The Reproductive Biology of Rock Bass, Ambloplites rupestris (Centrarchidae), in Lake Opinicon, Ontario

The reproductive behavior of rock bass in Lake Opinicon begins with an abrupt movement of mature males to inshore nesting areas. Inshore density increases several fold and males become territorial, defending a bottom area in which they construct a nest by anal and pectoral fin displacement of the substrate. This results in a bowl shaped depression about 1.9 times male size. Nests are typically in coarse substrate and at 1/2 to 3/4 m water depth. Reproductive males (mean age 7.5 years) tolerate concurrently nesting pumpkinseed (Lepomis gibbosus) within half the distance separating neighboring rock bass. Males can complete a nest in one day but usually spawn after two days. Spawning activity occurs in late May depending upon water temperature. Females showed peak reproductive behaviors at 21-23 C. Gravid females approach and enter nests without male courtship and spawn about 500 eggs. During the spawning act the male's body color darkens. Circling behavior in the nest is irregular and the female is often exterior to the nest center. The eggs are emitted over a 2-hour period with the horizontal dipping motions common to centrarchids. One female may spawn with several males and one male may spawn with several females. However, despite this apparent promiscuity, it appears that females may choose males with which to spawn the majority of their eggs. Following spawning females leave the nest area and males provide parental care for eggs by fanning with the pectoral fins, and for larvae through predator defense. Males are highly aggressive and often charge towards a territory intruder with spread opercles or displaying an open mouth. About 33% of nests are unsuccessful, possibly due to brood predation. Eggs hatch after 5 days and fry leave the nest 9-10 days later. Twenty-four % of the males respawn, usually in new nests but in the same area. Females can also spawn again, as indicated by modal egg classes in the ovary. Most reproductive activity is confined within a three week period, but males raising a second brood remain with nests until late June or early July. Following nesting, the inshore density of rock bass returns to a prereproductive level.