Comparing Smolt Production Estimation Methods in Atlantic Salmon (Salmo salar)

Abstract

Over the last 50 years the number of wild Atlantic salmon has declined by more than 50 %, and a better understanding of the survival and recruitment dynamics of wild populations are needed to ensure the right measures are taken to inform management authorities. River specific spawning targets which state how many adult salmon are needed to produce the highest amount of smolt the river can hold, i.e., the carrying capacity, is one measure to ensure a minimum number of salmon in the river. Smolt production is a common way to monitor population recruitment, and it is typically quantified by electrofishing, although this approach may not be as accurate as expected. This thesis estimated smolt production in the Etne river system by both electrofishing and mark-recapture methods and compared the estimates. The aim was to investigate the interannual and spatial variation in smolt production and how these are linked with roe deposition and spawning biomass. Previous electrofishing surveys conducted by Rådgivende Biologer in 2008 and 2014 were recreated to examine how the pre-smolt production has changed over time. In addition, the roe to smolt survival in the river was calculated and all estimates were related to the spawning target of the river.

10 stations distributed in Etneelva, Nordelva, and Sørelva were electrofished in 2023 to estimate total production and density per 100 m2 of pre-smolt, i.e., juveniles > 10 cm. Results from 2023 were then compared to electrofishing surveys previously carried out in 2008 and 2014. Nordelva had the highest density of pre-smolt each year, while 2014 was the year with the highest estimated total pre-smolt production of 31 442 pre-smolt, twice as much as in 2018 (16 598) and 2023 (17 290). The difference in the production between the river areas may be related both to habitat quality and temperature differences.

Since 2016, smolts leaving the river have been PIT-tagged and first-time spawners entering the river have been registered on the Resistance Board Weir fish trap that has been installed since 2013. Marine survival was thereafter estimated for each smolt year class and used to estimate the smolt production of the smolt year classes of 2016 to 2021. The estimated smolt production showed a tendency of increasing production through the years. In 2016 and 2017 the production was at its lowest, 40 325 and 42 395 smolts respectively, but increased to 72 149 in 2018 due to the increase in roe deposition between 2014 to 2015. The highest estimated smolt production was 116 066 for 2021.

Spawning biomass and roe deposition was estimated for 2013 to 2023 with data from the Resistance Board Weir fish trap and angling catch data. The estimates were investigated in the context of smolt production and the spawning target and showed that the smolt production seems to vary as a result of variation in roe deposition. Both roe deposition and spawning biomass were well above the spawning target most years, and as the smolt production showed a tendency of increasing annual production, this indicates that the spawning target set for the river is too low. By using the estimated roe deposition and smolt production as well as the age structure among the salmon, the roe to smolt survival was calculated and a mean survival of 1.89 % was estimated, which is close to half of what is assumed for the river.

In conclusion, the electrofishing seems to underestimate smolt production, and mark-recapture might be a better option to get more accurate estimates, yet the method is more costly and time consuming. There seem to be a close relationship between the roe to smolt survival and spawning biomass in the river, and the cumulative roe deposition and the annual smolt production. The smolt production of the river seem to be increasing, yet a longer time series of data is needed to identify the carrying capacity of the river. Future research should investigate how environmental variables, such as water temperatures during the different seasons, affect the smolt production and roe to smolt survival to get a fuller picture of the interannual variation in smolt production and the drivers behind this variation.