Douglas C. Braun

Burst Swimming in Areas of High Flow: Delayed Consequences of Anaerobiosis in Wild Adult Sockeye Salmon

Wild riverine fishes are known to rely on burst swimming to traverse hydraulically challenging reaches, and yet there has been little investigation as to whether swimming anaerobically in areas of high flow can lead to delayed mortality. Using acoustic accelerometer transmitters, we estimated the anaerobic activity of anadromous adult sockeye salmon (Oncorhynchus nerka) in the tailrace of a diversion dam in British Columbia, Canada, and its effects on the remaining 50 km of their freshwater spawning migration. Consistent with our hypothesis, migrants that elicited burst swimming behaviors in high flows were more likely to succumb to mortality following dam passage. Females swam with more anaerobic effort compared to males, providing a mechanism for the female-biased migration mortality observed in this watershed. Alterations to dam operations prevented the release of hypolimnetic water from an upstream lake, exposing some migrants to supraoptimal, near-lethal water temperatures (i.e., 24°C) that inhibited their ability to locate, enter, and ascend a vertical-slot fishway. Findings from this study have shown delayed post–dam passage survival consequences of high-flow-induced burst swimming in sockeye salmon. We highlight the need for studies to investigate whether dams can impose other carryover effects on wild aquatic animals.

Maternal and environmental influences on egg size and juvenile life-history traits in Pacific salmon

Life-history traits such as fecundity and offspring size are shaped by investment trade-offs faced by mothers and mediated by environmental conditions. We use a 21-year time series for three populations of wild sockeye salmon (Oncorhynchus nerka) to test predictions for such trade-offs and responses to conditions faced by females during migration, and offspring during incubation. In years when their 1100 km upstream migration was challenged by high water discharges, females that reached spawning streams had invested less in gonads by producing smaller but not fewer eggs. These smaller eggs produced lighter juveniles, and this effect was further amplified in years when the incubation water was warm. This latter result suggests that there should be selection for larger eggs to compensate in populations that consistently experience warm incubation temperatures. A comparison among 16 populations, with matching migration and rearing environments but different incubation environments (i.e., separate spawning streams), confirmed this prediction; smaller females produced larger eggs for their size in warmer creeks. Taken together, these results reveal how maternal phenotype and environmental conditions can shape patterns of reproductive investment and consequently juvenile fitness-related traits within and among populations.

Quantitative Links Between Pacific Salmon and Stream Periphyton

Species’ impacts on primary production can have strong ecological consequences. In freshwater ecosystems, Pacific salmon (Oncorhynchus spp.) may influence stream periphyton through substrate disturbance during spawning and nutrient subsidies from senescent adults. The shape of relationships between the abundance of spawning salmon and stream periphyton, as well as interactions with environmental variables, are incompletely understood and may differ across the geographic range of salmon. We examined these relationships across 24 sockeye salmon (Oncorhynchus nerka) spawning streams in north-central British Columbia, Canada. The influence of salmon abundance and environmental variables (temperature, light, dissolved nutrients, water velocity, watershed size, and invertebrate grazer abundance) on post-spawning periphyton abundance and nitrogen stable isotope signatures, which can indicate the uptake of salmon nitrogen, was evaluated using linear regression models and Akaike Information Criterion. Periphyton nitrogen stable isotope signatures were best described by a positive log-linear relationship with an upstream salmon abundance metric that includes salmon from earlier years. This suggests the presence of a nutrient legacy. In contrast, periphyton abundance was negatively related to the spawning-year salmon density, which likely results from substrate disturbance during spawning, and positively related to dissolved soluble reactive phosphorus prior to spawning, which may indicate phosphorus limitation in the streams. These results suggest that enrichment from salmon nutrients does not always translate into elevated periphyton abundance. This underscores the need to directly assess the outcome of salmon impacts on streams rather than extrapolating from stable isotope evidence for the incorporation of salmon nutrients into food webs.

Emergent stability in a large, free‐flowing watershed

While it is widely recognized that financial stock portfolios can be stabilized through diverse investments, it is also possible that certain habitats can function as natural portfolios that stabilize ecosystem processes. Here we propose and examine the hypothesis that free-flowing river networks act as such portfolios and confer stability through their integration of upstream geological, hydrological, and biological diversity. We compiled a spatially (142 sites) and temporally (1980-present) extensive data set on fisheries, water flows, and temperatures, from sites within one of the largest watersheds in the world that remains without dams on its mainstem, the Fraser River, British Columbia, Canada. We found that larger catchments had more stable fisheries catches, water flows, and water temperatures than smaller catchments. These data provide evidence that free-flowing river networks function as hierarchically nested portfolios with stability as an emergent property. Thus, free-flowing river networks can represent a natural system for buffering variation and extreme events.

Using watershed characteristics to inform cost-effective stream temperature monitoring

Water temperature is a key driver of aquatic processes. Monitoring stream water temperature is key to understanding current species distributions and future climate change impacts on freshwater ecosystems. However, a very small fraction of streams are continuously monitored for water temperature throughout North America, due to prohibitive logistical costs. We develop a framework that aids in developing cost-effective stream temperature monitoring by using stream habitat features to inform strategic site selection of temperature monitoring sites. We test this framework using sockeye salmon spawning streams as a model, which included 19 streams in the northern-most watershed of the Fraser River Basin, British Columbia, Canada. The objective of this framework is to evaluate the trade-off between cost (i.e., the number of streams monitored) and the effectiveness of monitoring scenarios at meeting different monitoring objectives. We compared monitoring scenarios that were informed by well-established relationships between variables and that are commonly collected or available as part of other monitoring activities (stream length, magnitude, order, gradient, wetted width, and spot temperatures) and water temperature metrics (maximum, mean, and variance during August) derived from continuously monitored streams to monitoring scenarios where streams were randomly selected. Informed scenarios included streams that were selected in order of watershed level and stream habitat characteristics (e.g., longest to shortest); ordering was based on the relationship between each habitat variable and temperature metrics. Informed monitoring scenarios were then compared to random selection of monitoring sites with regard to how well monitoring scenarios met two management objectives during the critical salmon spawning period: (1) identifying streams that exceed a temperature threshold and (2) identifying streams that represent the temperature regime of a complex of streams (e.g., mean and variance of streams within an aggregate of streams). Management objectives were met by monitoring fewer streams using the informed monitoring scenarios rather than the average of the random scenarios. This highlights how common inexpensive watershed level variables that relate to stream temperature can inform the strategic selection of sites and lead to more cost-effective stream temperature monitoring.