Connectivity between lentic and lotic freshwater habitats identified as a conservation priority for coho salmon

Abstract

Juvenile Pacific salmon exhibit diverse habitat use and migration strategies to navigate high environmental variability and predation risk during freshwater residency. Increasingly, urbanization and climate‐driven hydrological alterations are affecting the availability and quality of aquatic habitats in salmon catchments. Thus, conservation of freshwater habitat integrity has emerged as an important challenge in supporting salmon life‐history diversity as a buffer against continuing ecosystem changes. To inform catchment management for salmon, information on the distribution and movement dynamics of juvenile fish throughout the annual seasonal cycle is needed. A number of studies have assessed the ecology of juvenile coho salmon (Oncorhynchus kisutch) during summer and autumn seasons; catchment use by this species throughout the annual cycle is less well characterized, particularly in high‐latitude systems. Here, n = 3,792 tagged juvenile coho salmon were tracked throughout two complete annual cycles to assess basin‐wide distribution and movement behaviour of this species in a subarctic, ice‐bearing catchment. Juvenile coho salmon in the Big Lake basin, Alaska, exhibited multiple habitat use and movement strategies across seasons; however, summer rearing in lotic mainstem environments followed by migration to lentic overwinter habitats was identified as a prominent behaviour, with two‐thirds of tracked fish migrating en masse to concentrate in a small subset of upper catchment lakes for the winter. In contrast, the most significant tributary overwintering site (8% of tracked fish) occurred below a culvert and dam, blocking juvenile fish passage to a headwater lake, indicating that these fish may have been restricted from reaching preferred lentic overwinter habitats. These findings emphasize the importance of maintaining aquatic connectivity to lentic habitats as a conservation priority for coho salmon during freshwater residency.

A melting cryosphere constrains fish growth by synchronizing the seasonal phenology of river food webs

“…We assumed that individuals moved without energetic cost but were susceptible to density‐dependent limitations on foraging and growth. This formulation created a range of movement propensities that juvenile coho salmon have been shown to exhibit (Armstrong et al, 2013; Sethi et al, 2021), and is realistic when individuals reside in close proximity to other stream types (e.g., tributary junctions).…”

“…Coastal watersheds in this region frequently contain a mosaic of glacier‐, snow‐, and rain‐fed tributaries (Figure 1) that have distinct flow, temperature, and biogeochemical regimes and provide accessible year‐round habitat to salmon (Figure 2; Edwards et al, 2013; Fellman, Hood, et al, 2014; Hood & Berner, 2009). Juvenile salmon live up to 2 years in these streams before migrating to the ocean and can move long distances within watersheds to track favorable conditions for growth and survival (Armstrong et al, 2013; Sethi et al, 2021). Thus, the close spatial juxtaposition of meltwater and non‐meltwater streams could enhance juvenile salmon productivity by providing an intra‐watershed portfolio of streams with different resource phenologies (Moore et al, 2015).…”

Juvenile salmon habitat use drives variation in growth and highlights vulnerability to river fragmentation

“…The growth payoffs for juvenile coho salmon associated with lake overwinter habitat use strategies are dependent on seasonal migration corridors. In the Big Lake drainage, lentic overwintering fish frequently moved tens of kilometers upstream to transition from summer lotic rearing habitats to lentic overwinter habitats (Sethi et al, 2021). At such distances, these migration corridors are vulnerable to river fragmentation from dams and culverts, which could block passage to lentic overwinter grounds and constrain freshwater rearing to lower watershed and lotic habitats.…”

“…Fish habitat use strategies and growth outcomes were linked by first categorizing tagged fish movements during freshwater rearing, and subsequently by monitoring total growth between the time of initial tagging and at the time of outward smolt migration to the sea. Tagged fish distributions in the drainage were described in a previous study (Sethi et al, 2021), and here, we assess individual‐level habitat use strategies. Habitat use strategies were inferred by monitoring tagged fish movements using a combination of the minnow trap sampling outlined above for tag deployments and with a set of seven fixed pass‐through antenna installations that could detect fish as they swam by (specifications detailed in Ashline, 2016).…”

“…Thus, juvenile salmon move among locations during summer and fall to track seasonally shifting thermal regimes and capitalize on foraging opportunities (Baldock et al, 2016; Huntsman & Falke, 2019). Furthermore, juvenile salmon conduct wider seasonal migrations (Ebersole et al, 2006; Weybright & Giannico, 2018), with evidence of cross‐watershed movements to reach preferred overwintering grounds for some species (Sethi et al, 2021). To support connectivity restoration efforts in salmon‐bearing watersheds, understanding of the life history consequences of juvenile salmon movement and habitat use strategies is needed to assess population‐level impacts of river fragmentation and guide effective stream barrier mitigation (Flitcroft et al, 2019; Sethi et al, 2017; Silva et al, 2018).…”