Abstract. Few estimates of migration rates or descriptions of behavior or survival exist for wild populations of out-migrating Pacific salmon smolts from natal freshwater rearing areas to the ocean. Using acoustic transmitters and fixed receiver arrays across four years (2010-2013), we tracked the migration of >1850 wild sockeye salmon (Oncorhynchus nerka) smolts from Chilko Lake, British Columbia, to the coastal Pacific Ocean (>1000 km distance). Cumulative survival to the ocean ranged 3-10% among years, although this may be slightly underestimated due to technical limitations at the final receiver array. Distinct spatial patterns in both behavior and survival were observed through all years. In small, clear, upper-river reaches, downstream migration largely occurred at night at speeds up to 50 km/d and coincided with poor survival. Among years, only 57-78% of smolts survived the first 80 km. Parallel laboratory experiments revealed excellent short-term survival and unhindered swimming performance of dummy-tagged smolts, suggesting that predators rather than tagging effects were responsible for the initial high mortality of acoustic-tagged smolts. Migration speeds increased in the Fraser River mainstem (~220 km/d in some years), diel movement patterns ceased, and smolt survival generally exceeded 90% in this segment. Marine movement rates and survival were variable across years, with among-year segment-specific survival being the most variable and lowest (19-61%) during the final (and longest, 240 km) marine migration segment. Osmoregulatory preparedness was not expected to influence marine survival, as smolts could maintain normal levels of plasma chloride when experimentally exposed to saltwater (30 ppt) immediately upon commencing their migration from Chilko Lake. Transportation of smolts downstream generally increased survival to the farthest marine array. The act of tagging may have affected smolts in the marine environment in some years as dummy-tagged fish had poorer survival than control fish when transitioned to saltwater in laboratory-based experiments. Current fisheries models for forecasting the number of adult sockeye returning to spawn have been inaccurate in recent years and generally do not incorporate juvenile or smolt survival information. Our results highlight significant potential for early migration conditions to influence adult recruitment.

Temperature has a dramatic effect on the physiology of ectothermic animals, impacting most of their biology. When temperatures increase above optimal for an animal, their growth rate tends to decrease. The mechanism behind this growth rate reduction is unknown. Here, we suggest the aerobic scope protection hypothesis as a mechanistic explanation for the reduction in growth. After a meal, metabolic rate, and hence oxygen consumption rate, transiently increases in a process called specific dynamic action (SDA). At warmer temperatures, the SDA response becomes temporally compressed, leading to a higher peak oxygen consumption rate. This peak in oxygen consumption rate takes up much of the animal’s aerobic scope (the difference between maximum and resting rates of oxygen consumption), leaving little residual aerobic scope for other functions. We propose that animals will actively protect their postprandial residual aerobic scope by reducing meal sizes in order to regulate the peak SDA response. This hypothesis is consistent with the published literature and we suggest further predictions to test it.

Many ectotherms have shown a reduction in maximum body size in the past decades in parallel with climate warming. Indeed, some models forecast a maximum body size decline of 14%–24% by 2050 for numerous fish species. The gill-oxygen limitation (GOL) hypothesis is perhaps the most prominent concept regarding the physiological mechanisms underlying the observed trends, implicating oxygen uptake limitations in driving the decline in fish body size with warming. Current scientific debates, however, demonstrate a clear need for a synthesis of existing empirical evidence to test the fundamental assumptions of the GOL hypothesis. Here, we perform a systematic literature review of the intraspecific allometry of gill surface area (GSA) and metabolic rate. Additionally, we introduce a new parameter, the ratio S, which provides a measure of GSA in relation to the metabolic requirements for maintenance (SSMR) and maximum activity (SAMR). Support for the GOL hypothesis would be evidenced by a universal decline in S with increasing body mass within each species, such that gills become less equipped to supply metabolic requirements as fish grow. In contrast to the predictions of the GOL hypothesis, we show that the scaling exponents for SSMR and SAMR are consistently close to zero, with only a few exceptions where S either increased or decreased. These findings suggest that the GSA of each species is sufficient to meet its oxygen requirements throughout life, and that growth is not universally restricted by oxygen uptake limitations across the gills. We identify the need to investigate hypotheses other than the GOL hypothesis to help explain the observed declines in maximum fish body sizes concurrent with climate warming, in order to facilitate accurate predictions of fish community structure and manage fisheries in the face of climate change.

No abstract