-smolts that were detected at monitoring lines A and B for multiple days beyond the date when 75% of post-smolts had left the Irish Sea were removed from the analysis, as such detections were likely either a fish mortality, or a tag that was consumed by a predator.…”