COS 122-2 - Habitat diversity drives variation in juvenile salmon response to extreme drought

Background/Question/Methods

California is at the forefront of climate change, with a more variable precipitation season leading to droughts of increasing frequency and intensity. For Pacific salmon (Oncorhynchus spp.) at the southern end of their range, severe droughts can have drastic negative effects for juveniles rearing in freshwater over the summer; however, habitat heterogeneity within a watershed can potentially buffer populations from environmental variability. Diverse stream habitats may interact with drought in a variety of ways, and in fact some may serve as ecological refuges for juvenile salmon. Understanding how drought impacts vary across a watershed and the response of juvenile salmon is crucial to informing restoration and conservation efforts. This study explores temporal and spatial variation in spring-summer rearing conditions for endangered juvenile Coho Salmon (O. kisutch) in the Lagunitas Creek watershed in Marin County, California during the recent 2021 drought. We use a variety of field methods, including physical habitat sampling, snorkel surveys, underwater video footage, and macroinvertebrate collections to track changes in habitat across five stream sites throughout late-spring to summer and the consequences for resource availability, foraging behaviour and growth potential in juvenile Coho Salmon.

Results/Conclusions

Field observations revealed a gradient of habitat conditions across the five stream sites that got progressively more disparate as the summer wore on. Dissolved oxygen and connectivity of pools varied the most between habitats, and two streams contained pools that went dry or had lethal levels of dissolved oxygen by mid-summer. In contrast, the stream regulated by an upstream dam sustained high connectivity and dissolved oxygen levels throughout the summer, providing ideal physical habitat conditions. Snorkel surveys showed dramatic declines and presumed mortality of juveniles in impaired pools, fewer fish but of larger size classes in semi-impaired pools, and many fish widely ranging in size in the regulated stream. There were distinct shifts in macroinvertebrate biomass in drift and type of foraging by juvenile salmon based on physical pool habitat characteristics, with fish switching to more active foraging strategies as in-stream velocity declined. The high variation in habitat conditions during the drought and resulting impacts on juveniles demonstrates the importance of maintaining diverse habitats within a watershed to buffer salmon populations against the impacts of environmental extremes.