Quantifying the relative importance of stock level, river temperature and discharge on the abundance of juvenile Atlantic salmon (Salmo salar)

Abstract

Fisheries scientists need to understand the relationships between river temperature, discharge and production of juvenile salmonids to inform evidence‐based management and regulation of rivers and to understand the potential effects of climate change. These relationships can be determined by characterising interannual variability in abundance and environmental conditions from long‐term monitoring data and assessing their inter‐relationships. Two major challenges are (1) the requirement to separate the relative effects of stock level and environment which both affect interannual variability in abundance and (2) obtaining long‐term environmental time‐series that do not suffer from temporal biases. This study built on recent advances in hydrological, river‐temperature and juvenile salmonid modelling to investigate the influence of temperature and discharge on interannual variability in Atlantic salmon (Salmo salar) fry (age: 0 years) and parr (age: 1 year) production. The study used a unique long‐term dataset (>50 years) with detailed age‐differentiated census data collected for multiple life‐stages. The study shows that most of the interannual variability in recruitment was explained by stock level. Discharge had a comparatively small effect on fry recruitment, but a greater effect than artificial stocking. Discharge had no discernible effect on parr recruitment. Temperature had no effect on recruitment of either life‐stage. This study suggests that salmon are well adapted to current environmental variability in natural upland rivers in Scotland, but reductions in discharge during spawning and emergence could negatively affect fry recruitment with consequences for regulation of river flows. The study highlights the importance of high‐quality census data for accurately determining the effects of environmental variability on recruitment.

Can aspects of the discharge regime associated with juvenile Atlantic salmon (Salmo salar L.) and trout (S. trutta L.) densities be identified using historical monitoring data from five UK rivers?

“…Despite these caveats, this analysis identified a high discharge effect between spawning and fry emergence on 0+ salmon and trout densities, consistent with other studies (e.g. Bergerot et al., 2019; Glover et al., 2020), and thereby contributing to the development of a conceptual model of hydro‐ecological processes.…”

“…As the number of high discharge events rose from zero to four, the density marginal effect initially decreased from 10 (±6.7) to 9.5 (±6.4), followed by a subsequent increase to 19.9 (±14.3) as the number of high discharge events peaked at 18. Glover, Soulsby, Fryer, Birkel, and Malcolm (2020) showed that high discharges between spawning and emergence enhance 0+ salmon densities. Pre‐emergence covered the river‐specific period from peak autumn to winter adult spawning to spring fry emergence.…”

“…Salmonids might not be affected by these hydrological variables because they have evolved life‐history strategies in response to the discharge regime of their natal river (Bunn & Arthington, 2002; Lytle & Poff, 2004). Perhaps these hydrological variables operated at local spatial and temporal scales or interacted with variables not considered in this analysis, such as water temperature (Gibson & Myers, 1988), habitat availability (Armstrong et al., 2003), geomorphology (Moir, Gibbins, Soulsby, & Webb, 2004), within‐catchment location (Vollset et al., 2014), and adult abundance or egg deposition (Glover et al., 2020), for which comparable data were incomplete or unavailable. Definitions of hydrological variables used in this analysis might have differed from other studies.…”

Structural changes to forests during regeneration affect water flux partitioning, water ages and hydrological connectivity: Insights from tracer-aided ecohydrological modelling

“…Mean absolute error was used for discharge to avoid over-emphasis of high flows that can occur when using metrics based on mean squared errors (Krause et al, 2005). It was further used for all isotope simulations given the limited variability in observations and the daily time step of the model (see Gupta et al, 2009;Schaefli and Gupta, 2007). Root mean squared error was otherwise used as this is recommended if no information is available on error distributions (Chai and Draxler, 2004;Kuppel et al, 2018b).…”

A longer‐term perspective on soil moisture, groundwater and stream flow response to the 2018 drought in an experimental catchment in the Scottish Highlands

“…The latter question is particularly important in the context of the Girnock Burn, as the catchment is an important long‐term fisheries monitoring site, particularly for Atlantic salmon ( Salmo salar ) (Glover et al, 2020). In Scottish rivers, Atlantic salmon is a key biodiversity component which is also economically important as a sports fishing resource.…”