Managing for Atlantic Salmon Smolt Run Timing Variability in a Changing Climate

Abstract

The Gulf of Maine Distinct Population Segment of Atlantic Salmon Salmo salar is listed under the U.S. Endangered Species Act, with dams cited as a key threat to the species. Safe, timely, and efficient passage of emigrating smolts is critical for advancing recovery goals. However, climate change and the earlier (and more variable) onset of smolt migration have challenged the effectiveness of measures designed to protect smolts. Therefore, we used data from four long-term smolt trapping sites in Maine to characterize run duration, identify a standardized smolt run, and develop a predictive model for the initiation of smolt emigration for natural-and hatchery-origin smolts. We combined these data into a basinwide deterministic model that projects the movement of smolts from their point of origin to estuary entry, which we used to (1) evaluate duration and temperature triggers for supplemental spill periods used at hydroelectric dams to protect smolts and (2) investigate how knowledge of smolt emigration dynamics can guide protective measures in rivers where management actions are being developed. Timing of run initiation varied by over 14 d; however, mean air temperature for the first quarter of the year explained over 60% of the variance in the onset of emigration. Initiation temperature was linked with higher-elevation rearing areas where smolts originated and not with downstream dams where temperature was monitored. The shape of the smolt wave varied considerably; thus, our standardized smolt wave captured average dynamics but not the specific shape. Overall, 14-d spill windows are too short. To account for the variable timing and shape of the run, a 54-72-d protection window would be necessary to protect the earliest and latest emigrants in the absence of physical structures (e.g., turbine screens) and preserve the adaptive variation required for populations to respond to climate-driven changes in temperature and hydrology.Populations of Atlantic Salmon Salmo salar in Maine are listed under the U.S. Endangered Species Act, and the impacts of dams on these fish pose a threat to the species' recovery (National Research Council 2004;Fay et al. 2006; USFWS and NMFS 2018). Although both upstream and downstream passage at dams is essential, losses during the downstream migration of smolts in the springtime are particularly problematic. Estimates of smolt emigration from the Penobscot River-the largest watershed in Maine-from 1970 to 2012 indicated that of 19.7 million smolts stocked, only 7.7 million (39%) entered the ocean, and survival (63-95%) was most variable at dams (Stevens et al. 2019).

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

“…Trap‐and‐transport of salmon smolts is commonly used to mitigate the impacts of hydropower developments and other in‐river obstructions (Kock et al, 2021). Smolts are trapped at night when they are actively migrating, but smolt release typically occurs during daylight hours (Frechette et al, 2023; Lothian et al, 2018; Lundqvist et al, 2010) due to safety concerns and resource limitations. The use of consistent release locations, favoring vehicle access and ease of release, can become potential pinch‐points as predator aggregations are common in such areas where feeding opportunities are elevated (Jepsen et al, 2000; Kennedy et al, 2018; Larinier & Travade, 2002).…”

Potential Impact of Climate Change on Salmonid Smolt Ecology

“…Temperature and flow are environmental cues for smolt migration. Migration times differ between years and rivers; the temperature can be a good predictor of the timing [129][130][131]. Warmer temperatures result in earlier migrations [130].…”

“…For example, migration route selection at a hydropower plant intake has been found to be consistent with phenotypes, and those traits selecting turbines could potentially be eliminated from the population due to high mortality [152]. A long spill time is needed to protect the earliest and latest migrating phenotypes [131]. Low flows are expected to become more frequent, especially in the southern distribution area of salmonids, and during low flows, even small weirs can cause significant delays in smolt migration impacts [153].…”

The impacts of diel thermal variability on growth, development and performance of wild Atlantic salmon (Salmo salar) from two thermally distinct rivers

“…Smolts are juveniles preparing to transition from freshwater phenotype to saltwater phenotype during migration from freshwater streams to the ocean. The relationship between water temperature and initiation of smolt migration has been speculated to be related to a specific and/or cumulative temperature threshold but can also be impacted by day length and water discharge rates ( McCormick et al, 1998 ; Whalen et al, 1999 ; Zydlewski et al, 2005 ; Teichert et al, 2020 ; Frechette et al, 2022 ). Predictive models suggest that the mean temperature in the first 90 days of the year is a significant driver of the variation in onset of smolt migration timing across streams within a watershed ( Frechette et al, 2022 ).…”

“…The relationship between water temperature and initiation of smolt migration has been speculated to be related to a specific and/or cumulative temperature threshold but can also be impacted by day length and water discharge rates ( McCormick et al, 1998 ; Whalen et al, 1999 ; Zydlewski et al, 2005 ; Teichert et al, 2020 ; Frechette et al, 2022 ). Predictive models suggest that the mean temperature in the first 90 days of the year is a significant driver of the variation in onset of smolt migration timing across streams within a watershed ( Frechette et al, 2022 ). The survival success of the downstream migrants is also known to be dictated by environmental temperatures, and the smolts’ ability to meet certain ‘smoltification windows’ that are largely driven by temperature ( McCormick et al, 1997 ).…”