Students present at American Fisheries Society
Influence of Fine Sediment on Arctic Grayling Egg Survival: Field Experiments
The population of Arctic grayling (Thymallus arcticus) in the Red Rock Lakes drainage of southwestern Montana represents the last remaining population of native, adfluvial grayling in the contiguous United States. Biologists and managers have hypothesized that this population is limited by the availability of suitable spawning habitat. We conducted field experiments to examine how fine sediment levels in spawning reaches influence the suitability of spawning habitat, using grayling egg survival as a proxy for suitability. We simulated Arctic grayling redds by placing 200 fertilized eggs in each of 24 plastic buckets which contained a range of fine sediment and gravel/rubble compositions, and measured egg survival. We observed little to no egg and fry survival in Elk Springs Creek and Red Rock Creek, likely the result of fine sediment deposition and predation in experimental redd buckets, as well as fry escapement from buckets. With slight modifications this experimental design could shed light on the influence of fine sediment on Arctic grayling egg survival, although its practicality in high sediment-transport streams may be limited. We also estimated the amount of fine sediment in riffles throughout the Red Rock Lakes drainage in order to identify suitable spawning habitat for Arctic grayling. We are now using these data along with riffle/stream measurements to quantify the amount of suitable spawning habitat in various streams. This information can be used to estimate the amount of spawning habitat available per grayling, which could influence management actions in the coming years.
Wildfire frequency and severity in the western U.S. has increased in recent decades and is predicted to continue to increase with climate change. Wildfire can alter stream characteristics resulting in warmer water temperatures, higher sediment and nutrient loading, as well as shifts in aquatic benthic community composition. Few studies have examined long-term impacts of wildfires on fish populations or stream food webs. In the Bitterroot River basin, MT densities of Westslope Cutthroat Trout, Oncorhynchus clarkii lewisi, are higher in burned sites over a decade after severe fire. We predicted that burned sites would have a higher prey availability and/or energy availability to support these higher cutthroat densities. In August of 2014 and 2015, we examined diets and invertebrate drift in seven unburned streams and seven streams that experienced severe riparian burns in 2000. We observed a greater number of prey items in fish diets at burned sites. In the drift, total invertebrate biomass was higher and composition was different at burned sites versus unburned sites, which has implications for energy flow across trophic levels. The differences in drift and trout diets between treatments indicate decadal changes in food web structure that may alter production potential for local fish populations.
Rennie's work was selected as the Best Student Poster at the Western Division American Fisheries Society annual meeting in 2016.