Rock Island Steelhead Travel Time: 1995 & 1996 releases

The Data

The fish were tagged and released at Rock Island and observed at McNary and John Day dams. 15 cohorts were analyzed over the 2 year period 1995-1996. The migration period was mid April through early May.

These two years were chosen because the pattern of observations permitted analysis of multiple reaches.

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 the three models.

corresponds to observations at McNary Dam.

corresponds to observations at John Day Dam.

Tabular Results

Table 1 . Travel-time parameters for steelhead RIS releases(1995-1996).

model	parameter estimates (std. error)							resid. ss	mult. R2
	MIN	MAX	FLOW		TINFL	2	2
1	23.52 (1.27)	-	-	-	-	-	327.4	110.38	-
2	0.83 (3.24)	-	0.920 (0.149)	-	-	-	327.4	34.58	.687
3	3.16 (3.59)	-	0.927 (0.098)	0.093 (0.117)	110.0 (9.1)	-	327.4	28.97	.738
4	1.35 (8.80)	6.59 (6.77)	0.826 (0.359)	0.101 (0.093)	111.0 (7.3)	1.02 (18.608)	327.4	26.81	.757

* is fixed to prevent other parameters from straying into 'impossible' values while standard errors were calculated.

Discussion

As with other steelhead stocks, the more complicated models offer only limited improvement is captuing the variation in travel times. The steelhead appear to begin with high migration rates, extensively use the current, and not show much seasonal variation in migration rate.