Summary
Analysis of the piscivore guild in fish species-rich lake and stream systems in eastern Ontario showed the co-occurrence of three types: (1) specialists that became piscivorous at the age of a few weeks (Esox, Micropterus); (2) ‘secondary’ piscivores that are fish-eaters only later in life (Perca, Ambloplites); and (3) species in which fish consumption is limited to taking some larvae (Lepomis macrochirus). In the first group the basic series of dietary shifts that characterize many long-lived fish (i.e. zooplankton followed by small invertebrates then large invertebrates and finally fish); is greatly accelerated. Prey size increases with growth. Overall prey selection was on the basis of body size and abundance. Most piscivores took a range of fish prey. There was little evidence of specialization at the species level. Esox and Micropterus spawn some weeks ahead of their major prey species. This is seen as adaptive. Their young harvest the larvae of the latter. The ensuing predator/prey association with growth is highly advantageous to the piscivore as prey of optimum body size are thus continually available.
Methodology
The study sites were Lake Opinicon, a 2200 hectare, 10 m deep lake in Leeds County, Ontario ; and Jones Creek, a small 5 km long stream that enters the St . Lawrence River near Brockville, Ontario . The former has 18, and the latter 12, species of fish . Data were developed at four levels : (a) age at which each species became piscivorous and extent to which it developed the habit ; (b) the prey species eaten by each ; (c) changes in sizes of prey with age ; and (d) spawning times of piscivore relative to non-piscivore (i .e . prey) fish species. The first two sections are synthesized from previously published diet data (Keast 1965, 1966, 1968, 1977a, b, 1978a, b) . The prey eaten by each year class of each species was grouped into the broad categories of zooplankton, small-bodied invertebrates (length 10 mm), and fish . The last two sections are based on new data.
Food data relative to growth are from stomach analyses using standard procedures (see references above) . The fish were collected from three sites in monthly series, May-October, and grouped into year (age) classes . The numbers of stomachs processed were as follows : Esoc lucius 220; Micropterus salmoides 570; Pomoxis nigromaculatus 469 ; Ambloplites rupestris 1477; Lepomis machrochirus 3200; Perca flavescens 2200; Ictalurus natalis 320; Esox americanus vermiculatus 281 ; and Semotilus atromaculatus 654. The data on Esox lucius, Micropterus salmoides, and Ictalurus natalis, are previously unpublished. Spawning data on the piscivores and their prey were taken from Amundrud et al . (1974), Duckworth (1978), and Keast (1980) for Lake Opinicon, and Duckworth (1978), and Keast (unpublished) for Jones Creek . The Opinicon data is for the years 1969-1970, and Jones Creek, 1977 .