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McLaren, J. S., R. W. Van Kirk, A. J. Mabaka, S. Brothers, and P. E. Budy. 2023. Drawdown, habitat, and kokanee Oncorhynchus nerka populations in a western U.S. reservoir. North American Journal of Fisheries Management. DOI: 10.1002/nafm.10879. USGS FSP IP- 136834.
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Abstract
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January 2023
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Increased drought frequency and severity due to climate change can compound development pressures on water storage reservoirs throughout the western U.S. with uncertain effects on economically and ecologically valuable fisheries. Island Park Reservoir is a mid-sized hypolimnetic-release water storage reservoir on the Henrys Fork River in Idaho that supports a popular sport fishery for kokanee <i>Oncorhynchus nerka</i>. A region-wide drought in 2021 provided an opportunity to use Island Park Reservoir as a case study to examine how drought-induced drawdown affects kokanee habitat. We hypothesized that drawdown reduces the volume of water in the reservoir featuring “optimal” water quality for kokanee. We monitored water quality at five locations in Island Park Reservoir from spring turnover to fall turnover, from which we calculated the approximate volume of “optimal” (<17 C, >5 mg/L O<sub>2</sub>), “survivable” (<20C, >3 mg/L O<sub>2</sub>), and “lethal” (>20 C, <3 mg/L O<sub>2</sub>) conditions for kokanee. We found severe drawdown had a disproportionately negative impact on kokanee habitat availability. Although there was spatial and temporal heterogeneity in the availability of optimal and survivable conditions, drawdown for downstream water supply was the central mechanistic driver of declines in optimal and survivable conditions. Our findings explain interannual patterns in long-term kokanee spawning run return numbers, which were higher with higher reservoir carryover. Water conservation and precision management in the Henrys Fork watershed saved ~39 million m<sup>3 </sup>(32,000 acre-feet) in Island Park Reservoir in 2021, resulting in a ~150% increase in future spawning kokanee numbers from ~250 to ~650 in the 4,500 m study reach. Our study highlights the effects of drawdown on fisheries in water storage reservoirs and illustrates the degree to which hydrologic management, including projects to reduce water consumption, can benefit reservoir fish populations.
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McLaren, J. S., R. W. Van Kirk, P. Budy, and S. Brothers, and. In press. The reach-scale biogeomorphic effect of submerged macrophytes on trout habitat suitability. Hydrobiologia. https://doi.org/10.1007/s10750-024-05671-7. USGS FSP IP 157531 BAO Date: June, 18, 2023.
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Abstract
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August 2024
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Physical and biotic processes affecting trout habitat extent and quality often vary differentially across scales with the net effect oftentimes uncertain. For example, submerged macrophytes have conflicting effects on overall trout habitat by reducing preferred habitat extent within a river channel while simultaneously increasing a river channel’s productivity and growth potential. In the Henrys Fork River, Idaho, USA, we hypothesized higher submerged macrophyte coverage at the reach scale would exert a negative influence on the proportion of preferred trout habitats—defined as “habitat extent”—but would improve biotic habitat quality as measured by reach-scale models of Net Rate of Energy Intake (NREI). We therefore hypothesized a neutral or positive relationship between reach-scale submerged macrophyte coverage and a measure of net trout habitat suitability. Through random habitat sampling, aerial imagery, and snorkel surveys, we found macrophyte coverage resulted in a complementary riverscape of five habitat types. Macrophyte clumps created geomorphic forces that gave rise to intermacrophyte spaces and contributed to the formation of the remaining habitat types: gravel bars, emergent margins, and the thalweg. When testing the relationship between submerged macrophyte coverage and trout habitat, we found our hypothesized models of how habitat metrics interact with submerged macrophyte coverage appeared to fit the data well. Habitat extent declined quadratically with increasing submerged macrophyte coverage, whereas habitat quality increased exponentially. As a result, weighted trout habitat suitability changed little. As our study only featured a moderate range of macrophyte coverage (from 32 to 94% available surface area), future testing across a wider range of coverages may provide a more thorough test of our hypotheses. Nonetheless, our results highlight the importance of submerged macrophyte coverage to trout habitat via geomorphic effects. Our results also highlight the importance of trout habitat plasticity as well as habitat quality in determining overall habitat suitability.
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**McLaren, J. S., P. E. Budy, S. Brothers, R. W. Van Kirk. 2023. The scale-dependent role of submerged macrophytes on trout habitat. Canadian Journal of Fisheries and Aquatic Sciences 80:1533–1546. doi.org/10.1139/cjfas-2022-0182. USGS FSP IP-139190.
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Abstract
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July 2023
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Submerged macrophyte communities are prevalent in low-gradient rivers around the world and interact with rivers and their biota in complex ways; the net effect of these interactions on trout habitat provision remains poorly understood. We used snorkel surveys and bioenergetics to study trout habitat selection and potential mechanisms driving habitat selection in the submerged macrophyte-rich Henrys Fork, ID, USA. At smaller spatial scales (i.e., individual microhabitats), we hypothesized trout would select for locations with more submerged macrophytes that provide overhead cover from predators, drifting macroinvertebrates as food, and lower swimming costs. In contrast, we found individual trout selected for microhabitats without submerged macrophytes (, with deeper water ( and with greater bioenergetic potential (. At larger spatial scales (i.e., reach-scale), submerged macrophytes acted as a living substrate, indirectly creating a riverscape of complimentary microhabitats by increasing depth (), bioenergetic potential (), and by influencing reach-scale ecosystem dynamics which modulated trout habitat selection. Our study improves understanding of the complex relationship between submerged macrophytes and trout habitat, explaining conflicting results among previous studies and revealing potential management actions to improve trout productivity. Continued study of abiotic and biotic processes across multiple scales may continue to reveal unexpected scale-dependent effects and mechanisms controlling trout habitat quality, quantity, and preferences.
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