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Distribution and habitat responses of the coastrange sculpin (Cottus aleuticus) and prickly sculpin (Cottus asper) in the Little Campbell River, British Columbia Taylor, Gerald David

Abstract

Differential responses to water current and substrate, exhibited by populations of Cottus aleuticus and Cottus asper in the Little Campbell River, were studied both in the field and in experimental laboratory apparatus. Both broad-scale and local distributions of C. aleuticus and C. asper appear to be closely associated with available habitat. The slow moving areas of the lower river appear to provide adequate habitat for fry of both species. These fish have the least decided preferences for current and substrate conditions. As yearlings the two species show almost complete divergence of habitat choice with most C. asper inhabiting slow areas of the lower river and most C. aleuticus moving upstream to riffles and other areas of distinct current. As adults, C. asper inhabit areas of non-current, decreasing in numbers from mouth to source of the stream, coincident with a decrease in occurrence of pool environments. The upper river, which is entirely pool habitat, may have a reduced C. asper population due to isolation for six months of the year and suspected low oxygen concentrations. Cottus aleuticus yearlings and adults are found principally in the middle river where areas of current are present. Day and night collections reveal that yearling and adult C. aleuticus and adult C. asper are largely segregated into different microhabitats during daylight but may occupy similar areas during darkness. Formation of pool environments by tidal inundation of former current and non-current areas appears to allow fry of both species, previously largely segregated, to occupy similar areas regardless of time of day. Both species were found to feed principally at night as yearlings and adults. Responses of field fish displaced to habitats not normally occupied strongly suggests the importance of environmental factors in determining distribution and abundance and, in segregating the two species under natural conditions. Fish were given a choice of current and non-current areas in one series of laboratory experiments and coarse and fine substrates in another series. Tests were performed on fish taken directly from the field, on fish held in bare hatchery troughs for 60 days, and on fish exposed to simulated natural conditions of flow and substrate for 60 days. At current speeds of 45 centimeters per second, C. aleuticus fry (less than 40 millimeters) in the laboratory showed no preference for either current or non-current areas, while fish greater than 60 millimeters (yearlings and adults) tended to select current. Cottus aleuticus, at all size groups tested, preferred coarse rather than fine-textured substrate. In the field, C. aleuticus showed a preference for current and coarse substrate although fry often occupied areas of fine substrate and non-current when C. asper were absent or scarce. Cottus aleuticus, as yearlings and adults, were rarely found over fine substrate. In laboratory experiments C. asper generally preferred non-current to current and showed no substrate preference except fry which preferred coarse substrate. Fish between 80 and 100 millimeters showed no preference for either current or non-current areas in the laboratory. In the field, C. asper were rarely associated with current but were found over a wide range of fine and coarse substrate. When a choice of flow and substrate conditions were offered simultaneously, C. aleuticus preferred current and coarse substrate. C. asper showed no preference for either flow condition and had no substrate preference. There was no significant difference in a species response to flow and substrate when tested singly or in groups. Holding conditions (bare troughs and simulated natural conditions) did not change responses of either species from those outlined for fish tested directly from the field. Field and laboratory results suggest habitat segregation to be most pronounced in fish between 40 and 80 millimeters long. The relevance of these results to habitat segregation by the two species in a stream is discussed. Food, reproduction, fish associations and densities are also discussed as factors affecting spatial and temporal distribution of the species.

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