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Factors involved in the predator-prey relationship of rainbow trout (Salmo gairdneri Richardson) and redside shiners (Richardsonius balteatus (Richardson)) in Paul Lake, British Columbia Crossman, Edwin John

Abstract

The role of such factors as distribution and movements of predator (Salmo gairdneri) and prey (Richardsonius balteatus), the activity involved in predation and the contribution of prey to the diet of the predator were studied in order to answer where, when, how and to what extent trout preyed on shiners. Data for 1955 and 1956 are compared with data for years when trout alone inhabited the lake. Predation became significant in 1950, approximately five years after the introduction of the prey species and has increased steadily, especially in trout over 10 inches in length, since that time. Movements of shiners in Paul Lake are complex and tend to bring this species into contact with trout during July, August and September. At this time shiners constitute over 90 percent of the diet of trout over 14 inches in length and lesser volumes in smaller trout. Trout under six inches in length prey on shiners to an insignificant degree. In winter shiners form a negligible part of the diet of trout of all sizes. Movement patterns seem to indicate that these two species might be separated in winter, and as a result predation is almost nil. It is inferred from the study that predation by trout is not a control of the number of shiners in the lake. The growth rate of trout under eight inches in length is still depressed as a result of competition with shiners for food. The contribution of shiners through predation, to the diet of larger trout, appears to have elevated the growth rate of trout eight to twelve inches in length, somewhat above that for years when trout alone inhabited the lake. The casual rather than causative nature of this predator-prey interaction is compared with the more stylized, obligate relationships of predator and prey in models of predation in the published literature. This relationship between rainbow trout and redside shiners is also discussed as it applies to management of lakes in which "sport fish" and "coarse fish" exist together.

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