To experimentally evaluate the potential for fish stranding during water surface elevation fluctuations, field trials using fish placed in experimental enclosures can be conducted. For this approach, fish are enumerated, measured, and released into enclosures during periods of water surface elevation fluctuations in reservoirs or river reaches affected by project operations (Figure 10). Enclosures may have a live box to capture all fry that move toward the channel as flows recede. Repeated visual observations are used to continually monitor the effects of stranding on individuals during the experiment. Emigration, direct mortality, delayed mortality, and injury are recorded for each trial. Experiments may be repeated at different locations or periods to assist in accounting for variables such as rate and magnitude of water fluctuations, time of day, season, topography, substrate characteristics, water quality, and water temperature.
4.2.3.1 Advantages and disadvantages of approach
Use of experimental net enclosures to assess stranding has advantages and disadvantages relative to the other approaches evaluated, as discussed below:
Advantages
- Specific geomorphic features and project operations can be evaluated in separate trials.
- Species- and life-stage-specific trials can be conducted.
- Fish behavior and delayed effects of flow fluctuations on fish can be evaluated.
- Statistically robust, since the number of fish that are susceptible to stranding is known, and probability of detection is high.
Disadvantages
- Fish used for the experiment (typically from hatcheries) may not adequately simulate species composition, behaviors, and abundance in the system.
- Nets are difficult to deploy, and therefore typically only a few sites are evaluated. Results from the experiment may be difficult to extrapolate spatially and temporally.
- If multiple replicates are conducted to increase statistical robustness, other environmental factors such as weather and water temperature and quality affect results. In addition, it may not be possible to reproduce water surface fluctuations of the same rate and magnitude, which may also affect results.
4.2.3.2 Site-specific considerations and applicability
Many of the advantages and disadvantages of this approach will depend on site-specific considerations, as discussed below:
- In systems where few fish are expected to be stranded, this approach can account for detection probability, since a known number of fish are used.
- This approach works well if areas with a potential for stranding are homogeneous, such that results from focused test in one area can be applied to other areas.
- Has been applied at the Bratsberg power station, on the Nidelva River, Norway.
4.2.3.3 Selected references
Bradford, M. J. 1997. An experimental study of stranding of juvenile salmonids on gravel bars and in sidechannels during rapid flow decreases. Regulated Rivers 13: 395-401.
Bradford, M. J., G. C. Taylor, J. A. Allan, and P. S. Higgins. 1995. An experimental study of the stranding of juvenile coho salmon and rainbow trout during rapid flow decreases under winter conditions. North American Journal of Fisheries Management 15: 473-479.
Halleraker, J.H., Saltveit, S. J., Harby, A., Arnekleiv, J.V., Fjeldstad, H.P. and Kohler, B. 2003. Factors influencing stranding of wild juvenile brown trout (Salmo trutta) during rapid and frequent flow decreases in an artificial stream. Journal of Rivers Research and Application 19: 589-603.
Saltveit, S. J., J. H. Halleraker, J. V. Arnekleiv, and A. Harby. 2001. Field experiments on stranding in juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) during rapid flow decreases caused by hydropeaking. Regulated Rivers: Research and Management 17: 609-622.
