Mill Creek Fisheries Monitoring Program: Ten Year Assessment and Report on the Stability and Health of Mill Creek’s Anadramous Fisheries Population
The coho salmon population in Del Norte County’s Mill Creek depends heavily on the quantity and quality of winter habitat for survival, according to a study by The Rowdy Creek Fish Hatchery and a team of fisheries biologists.
Adult coho spawn in the fall and the eggs hatch the next spring. While conventional wisdom holds that the more eggs salmon lay in a stream during late summer and early fall spawning, the bigger that stream’s salmon population will be, the fisheries biologists say this is not true. Just a few successful spawning nests hatch many more fry than very good summer habitat can support. Roughly the same number of fry will survive through the summer whether parent coho produce more eggs or not. And for strong fry, survival during the summer is relatively easy—river water moves leisurely and isn’t so icy cold. The real test for survival takes place in the winter, when heavy rains increase water velocity and temperatures take a dive. It’s the availability of good quality winter rearing habitat to foster the development of surviving fry that puts a limit on coho population.
During winter and spring freshets—sudden overflows in a stream resulting from a heavy rain or a thaw—coho seek the shelter and foraging benefits of habitats with slower moving, warmer water, such as floodplains, sloughs, off-channel water bodies, beaver ponds, and, most often, complex habitats within the channel formed by fallen logs. A benefit to salmon, fallen logs trap and store sediment and in the process diversify the stream’s habitats. The team’s research, conducted on the Save-the-Redwoods League-acquired Mill Creek property, has revealed that the livelihood of a coho population depends specifically on the presence of these habitats throughout the winter.
It’s likely that such habitats were historically much more profuse in northern California’s coastal streams. Since the advent of commercial forestry, logging that removes trees from stream banks has reduced the amount of fallen logs in streams and, by association, the slower moving, warmer pools and pockets that coho depend on. From fish counts before and after winter freshets, researchers were able to predict that doubling the current winter habitat would increase the population by up to 15%.
In their study, the researchers considered how the coho population size might vary with respect to the availability of three kinds of habitat: spawning, summer, and winter. They collected data by sending snorklers into the stream for visual surveys and by live trapping and counting outmigrating fish in several sections of the stream.
In addition to finding that coho depend on winter habitat, the researchers found that Chinook salmon, another species of salmon in Mill Creek, depend most on spring rearing habitat for survival. This habitat is also characterized by slower moving, warmer water and is created by fallen logs in the stream.
In light of their finding that the availability of winter and spring habitat limits the number of coho and Chinook that can survive in the stream, the researchers recommend introducing fallen logs into the water channel to restore low-velocity stream habitat.