Introduction

As detailed in Chapters 2 and 4, we manipulated individual fish populations to change food web structure. In addition, we were able to follow dramatic responses in the fish community structure. Shifting a system from dominance by planktivores to one dominated by piscivores was accomplished relatively simply by introducing large numbers of piscivores. Establishing a planktivore-dominated system by eliminating piscivores first and then introducing planktivores has also worked well. These techniques were used in the manipulations in Tuesday Lake (Chapters 2 and 4). The resulting dominance by planktivores or piscivores provided dramatic contrasts in food web structure; however, most natural lake systems (such as those regularly subjected to fisheries exploitation) have mixed assemblages of piscivores and planktivores. The greater diversity and complexity of many fish communities encourages a search for general ecological principles (Werner & Gilliam, 1984). Fish communities are often further influenced by stocking and/or species- and size-selective harvest regulations. Thus, the interests of managers in facilitating or diminishing the relative abundance of selected species suggests that we must better understand the interactions among piscivorous and planktivorous fishes, and the resulting effects on other trophic levels.

In Peter Lake, attempts to establish dominance by planktivorous fishes in 1985 and 1989 failed (Chapters 2 and 4). The minnow population introduced in 1985 declined much more rapidly than could be accounted for by direct predation by known numbers of largemouth bass in the lake.