A Laboratory Study of Interactive Segregation between two New Zealand Stream-Dwelling Fish

Abstract

The common river galaxias (Galaxias vulgaris Stokell) and the upland bully (Gobiomorphus breviceps (Stokell)), are common freshwater fish in the South Island of New Zealand. The habitat requirements of Galaxias vulgaris appear to be more stringent than those of Gobiomorphus breviceps which occurs in all types of water, both lotic and lentic (Cranfield 1962; Woods 1967; Lane & Skrzynski 1972). Stokell (1955) noted that G. breviceps showed 'remarkable adaptability to different types of habitat'. He transferred specimens from a rapid alpine stream to a small landlocked lake which previously contained no native fish and found that they immediately adapted themselves to the new conditions and 'multiplied so rapidly that the lake was fully stocked in three years'. On the other hand, Galaxias vulgaris is found only in rivers and streams, and does not occur in lakes (Stokell 1949,1955; Hopkins & McDowall 1970; McDowall 1970; Lane & Skrzynski 1972). Stokell (1955) transferred 'some hundreds of specimens' of G. vulgaris into a small lake fed by seepage from a number of small streams which did not normally flow directly into the lake. He found that the fish existed in the lake for about one year, but 'escaped' into the largest tributary during a period of connected water. Both G. vulgaris and Gobiomorphus breviceps occur together in a number of rivers and streams, e.g. the Glentui River in the Province of Canterbury. In the Glentui, adult Galaxias vulgaris occupy the more turbulent parts of riffles, whereas adult Gobiomorphus breviceps occur in quiet stretches including the quieter parts of riffles. During an investigation into the ecology of Galaxias vulgaris in the Glentui (Cadwallader 1975c, d, e), monthly collections were made over a period of one year. Although no estimates of population numbers were made, it appeared that adult Gobiomorphus breviceps increased in numbers in areas from which Galaxias vulgaris was removed. This suggested that interactive segregation (Nilsson 1967) was occurring within the river. The present study was undertaken to investigate interactive segregation between adults of the two species in a laboratory stream tank.