The Movements and Population Densities of Roach (Rutilus rutilus (L.)) and Gudgeon (Gobio gobio (L.)) in the River Mole

Abstract

The movements of coarse (i.e. non-salmonid) fish have been little studied, although recently Williams (1965) has produced evidence to show that in the Thames most of the roach, perch (Percafluviatilis L.) and dace (Leuciscus leuciscus (L.)) ranged over a limited area. Lelek (1962) tentatively concluded that gudgeon were not tied to one part of a river, being free to move into any area where food was available, but he subsequently formed the opinion (Lelek, private communication) that the sections of river in which he worked were perhaps too small to satisfy the requirements of the species. His colleague, Lusk (1963), who worked on a larger section of river, found that 770 of the gudgeon he marked were still in the same area 100 days later and concluded that they occupied a 'home range' (Gerking 1953). Other evidence also points to the restricted movements of native fish; Worthington (1950) quotes marking experiments carried out by Le Cren in Windermere which indicated that perch showed little movement along the shore line and that there was a strong tendency for artificially displaced fish to return to their original locations. Catches taken in a trap for upstream migrants (Fishery Officer's report to Lancashire River Board, 1951) show that, apart from an occasional chub (Squalius cephalus (L.)) which appeared to move in response-to increases in water flow, very few coarse fish travelled upstream from spring to early winter except between 17 and 30 May, when there was a marked movement of roach which might have been associated with spawning. Roach spawning migrations are mentioned by Yarrel (1841) and, according to Svardson (1952), they migrate into shallow water before spawning. Hartley (1947) reported a similar phenomenon although he regarded it as unproven because his data came from traps whose catching power was possibly dependent on an activity factor in the fish. The present investigation was carried out partly in the River Mole and partly in a convoluted channel built in the laboratory to conduct experiments in an attempt to explain the field observations. A preliminary analysis of the movements of roach in the river has already been published (Stott 1961) and these data are further examined in this paper together with data for gudgeon. Mark-recapture estimates of population densities have also been made from the movement data.