Above parr: Lowland river habitat characteristics associated with higher juvenile Atlantic salmon (Salmo salar) and brown trout (S. trutta) densities

Abstract

Understanding juvenile salmonid habitat requirements is critical for their effective management, but little is known about these requirements in lowland rivers, which include important but unique salmonid habitats. We compared the relative influence of in‐stream Ranunculus cover, water depth, prey abundance, distance upstream and two previously unexplored factors (water velocity heterogeneity and site colonisation potential) on summer densities of juvenile Atlantic salmon and brown trout. We applied electrofishing, habitat surveys and macroinvertebrate kick sampling, and calculated the site colonisation potential from salmon redd surveys across 18–22 sites in a lowland river in 2015–2017. Due to a recruitment crash in 2016, models including and excluding this unusual year were explored. Excluding 2016 data, juvenile salmon densities showed a positive association with Ranunculus cover and numbers of nearby upstream redds, and a negative association with distance upstream from the tidal limit. Trout densities were positively associated with velocity heterogeneity, indicating a potential indirect influence of Ranunculus mediated by water velocity. When including 2016, year had the largest effect on densities of both species, highlighting the impact of the recruitment failure. These findings uncover interspecific differences in the habitat requirements of juvenile salmonids in lowland rivers. Velocity heterogeneity and site colonisation potential had high explanatory power, highlighting that they should be considered in future studies of habitat use. These findings demonstrate that temporal replication and recruitment dynamics are important considerations when exploring species–habitat associations. We discuss potential management implications and argue that Ranunculus cover could be an important management tool in conservation of lowland salmonids.

High summer macrophyte cover increases abundance, growth, and feeding of juvenile Atlantic salmon

“…The steepness in velocity gradients between a quadrat and its neighboring quadrats was calculated using the Terrain Ruggedness Index (TRI) and absolute values of TRI were averaged to represent site‐level velocity heterogeneity (Marsh et al. 2020).…”

Relationship between Morphological Characteristics and Quality of Aquatic Habitat in Mountain Streams of Slovakia

“…However, fish are considered the best bioindicators and are the most sensitive to the morphology of the river channel, including regulated reaches. This has been confirmed by numerous studies [19][20][21]. The methodology of the Water Framework Directive 2000/60/EC [22] identifies fish as the appropriate bioindicator of morphological changes.…”

Medium‐term environmental changes influence age‐specific survival estimates in a salmonid population

“…A mosaic of riparian canopy cover achieved from tree planting or management could reduce macrophyte cover with limited in‐river disturbance, whilst also providing shade to alleviate high summer temperatures (Cole et al., 2020). This might also promote macrophyte cover in unshaded areas during the summer months to support other salmonid species that benefit from macrophyte cover (Marsh et al., 2020), and ensure abundant macroinvertebrate prey to improve survival of both juvenile and adult grayling. Although not directly tested in our model, we considered the negative impact of reduced winter flows on juvenile recruitment was likely to be linked to increased sediment within spawning gravels (Chapman, 1988).…”