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Ecology of the leopard dace Rhinichthys falcatus and its ecological relationships with the longnose dace Rhinichthys cataractae Gee, John Henry

Abstract

Sympatrically occuring Rhinichthys falcatus and R. cataractae were collected from four areas of the Fraser River drainage system. A life history examination of R. falcatus revealed that it is an omnivorous feeder, spawning in early July at age III and older, with some males of this age developing a permanent spawning color. Both sexes develop nuptial tubercles at age I. Females are heavier and longer than males, the greatest differences occuring at age III and older. Field collections from a variety of defined habitats showed that R. falcatus were most numerous during daylight in water velocities of less than 1.5 ft./sec. Adults occupied primarily depths of one foot or greater, the yearling and young-of-the-year fish were most numerous in depths of less than one foot. R. cataractae were most numerous during daylight in depths of less than one foot, the adults occupying water velocities of greater than 1.5 ft./sec, while yearling and young-of-the-year fish were distributed almost equally in velocities both greater and less than 1.5 ft./sec. Both species remained unchanged in their current distribution after dark but some young-of-the-year and yearling fish of both species moved offshore to areas deeper than one foot while adult R. falcatus became more numerous at night in onshore shallows of less than one foot in depth. Field observations on current distribution of both species were confirmed in laboratory current preference experiments in a stream tank. The preference of three size groups of both species for areas of little or no current and areas of either 1, 2, 3, or 4 ft./sec. water velocity in the stream tank were tested In 30 experiments. In all experiments but one, there was a significant difference between the ratio of R. falcatus to R. cataractae in the current areas; R. cataractae were more numerous. Morphology and function of the airbladder of both species were analysed, and differences in volume were explained on the basis of diverging habitat selection of yearling and adult fish. The habitat divergence between yearling and older R. falcatus and R. cataractae is in accordance with Gause's contention.

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