Modification of the response to high density conditions in the guppy, Poecilia reticulata (Peters)

Abstract

An examination was made of some of the factors which influence the response to density in the guppy. Water from high density fish was given to fish populations kept at high and low densities. This induced high density behaviour in low density fish by increasing aggression and decreasing courtship. But giving ‘fresh’ water to high density fish failed to alleviate the response to density. ‘High density water’ slightly reduced the numbers of young found in low density groups. High density fish kept in the dark had more young than high density groups in the light; and the number of young found was decreased when ‘high density water’ was given to low density fish. The number of ovarian stages was decreased in the dark in high density and was not overridden by the type of water. Giving ‘high density water’ to low density fish decreased the number of oocytes stage III, but the interaction between water and ‘light’ was confounded by the total numbers of ovarian stages. Increasing the ‘visual space’ of high density fish with mirrors partially decreased aggression and increased courtship and number of young found; but had little affect upon the number of ovarian stages. Decreasing the number of physical contacts by keeping the fish chronically tranquillized reduced the activity of the fish. This reduced the aggression of the high density fish and also reduced the differences between the high and low density fish on the basis of their courtship scores, number of young found and number of stages in the ovary. Fish from differing population sizes were given a choice between varying fish densities. The fish from a ‘normal’ sized population and with an extended visual field, tended to make a choice conforming to the overall selections compared with fish from populations of extreme size and with a reduced visual field.