Growth trajectories of wild California steelhead parr
David Swank1, Will Satterthwaite1, Michael Beakes1, Rob Titus2, Joe Merz3, Susan Sogard4, and Marc Mangel1
1UC Santa Cruz, Santa Cruz, CA
2California Department of Fish and Game, Sacramento, CA
3SP Cramer Fish Sciences
4National Marine Fisheries Service, Santa Cruz, CA.
Thorpe (1987) showed that in Atlantic salmon (Salmo salar), juveniles have annual decision windows during which they decide whether to remain immature parr, mature as stream residents, or emigrate to the ocean. These decisions take the form of a series of comparisons between realized size and an expected size relative to a threshold. We hypothesized that steelhead (Oncorhynchus mykiss) follow similar decision rules for this trait, and that northern and southern California steelhead stocks should show differences in the timing of this decision based on habitat variables influencing growth rates, including stream temperature, stream flow, and prey availability. To test this hypothesis, we tagged wild steelhead parr from two Central Valley rivers and two coastal California streams, and measured individual growth rates. We monitored emigration using PIT tag antennae on the coastal streams, and are currently implanting wild smolts with acoustic transmitters in a Central Valley river. Invertebrate biomass in each stream was measured using drift nets and substrate samples. Early results show much higher food availability, faster growth, and earlier ages at smolting in Central Valley rivers compared to coastal streams. PIT tag recaptures from coastal streams show that individual steelhead parr can have little to no growth from spring through fall but still emigrate the following spring.
2008 Pacific Coast Steelhead Meeting Boise, ID
March 4-6, 2008