Challenges for Diadromous Fishes in a Dynamic Global Environment

Using Physiological Telemetry and Intervention Experiments to Examine the Maladaptive Shift in Fraser River’s Late-Run Sockeye Salmon Spawning Migration

Scott G. Hinch, Anthony P. Farrell, Steven J. Cooke, David A. Patterson, Mike F. LaPointe, David W. Welch, Karl K. English, Glenn T. Crossin, Kristi Miller, Richard E. Thompson, Glen van der Kraak, Ivan Olssen, Mark Shrimpton, and Michael S. Cooperman

doi: https://doi.org/10.47886/9781934874080.ch70

The Fraser River drains one-third of British Columbia as it flows to the Pacific Ocean and is the largest salmon-producing river in Canada. Sockeye salmon Oncorhynchus nerka are the most economically important species in the Fraser River system, and the population is composed of more than 150 stocks. It is managed as four groups (Early Stuart, Early Summer, Summer, Late-Run), based on timing of adult salmon returning to freshwater. Late-Run stocks were unique, historically, in that upon arrival to the Strait of Georgia (SOG), maturing adults paused for 3–6 weeks on their spawning migration prior to entering the Fraser River. However, since 1995, increasing proportions of Late-Run salmon have entered the Fraser River without delaying in SOG, despite early river entry causing high river migration mortality (60–96% per year compared to 20% per year, historically; Cooke et al. 2004). Mortality of early-timed migrants is thought to be caused by salmon encountering warmer river temperatures than normal-timed migrants, resulting in higher energetic costs, more severe infections (e.g., kidney parasite Parvicapsula; Wagner et al. 2006), and, in some years, the collapse of metabolic scope. The high mortality has caused a conservation crisis, including emergency listings of stocks, fisheries closures, and lost recreational and cultural fisheries (Cooke et al. 2004). We formed a research team, in 2001, involving ecologists, physiologists, oceanographers, geneticists, and resource managers, to explore the causes and consequences of this maladaptive shift in migration behavior.

Our initial efforts focused on identifying the environmental and physiological correlates of Late- Run sockeye migration behavior and fate (Cooke et al. 2006a, 2006b). The strongest environmental correlate of river entry timing is the level of surface water mixing occurring within SOG at the time of Late-Run sockeye arrival. Our data indicate that Late-Run sockeye that enter the SOG at times of low mixing encounter low salinity surface waters and are more likely to enter the Fraser River without delaying migration than are Late-Run sockeye that enter the SOG during well-mixed conditions (i.e, absence of low salinity surface water) (linear regression of mean river entry date and mixing strength, N = 9 years, r² = 0.89, P < 0.00001; R. Thomson, unpublished data). Exposure to low-salinity water may accelerate osmoregulatory preparedness for freshwater entry and thereby cause fish to enter the Fraser River prematurely—our salinity intervention study, described below, demonstrated that freshwater- exposed marine fish could survive for periods of time in saltwater. We used physiological biopsy telemetry (Cooke et al. 2005) to relate individual condition to the behavior and fate of Late-Run sockeye at various locations along marine and freshwater portions of the spawning migration. The biopsy involved collecting blood plasma for ions (indicators of osmoregulatory function and stress), reproductive and stress hormones (indicators of maturation level), gill tissue for Na⁺ K⁺ ATPase (osmoregulation status) and genomic analysis, somatic tissue for genomic analysis and stock identification using DNA, scales for stable isotopes, and microwave interrogation of somatic water (index of energy reserves). To date, we have biopsied more than 3,000 Late-Run sockeye, and the results indicate that sockeye that do not hold in the SOG are characterized by more variable osmoregulatory condition, lower energy reserves, and higher stress and reproductive hormone concentrations (Table 1).