Bull Trout Movements Match the Life History of Sockeye Salmon: Consumers Can Exploit Seasonally Distinct Resource Pulses

Abstract

Resource pulses provide consumers with opportunities to feed at high rates. The spawning and die-off of semelparous Pacific salmon provide consumers with feeding opportunities that contribute substantially to annual energy budgets. The life history of Pacific salmon also includes the downstream migration of smolts, which could provide similar opportunities for consumers to exploit. We tracked seasonal movements of adult Bull Trout Salvelinus confluentus in and out of the Chilko Lake-River outlet in interior British Columbia, which is characterized by a large Sockeye Salmon Oncorhynchus nerka population. Use of the outlet, lake, and river habitats were compared with the timing of out-migrations of Sockeye Salmon smolt and the spawning events of Sockeye Salmon, Chinook Salmon O. tshawytscha, and Coho Salmon O. kisutch. Bull Trout activity and residency in the outlet increased during the spring Sockeye Salmon smolt out-migration, and 40% of Bull Trout were found to return to the outlet in successive years. During Sockeye Salmon spawning, Bull Trout residency was increased in the river where most Sockeye Salmon spawning occurs. Therefore, Bull Trout may exploit both life history events, although direct consumption of eggs was not observed. Spawning by Chinook Salmon and Coho Salmon in the river did not appear to attract Bull Trout; Bull Trout residency increased in the lake (where spawning does not occur) during these times. Bull Trout residency in the lake also increased when outlet water temperatures were >16°C, and thus the lake may provide thermal refuge when river temperatures are above optima for this coldwater char. This research demonstrates that movements of aquatic consumers can depend upon smolt out-migrations, and such movements can allow exploitation of multiple pulses that are seasonally distinct. These results also highlight the ecological significance of Pacific salmon for consumers in freshwater systems.

New Insights into the Ecology of Adfluvial Bull Trout and the Population Response to the Endangered Species Act in the North Fork Lewis River, Washington

“…(). Together, these results demonstrate that the complex interactions of in situ conditions and foraging opportunities (e.g., Johnston and Post ; Furey and Hinch ) likely drive interpopulation and life history differences in growth rates and suggest the importance of local data when describing Bull Trout life history expressions.…”

“…For example, Bull Trout lengths at age from the North Fork Lewis River are generally similar to fluvial and adfluvial Bull Trout populations in northwestern Montana and Idaho (Fraley and Shepard 1989;Erhardt and Scarnecchia 2016;McCubbins et al 2016) but are considerably lower than those reported in Oregon by Ratliff et al (1996). Together, these results demonstrate that the complex interactions of in situ conditions and foraging opportunities (e.g., Johnston and Post 2009;Furey and Hinch 2017) likely drive interpopulation and life history differences in growth rates and suggest the importance of local data when describing Bull Trout life history expressions.…”

Migratory salmon smolts exhibit consistent interannual depensatory predator swamping: Effects on telemetry‐based survival estimates

“…The downstream migration to sea can be high‐risk for smolts particularly via exposure to predators. Predators aggregate to exploit smolts in rivers and estuaries (Zimmerman, 1999; Petersen, 2001; Furey, Hinch, Mesa, & Beauchamp, 2016; Furey & Hinch, 2017), influencing location‐specific mortality (Evans et al., 2012; Osterback et al., 2013; Schreck, Stahl, Davis, Roby, & Clemens, 2006). Use of acoustic telemetry has identified landscapes of high mortality for smolts (Michel et al., 2015; Stich, Zydlewski, Kocik, & Zydlewski, 2015; Thorstad et al., 2012), including small unimpacted rivers (Clark et al., 2016; Melnychuk, Welch, Walters, & Christensen, 2007).…”

Opening the tap: Increased riverine connectivity strengthens marine food web pathways

“…Thus, seasonal dependence on specific forage species is often underestimated. However, there are plenty of examples of species that heavily rely on short bursts of single prey species [9,17,75]. A new paradigm is emerging, which considers spatial and temporal variations in the forage base, and contrasts availability versus food quality in predator diets.…”

“…Whereas questions remain about the importance of single predator-prey linkages in driving productivity across larger ecosystems [15], complex life histories likely contribute consistency to predator-prey relationships [16,17]. For instance, capelin have two spawning modes, both of which contribute to stock productivity [18,19].…”