Abstract
Spada Lake is a reservoir that supports wild populations of rainbow trout Oncorhynchus mykiss, cutthroat trout Oncorhynchus clarki, and hybrids of these species, which sustain a locally popular sport fishery. Creel surveys were conducted in 6 years between 1979–1988 to assess the effect on the fishery of enlargement of the reservoir in 1984. We synthesized the available creel survey and water quality data to evaluate the effect of enlargement and other factors on the yield of wild trout. Yield increased from less than 1,000 kg/year before enlargement to nearly 5,000 kg/year immediately after enlargement and then declined. Harvest per unit of effort was variable, but mean length of trout in the harvest declined steadily after enlargement until a minimum‐length limit was imposed in 1988. Analysis of year‐class abundance indicated that natural production in stream spawning and rearing areas after lake enlargement remained sufficient to sustain the fishery, but that harvests exceeded the sustainable level of lacustrine production. Estimation of lake production capacity from the morphoedaphic index (total dissolved solids/mean lake depth) indicated that, although total potential yield (kg/year) may have increased 84% after enlargement, sustainable yield was exceeded in both periods until harvest was reduced by the change in fishing regulations. Only moderate trout growth rates, compared with those in other lakes, and a diet consisting primarily of small invertebrates pointed to a limited production potential for Spada Lake. The yield of rainbow trout was further limited, relative to that of cutthroat trout, by higher natural mortality offish older than age 2.