DECLINE OF SNAKE RIVER SUMMER STEELHEAD

Yuen, Henry. 2005. Decline in Snake River summer steelhead. N. Am. J. Fish. Mgt. 25: 446-463.

ABSTRACT

We collected adult and juvenile spawner recruit data on wild summer steelhead Oncorhynchus mykiss for the Snake River and estimated parameters for fisheries management by partitioning the data into predam and postdam periods and fitting the Ricker and Beverton-Holt models to those time series. The results showed a decline in productivity irrespective of the model chosen and the way in which the pre- and postdam periods were defined. However, the data were noisy and the confidence bounds on parameter estimates were fairly large. To reconcile the different management goals derived from the different data sources (adult or juvenile data) or model choice (Ricker or Beverton-Holt), we used simulation techniques and Bayesian algorithms. The combined approach suggests a recovery management goal (i.e., spawning stock associated with the maximum sustained yield) of 60,000 steelhead above Lower Granite Dam. At current smolt-to-adult survival rates, the data indicate optimal escapement of between 20,000 and 27,000 adults. We note that Snake River steelhead stocks cannot be managed for recovery escapement levels given current estimates of smolt-to-adult survival rates, and we discuss alternatives for present-day management and rebuilding over time.

Quotes From Text:

Rebuilding goals must address productivity and carrying capacity.

Carrying Capacity = capacity of the stock (Smsy), the spawning stock size at which the yield is maximized and maximized recruitment (Smsp). The spawning stock size at which recruitment is maximized as well as abundance.

Productivity = in-river habitat quality for juvenile life stages. (Extinction riks > as habitat quality <)

Snake Basin = 25 independent summer steelhead populations (TRT 2003).

The inherent productivity of individual populations varies greatly as does freshwater habitat.

The value of Smsy and Smsp for an aggregate of populations with varied productivity and capacity tend to be biased downward (Hilborn and Walters 1992).

The only data available to fish managers are for aggregates of all populations. This means there is uncertainty about productivity and capacity.

The number of hatchery steelhead that spawned naturally in the Snake Basin is unknown.

1957: The Dalles Dam floods Celilo Falls and displaces the tribal fishing and the non-Indian fishery above Bonneville Dam was closed and there is a spike of summer steelhead run size and escapement (170k in 1968).

Before completion of Lower Granite Dam (1974), harvest rates on wild steelhead were high. After 1974 harvest rates dropped but adult dam passage mortality increased.

The post dam smolt and adult numbers are one half of the pre-dam period up to 1975. Average wild smolt declined from 1,470,308 to 845,343 per year. Wild adult declined from an average of 52,310 to 22,925, and the escapement index declined from 36,524 to 16,434.

As the average spawner escapement < the average smolts per spawner >, but the average number of smolts< and the smolts/adult survival < (4% to 2%). The result is ½ as many adults escaping.

Snake: Assuming a 5% SAR maximum spawning potential ranges from 42,000 to 143,500.

Snake summer steelhead smolt to adult survival (SAR)

- Predam average SAR = 3.6%

- Predam maximum SAR= 6.4%

- Postdam average SAR= 2.7%

- Keogh River SAR maximum 20.0 %

“At present, data on hatchery straying rates are not collected in a systematic manner.” (page 455)

“A-run summer steelhead rebounded more than B-run steelhead when ocean conditions recently became favorable.”

“Working with the aggregate population is the only management option at this time.” (page 457)

“Managing for an aggregate goal ignores individual subpopulations. (page 456)

“For rebuilding and management, both estimates of productivity and capacity are essential.”

“While very low escapement goals are attainable, they do not contribute to rebuilding the population.” (page 457)

“The problem with using post dam data sets models is they will generate extremely low management goals for productivity.” (page 457)

“The inherent problems with derived escapement goals based on standard spawner-to-adult estimates will recur in fisheries management as long as we lump survival over different life cycle stages. The juvenile data indicate that freshwater productivity is high. The bottleneck is clearly smolt-to-adult survival.” (page 457)

“…changes in freshwater spawning and rearing survival could not explain the overall decline in productivity for Snake River summer steelhead. The same conclusion was reached by Petrosky (2001) and Wilson (2003) for spring and summer chinook.

“Thus rebuilding efforts should closely examine the juvenile to adult survival rates.” (page 458)

“Supplementation may also provide a boost in natural production for these stocks (Phillips et al. 2000), but they will not be sustainable without increases in overall productivity (Sharma et al. 2005). Page 458.)