The roles of extrinsic and intrinsic factors in the freshwater life‐history dynamics of a migratory salmonid

Abstract

Key life‐cycle transitions, such as metamorphosis or migration, can be altered by a variety of external factors, such as climate variation, strong species interactions, and management intervention, or modulated by density dependence. Given that these life‐history transitions can influence population dynamics, understanding the simultaneous effects of intrinsic and extrinsic controls on life‐history expression is particularly relevant for species of management or conservation importance. Here, we examined how life histories of steelhead (Oncorhynchus mykiss) are affected by weather, pink salmon abundance (Oncorhynchus gorbuscha), experimental nutrient addition, and density‐dependent processes. We tested for impacts on the size of steelhead smolts (juveniles migrating to the sea), as well as their age and abundance across four decades in the Keogh River, British Columbia, Canada. Larger steelhead smolts were associated with warmer years and artificial nutrient addition. In addition, higher pink salmon abundance and artificial nutrient addition correlated with juvenile steelhead migrating at younger ages. While density dependence appeared to be the primary factor regulating the abundance of steelhead smolts, nutrient addition and temperature were positively and negatively associated with smolt production, respectively, prior to 1991, and pink salmon spawning abundance was positively associated with smolt production after 1990. Thus, this study provides evidence that the temporal dynamics of one species of salmon is linked to the juvenile life history of co‐occurring steelhead. A complex interplay of species interactions, nutrient subsidies, density dependence, and climatic variation can control the life‐history expression of species with complex life cycles.

Density‐dependent marine survival of hatchery‐origin Chinook salmon may be associated with pink salmon

“…While we show here that pink salmon abundance is associated with lower survival of Salish Sea hatchery Chinook salmon, it is important to note that pink salmon are an essential part of the Salish Sea ecosystem. Given that they are often the most numerous salmonid in the Salish Sea, especially in recent decades (Losee et al 2019), they provide essential marine-derived nutrients and food resources to many freshwater systems (Ward and Slaney 1988, Cederholm et al 1999, Marston 2017, Bailey et al 2018. Pink salmon rely minimally on freshwater habitats compared to other salmonids (Quinn 2005), which may serve them especially well in the future given the potential impacts of climate change on freshwater ecosystems (Ward et al 2015).…”

Resource pulses increase the diversity of successful competitors in a multi‐species stream fish assemblage

“…Thus, our study adds to the growing body of research that illuminates the potential trade‐offs between harvest and the ecosystem benefits that salmon provide (Levi et al 2012). Increasing the number of eggs (a high quality, energy‐dense food) a stream fish consumes can increase resident fish growth and survival (Bentley et al 2012, Bailey et al 2018). Our results indicate that there were diminishing returns for increasing egg abundance increasing egg consumption (i.e., type II functional response) for individual size classes of fish, but that higher egg numbers were needed to benefit diverse consumers.…”

“…The Keogh River is a small (31.2 km long with a watershed area of 130 km 2 ; Smith and Slaney 1980) coastal stream with low nutrient concentrations (Johnston et al 1990) that drains into Johnstone Strait on the east coast of Vancouver Island, just south of the town of Port Hardy. Since 1972, salmonid monitoring has been ongoing and several habitat restoration projects have been completed at the Keogh River (Ward and Wightman 1989, Smith and Ward 2000, McCubbing and Ward 2002, Atlas et al 2015, Bailey et al 2018). Stream fish species that may prey on salmon eggs in the Keogh River include juvenile steelhead trout ( O. mykiss ), coho salmon ( O. kisutch ), cutthroat trout ( Oncorhynchus clarkii clarkii ), Dolly Varden char ( Salvelinus malma ), and sculpins Cottus spp.…”

“…( C. asper and C. aleuticus ). Estimated pink salmon spawner abundance has ranged from as few as ~75 to >800,000 fish over the last 40 yr (Bailey et al 2018, Fisheries and Oceans Canada 2020), generating resource pulses that span many orders of magnitude.…”

“…Energy acquired during the brief salmon spawning period can dominate the annual energy budget for stream fish consumers (Scheuerell et al 2007, Moore et al 2008, Armstrong and Bond 2013); for example, through bioenergetic simulations, Moore et al (2008) predicted that resident rainbow trout ( Oncorhynchus mykiss ) in their study would run an energetic deficit during the summer without salmon subsidies. Increased spawning salmon abundance can increase the growth (Rinella et al 2012, Swain et al 2014) and abundance (Nelson and Reynolds 2014, Swain and Reynolds 2015) of stream fishes, and alter their life histories (Bailey et al 2018). The availability of this important food resource depends on stream fishes’ ability to consume dislodged or unsuccessfully buried salmon eggs (Collins et al 2016).…”