Phenological Diversity of Salmon Smolt Migration Timing within a Large Watershed

Abstract

Although there is growing concern that climate change might drive phenological mismatches between predators and prey, it is possible that within‐ and among‐species phenological variation provides resilience against such a mismatch. One key life history event that may be vulnerable to climate‐induced mismatch is the seaward migration of juvenile salmon relative to the spring bloom of their marine zooplankton prey. Here, we quantified phenological diversity of out‐migration timing among salmon populations within a large watershed and its implications for climate mismatches with marine zooplankton. Specifically, we sampled juvenile Sockeye Salmon Oncorhynchus nerka throughout the spring and early summer in the estuary of the Skeena River, a vast watershed with numerous locally adapted salmon populations that support commercial, recreational, and First Nations fisheries, and we used genetic stock identification to link the fish to their population of origin. We found that Sockeye Salmon were migrating through the estuary for more than 50 d, with peak emigration for different populations varying by over 5 weeks. The out‐migration timing of specific populations was related to geographic factors, including elevation of the rearing lake and the river distance between individual rearing lakes and the estuary, with different populations arriving 1.5 d later for every 100 m of elevation or 3 d later for every 100 km of river distance. Concurrent with sampling, we quantified the estuarine prey of juvenile Sockeye Salmon; zooplankton species composition and abundance varied throughout the smolt migration period, and the different salmon populations encountered different prey abundances upon ocean entry. Together, these results indicate underappreciated phenological diversity in this harvested metapopulation, which may contribute to response diversity and metapopulation‐level resilience to climate change.

Integrating prey dynamics, diet, and biophysical factors across an estuary seascape for four fish species

“…In contrast, multiple abiotic variables covaried with the abundance of certain prey in juvenile coho salmon diets, suggesting that there might be stronger impacts on their foraging success with abiotic changes. In addition to integrating with prey across the biophysical dynamics of the seascape, populations of juvenile salmon enter the Skeena River estuary at a diversity of times and may interact with different peaks of zooplankton abundance in different seascape conditions (Carr Harris et al 2018). Thus, different populations' diets may be affected differently depending on when they enter the estuary.…”

“…The estuary region we focus on is within the larger Skeena River estuary, an area previously identified as having particularly high abundances of juvenile salmon during their migration: 2-to 8-fold greater abundance of juvenile salmon than other regions over several years of observation (Carr- . The region supports juvenile salmon from throughout the Skeena watershed, with at least 40 different populations identified in the estuary ) that enter at different times (Carr Harris et al 2018), most likely forage there, and reside for vari-able amounts of time (Moore et al 2016). The average estimated residencies for sockeye and coho salmon were 2 and 14 d, respectively (Moore et al 2016).…”

Estuary habitat associations for juvenile Pacific salmon and pelagic fish: Implications for coastal planning processes

“…Previous research mapping juvenile salmon use of estuary regions found particularly high abundances of juvenile sockeye, Chinook, and coho salmon near the Skeena River mouth, around the Lelu Island and Flora Bank region (Figure ) (Carr‐Harris et al, ). In addition, this area supports more than 40 populations of sockeye and Chinook salmon from throughout the Skeena River watershed (Carr‐Harris et al, , ; Moore, Carr‐Harris, Gottesfeld, et al, ) and encompasses the locations of seven of the eight LNG developments that were proposed for the greater Skeena River estuary (Aurora LNG, ; Exxon Mobil, ; Pacific Northwest LNG, ; Pembina Pipeline, ; Prince Rupert LNG Ltd, ; Province of British Columbia, , ). This study builds on these previous findings to examine how salmon and other pelagic fish are using specific estuary habitat types and locations.…”

“…A 2‐year field study of juvenile salmon and pelagic species and their habitats was conducted in the Skeena River estuary in 2015 and 2016. This research is part of a collaborative research programme with the Lax Kw'alaams Fisheries Program, Skeena Fisheries Commission, and Simon Fraser University examining the estuary phase of juvenile salmon migration in the Skeena River (Arbeider, Sharpe, Carr‐Harris, & Moore, ; Carr‐Harris et al, ; Carr‐Harris, Gottesfeld, & Moore, ; Moore et al, ; Moore, Carr‐Harris, Gottesfeld, et al, ). Previous research mapping juvenile salmon use of estuary regions found particularly high abundances of juvenile sockeye, Chinook, and coho salmon near the Skeena River mouth, around the Lelu Island and Flora Bank region (Figure ) (Carr‐Harris et al, ).…”

“…Estuary habitat degradation is associated with decreased survival of salmon and can have population‐level consequences for these species (Magnusson & Hilborn, ; Meador & MacLatchy, ; Toft et al, ). Estuaries at the base of large watersheds may support juvenile salmon from many different populations that are originating from across vast areas (Carr‐Harris et al, ; Moore et al, ). Understanding and identifying juvenile salmon estuary habitat is particularly relevant given their enormous economic and cultural importance.…”