In the last issue of the Chronicle, we introduced a "movie" of the interflows in Shasta Lake, a reservoir in northern California. That video animation was the result of a two-dimensional simulation model, CE-QUAL-W2, that tracks the hydrodynamics in Shasta resulting from thermal gradients, inflows and outflows, and wind-caused circulation. In this issue, we have included another movie of Shasta Lake that illustrates a preliminary bioenergetics analysis of the effects of the thermal dynamics within the reservoir on rainbow trout, one of Shasta's most important sport fish species.
The bioenergetics approach used to create this movie involves the calculation of scope for growth, which represents the opportunity for growth of a species, assuming that food is abundant and no other competitors or predators are present. It is the energy remaining for growth and reproduction after metabolic requirements are met (Wootton 1990). The general procedure for calculating scope for growth is to first determine how much energy the fish consumes using a set of consumption equations. Energy used for maintenance requirements, including respiration and waste losses due to egestion and excretion, is then calculated and subtracted from the energy consumed. The remaining energy is the estimate of energy available for the fish to grow and reproduce, or the scope for growth, which is expressed in grams of growth of the predator per gram of prey consumed per day, or g g-1d-1. To calculate scope for growth for fish species in Shasta Lake, equations and coefficients were used from a bioenergetics model developed by Hanson et al. (1997).
Water temperature output from CE-QUAL-W2 was used with these equations for specific species and size classes of fish to evaluate the effects of different operating scenarios and hydrologic conditions on the growth potential of these fish in Shasta Lake. The movie shown here is for a 100-g rainbow trout under dry-year hydrologic conditions simulated using measured climatic and flow conditions for 1968. The operating condition simulated involves the use of a temperature control device (TCD) that regulates downstream temperatures by releasing water through the highest outlet elevation during the winter months, a middle outlet elevation during the late spring/early summer months, and the lowest outlet elevation during the late summer and fall.
As with the interflow animation, this movie summarizes a 365-day simulation for Shasta Lake. Each image represents a single simulation day's output, with contours indicating relative scope for growth for a 100-g rainbow trout in a vertical slice of the reservoir along its longest dimension (i.e., approximately 40 kilometers from the reservoir's upstream reaches of the Pit River arm to the dam). The dam is located on the left side of the image, with the red triangles indicating the approximate locations of the TCD's outlets. The stair step-like bottom of the reservoir denotes the physical geometry seen by the hydrodynamic model (CE-QUAL-W2). Note that anomalies introduced by the software used to create the movie cause the bottom to appear blue throughout the year. The date, starting January 1, is shown in the upper left portion of the screen as the movie plays. The legend indicates the relative scope for growth for the rainbow trout, with blue indicating very little or even negative growth potential, and red indicating optimal or maximum growth potential. Negative scope for growth values mean that the trout will actually lose weight, rather than grow, if confined to those regions of the reservoir.
The scope for growth calculated in this preliminary analysis is strictly a function of water temperature for a given species and size class. Thus, the movie shows the effects of the changing water temperatures over time on growth potential as it plays through the year. Beginning on January 1, the water temperatures throughout the reservoir indicate moderate growth potential for a 100-g rainbow trout. As the movie proceeds into spring, the growth potential continues to improve, especially at the water surface. Optimal scope for growth values are seen in May in the top 10 to 20 meters of the reservoir. However, in June, surface water temperatures begin to exceed optimal conditions for trout, and the optimal region of growth indicated by the deep red color drops to 20 to 40 meters below the water surface. Note that as the summer progresses, this region of optimal scope for growth continues to drop deeper into the reservoir, while the surface water temperatures become so high that trout scope for growth values become negative near the water surface. In September, the reservoir begins to mix, causing the band of optimal scope for growth to expand vertically. By mid-October, the water temperature throughout the reservoir is near-optimal for trout growth. As the movie progresses through the fall and into the winter, the Pit and other tributaries supply ever cooler water, and along with cooling air temperatures, decrease the reservoir's temperature and the scope for growth values for the trout. By the end of the year Shasta's temperature again promotes moderate growth potential for the trout.
While the scope for growth analysis has been useful in interpreting water temperature effects on growth potential for fish species in Shasta Lake, we are continuing to investigate the effects of in-reservoir food production on fish growth potential. Stay tuned for more in the next issue of the Chronicle.
Some users have had problems viewing or playing the .avi animation so we have provided a .gif version as an alternative. Note that the GIF version, while smaller, may not play as smoothly as the AVI.
Hanson, P.C., T.B. Johnson, D.E. Schindler and J.F. Kitchell. 1997. Fish bioenergetics 3.0 for Windows. University of Wisconsin-Madison. 86 p.
Wootton, R.J. 1990. Ecology of teleost fishes. Chapman and Hall. London. 404 p.
U.S. Department of the Interior | U.S. Geological Survey
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