The direct observation, or empirical, approach to assessing stranding consists of direct observations to search for fish and/or invertebrates that have become isolated from the primary water body, or redds or egg masses that have become dewatered during periods when the water surface elevation is decreasing (i.e., down ramping and drawdown events). Surveys are typically focused in areas believed to be sensitive to flow fluctuations. Surveys are often conducted during down ramping and drawdown events caused by hydroelectric project operations during susceptible time periods for specific species and life stages. As flows recede during down ramping and drawdown events, biologists can conduct surveys from banks, or in-water snorkel surveys, or in-water electrofishing, in areas that are being dewatered and in vegetation and substrates. Transects can be established (Figure 9) for repeated surveys to increase the power of statistical analysis, and allow overall magnitude of stranding to be estimated.
4.2.2.1 Advantages and disadvantages of approach
Use of direct observation surveys to assess stranding has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
Advantages
- Allows direct observations of stranding events, including the species and life-stages potentially affected.
- Is compatible with study designs to estimate total magnitude of stranding.
- Ancillary information can be gained, such as lag times between operational changes and flow fluctuations, and the types of features or river stages most susceptible to stranding.
Disadvantages
- Accessibility to sites may be difficult in some systems.Potential for loss due to predation is difficult to quantify.
- In order to calculate annual mortality values, multiple observations are needed to account for differences in stranding potential due to seasonality, daily behavioral patterns, and project operations; it may be difficult to capture all of these differences.
- Can only be used to quantify stranding during the conditions that are observed, and predicting effects under alternate operations is difficult.
- There are limitations to the ability to observe stranded organisms, especially early life-stages of fish that preferentially select shallow habitat with cover. The error associated with observational accuracy is difficult to quantify, but can be estimated with separate trials.
4.2.2.3 Selected references
Nugent, J., T. Newsome, P. Hoffarth, M. Nugent, W. Brock, and Jr. M. Kuklinski. 2002. 2001 Evaluation of juvenile fall Chinook salmon stranding on the Hanford Reach of the Columbia River. BPA Contract Number 9701400, GCPUD Contracts Document 97BI30417. Prepared by Washington Department of Fish and Game for the Bonneville Power Administration, the Public Utility District Number 2 of Grant County.
USFWS (U. S. Fish and Wildlife Service). 2002. Trinity River juvenile fish stranding evaluation May-June, 2002. Report Number AFWO-F-01-03.
Stillwater Sciences. 2006. Flow fluctuations and stranding at the Carmen-Smith Hydroelectric Project, upper McKenzie River Basin, Oregon. Final report. Prepared by Stillwater Sciences, Arcata, California for Eugene Water & Electric Board, Eugene, Oregon.
