Trout under drought: A long-term study of annual growth and condition of stream-living Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii)

Abstract

Quantifying the dynamics of natural populations is a central issue in ecology. In the Pacific Northwest of North America, climate extremes are becoming more frequent and severe with projections of increasing winter floods and prolonged droughts during summer. Using a 13-year dataset of adult (Age 1+) Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii), we evaluated the effects of three droughts on annual growth and condition in two stream reaches of Mack Creek, H.J. Andrews Experimental Forest, Oregon, USA. In the three drought years, the onset of seasonal low flow consistently started earlier than reference water years and extended longer, from mid June until the end of September. We found consistent evidence of slower individual growth rates across sizes in drought years relative to reference years, with an apparent greater effect in larger trout. The median annual responses of trout were highly synchronous between stream reaches. There was evidence of slower growth and reduced condition associated with higher trout abundances in the two reaches. In addition, we found that growth rate and condition were associated with timing (annual maxima) and frequency (days >14 °C) of warm events, and habitat size (pool depth). Faster growth rates, higher abundance, and improved condition occurred in the second-growth forest reach compared to the old-growth forest reach. These results illustrated that a combination of density-dependent and density-independent processes can explain observed patterns in growth and condition over time. Each drought year had different climatic characteristics compared to reference years, including differences in the timing of precipitation, timing and magnitude of winter peak flows, and stream temperatures (especially in winter). Collectively, our findings suggest that growth and condition of adult trout are influenced by a complex interplay between density-dependence and density-independent factors. Thus, predictions about the effects of droughts on growth and condition in stream salmonids are difficult to generalize across regions. Our study highlights the value of long-term datasets because we can weigh the importance of processes that occur over both the short- and long-term.

Citation

Arismendi, Ivan; Penaluna, Brooke E.; Gregory, Stanley V. 2024. Trout under drought: A long-term study of annual growth and condition of stream-living Coastal Cutthroat Trout (Oncorhynchus clarkii clarkii). In: Lobon-Cervia, Javier; Budy, Phaedra; Gresswell, Robert, eds. Advances in the ecology of stream-dwelling salmonids. Springer, Cham. Fish & Fisheries Series. 44:343-369. https://doi.org/10.1007/978-3-031-44389-3_16.