Snake River Yearling Chinook Travel Time

The Data

The fish were tagged and released at the Snake Trap and observed at Lower Granite Dam, Little Goose Dam and McNary Dam. 78 cohorts were analyzed over the 7 year period 1989-1996. The migration period was early April through early May.

Models

We applied four nested models of increasing complexity to the data:

• Model 1 uses a constant migration rate.
• Model 2 incorporates a linear flow relationship.
• Model 3 adds a seasonal/flow interaction - fish use more of the flow for migration as the season progresses.
• Model 4 adds an experience factor - flow independent migration speed increases the longer the fish have been in the river.

Model details are provided by Zabel, et al. (1996).

Graphical Results

The plots below show observed average travel times versus modeled average travel times to each of the three observation sites for all four models.

corresponds to observations at Lower Granite Dam.

corresponds to observations at Little Goose Dam.

corresponds to observations at McNary Dam.

Tabular Results

model	parameter estimates (standard error)					2	resid. ss	R2
	MIN	MAX	FLOW		TSEASN
1	6.887 (0.235)	-	-	-	-	121.55 (3.63)	8876.91	-
2	-1.779 (0.330)	-	1.424 (0.069)	-	-	78.44 (2.11)	4278.24	0.518
3	1.722 (0.202)	-	0.635 (0.046)	0.177 (0.031)	110.82 (1.22)	52.69 (1.23)	1949.91	0.780
4	0.643 (0.043)	9.398 (0.601)	0.484 (0.023)	0.128 (0.009)	110.96 (0.91)	61.57 (1.30)	932.06	0.895

Discussion

For the fish analyzed here, the more complex models are supported by the data. The model with the seasonal flow term (model 3) confers a considerable improvement in model fit over the model with the simpler linear flow term (model 2), indicating the effect of flow on the Snake River yearling chinook has a seasonal component. Model 4 offers improvement in model performance over model 3 by correcting the bias present in the simpler model. This is achieved by speeding up the fish as they move downstream - a difference in migration rate of almost 9 miles per day in the non-flow related terms (_MIN and _MAX).