Survival Of Atlantic Salmon Smolts Research Articles

The effect of trapping on the migration and survival of Atlantic salmon smolts

AbstractElectronic tags are often used to track the freshwater‐marine migrations of smolts, where smolts are captured for tagging pre‐migration (e.g., via electrofishing) or during‐migration (e.g., via traps). Pre‐migration capture allows smolts to initiate and complete their downstream migration unhindered, but risks smolt loss before the migration commences. The contrary is the case for during‐migration trap‐caught smolts, but trapping smolts temporarily halts their seaward journey which may negatively impact their progress. This study investigated the effect of trapping on the behaviour and survival of migrating Atlantic salmon (Salmo Salar) smolts using acoustic telemetry. We compared the movements and survival of smolts tagged before the smolt run captured by electrofishing (“comparator”) with smolts trapped and tagged during the smolt run (“trapped”). A total of 478 smolts were tagged and released in River Skjern (2020 and 2022), Denmark, and 82 smolts in River Ballycastle (2022), Northern Ireland, and their seaward movements were monitored using acoustic receivers deployed in the river, fjord, and coastal area. In River Skjern in 2022, comparator smolts migrated earlier than trapped smolts, likely because these constituted more of the larger‐sized, earlier migrating individuals. We found no differences in descent trajectories, diel patterns, progression rates, or survival between trapped smolts and comparator smolts in any of the rivers or study years. Thus, our results support the use of during‐migration trapping as a low‐impact method to capture smolts for telemetry studies, with trapped samples (if held <24 h) yielding comparable results in terms of behaviour and survival with non‐delayed pre‐migration tagged fish.

Effects of dam-induced delays on system-wide survival of Atlantic salmon smolts during high-flow, high-survival years in the Penobscot River, Maine, USA

System-wide survival of hatchery-reared Atlantic salmon ( Salmo salar) smolts was evaluated (2017–2019) in the Penobscot River and compared to survival estimates from previous years that spanned major changes (2005–2016). This system was transformed through two dam removals in 2012 and construction of a nature-like passage structure at a third. The main stem had three dams (five prior to 2012), while the main tributary had four dams (one with the new nature-like passage). We estimated survival using acoustic telemetry mark–recapture ( n = 1482) from 2017 to 2019. Six release sites and two release dates were included to assess system-wide survival. Survival from 2017 to 2019 was higher than previous years, with total cumulative survival > 0.75, independently of year and release sites, compared to survival < 0.5 in previous years. These years coincided with exceptional high flows not seen previously. We found an effect of delays on survival, longer delays associated with lower survival. Overall, survival in these years increased in all reaches relative to previous years except for one dam, Weldon Dam, which was a site of sustained high mortality.

Acute effects of gas supersaturation on Atlantic salmon smolt in two Norwegian rivers

Total dissolved gas (TDG) supersaturation downstream of hydropower plants may cause gas bubble disease (GBD) and harmful effects in fish. Little is known about tolerance levels of TDG supersaturation on Atlantic salmon (Salmo salar Linnaeus, 1758) in natural rivers. The present study investigated the effects of TDG supersaturation on the survival of Atlantic salmon smolts at two field sites in Norway. Here, we kept smolts in cages at increasing distances from hydropower plants known to cause TDG supersaturation and at control sites. We recorded fish mortality and examined for GBD using a stereo microscope. Mortality and symptoms of GBD commenced in fish exposed to an average of 108.3% TDG (maximum 111.0%, water depth 0.55 m) for 2 days. Significant differences in time before mortality at the control sites and test sites commenced at 110.2% TDG (maximum 111.8%) for 3 days. The study indicates that Atlantic salmon may be more vulnerable to TDG supersaturation than Pacific salmonids, which are considered at risk when the TDG is above 110%. In addition, the study provides important data to link effects caused by TDG in the laboratory and in the field.

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

AbstractDecreased marine survival is identified as a component driver of continued declines of Atlantic Salmon Salmo salar. However, estimates of marine mortality often incorporate loss incurred during estuary migration that may be mechanistically distinct from factors affecting marine mortality. We examined movements and survival of 941 smolts (141 wild and 800 hatchery‐reared fish) released in freshwater during passage through the Penobscot River estuary, Maine, from 2005 to 2013. We related trends in estuary arrival date, movement rate, and survival to fish characteristics, migratory history, and environmental conditions in the estuary. Fish that experienced the warmest thermal history arrived in the estuary 8 d earlier than those experiencing the coolest thermal history during development. Estuary arrival date was 10 d later for fish experiencing high flow than for fish experiencing low flow. Fish released furthest upstream arrived in the estuary 3 d later than those stocked further downstream but moved 0.5 km/h faster through the estuary. Temporally, movement rate and survival in the estuary both peaked in mid‐May. Spatially, movement rate and survival both decreased from freshwater to the ocean. Wild smolts arrived in the estuary later than hatchery fish, but we observed no change in movement rate or survival attributable to rearing history. Fish with the highest gill Na+, K+‐ATPase activity incurred 25% lower mortality through the estuary than fish with the lowest gill Na+, K+‐ATPase activity. Smolt survival decreased (by up to 40%) with the increasing number of dams passed (ranging from two to nine) during freshwater migration. These results underscore the importance of physiological preparedness on performance and the delayed, indirect effects of dams on survival of Atlantic Salmon smolts during estuary migration, ultimately affecting marine survival estimates.Received October 25, 2014; accepted January 8, 2015

Use of Inclined-Plane Traps to Study Movement and Survival of Atlantic Salmon Smolts in the Connecticut River

Abstract We determined the utility of inclined-plane traps for capturing smolts of Atlantic salmon Salmo salar and studied the timing of the downriver run and survival of smolts in the Connecticut River in 1985 and 1986. We captured 22 smolts with one trap in 1985, and 45 with three traps in 1986. Although these numbers were small, the results from both years were similar. Mean total lengths of smolts (ranges in parentheses) were 186 mm (152–206 mm) in 1985 and 178 mm (152–201 mm) in 1986. The migration period in both years extended from early April to mid-May. Mean river temperatures when smolts were captured were 10.5°C (4.5–16.0°C) in 1985 and 10.3°C (4.5–15.0°C) in 1986. Most smolts were captured several weeks after the period of peak spring flows that probably would have provided the safest passage past electricity generating stations. The recapture rates of tagged hatchery smolts stocked 8 km upstream from the trap were 1 of 4,400 (0.023%) in 1985 and 1 of 2,900 (0.034%) in 1986. These rates were si...

The acute lethal toxicity of mixtures of cyanide and ammonia to smolts of salmon, Salmo salar L. at low concentrations of dissolved oxygen

The survival of Atlantic salmon smolts on exposure to constant concentrations of cyanide and ammonia, singly and together, has been measured under laboratory conditions at a concentration of 5 mgl‐1 of carbon dioxide. The 24‐h LC50 values of cyanide and of un‐ionised ammonia, in fresh water, were 0·073 mg HCN l‐1 and 0·20 mg NH3l‐1 respectively at a concentration of dissolved oxygen of 10 mg l‐1, and 0·024 mg HCN l‐1 and 0·08 mgNH3l‐1 respectively at a concentration of dissolved oxygen of 3·5 mg l‐1. In 30% sea water the corresponding values were similar for cyanide but markedly higher for ammonioa. In 80% sea water the values were intermediate between those of fresh water and 30% sea water. Prior acclimation of the fish to the respective toxicant increased the resistance of the fish only slightly to cyanide, but with ammonia the 24‐h LC50 was increased between 1·4 and 2‐fold after acclimation for 1–3 days to between 0·2 and 0·5 of the 24‐h LC50 value. Mixtures of cyanide and ammonia were between 0·6 and 1·25 times as toxic as expected, assuming simple additivity of toxicity.

The effect of dissolved oxygen and salinity on the toxicity of ammonia to smolts of salmon, Salmo salar L.

The survival of Atlantic salmon smolts on exposure to constant concentrations of ammonia has been measured under laboratory conditions. At concentrations of dissolved oxygen close to the air‐saturation value, the 24‐h LC50 of un‐ionised ammonia is 0.15 mg NH31−1 in fresh water (hardness 264 mg 1−1 as CaCO3) and 0.3 mg NH31−1 in 30% sea water; at concentrations of dissolved oxygen of 3.5 mg 1−1 in fresh water and 3.1 mg 1−1 in 30% sea water, the 24‐h LC50 is 0.09 mg NH3 1−1 and 0.12 mg NH3 1−1 respectively; for fish acclimated for 1 day to a concentration of ammonia close to the 24‐h median for un‐acclimated fish, the median is increased between 38 and 79%, depending on test conditions.

The survival of smolts of salmon Salmo salar L. at low concentrations of dissolved oxygen

The survival of Atlantic salmon smolts on sudden exposure to low constant concentrations of dissolved oxygen (DO) has been measured under laboratory conditions; the 3‐d LC50 is about 3 mg l−1 in freshwater and about 2.5 mg l−1 in 30% and 80%‘sea water’, even for fish acclimated for several days to low concentrations of DO and high salinity; median threshold concentrations of DO are probably close to 3.3 and 2.6 mg l−1 respectively.