Morphological changes in a marine population of threespined stickleback, Gasterosteus aculeatus, recently isolated in fresh water

Abstract

A marine population of Gasterosteus aculeatus recently isolated in fresh water was found in Bergen, western Norway. This discovery offered the opportunity to study early changes in a population isolated in an environment quite different from what it previously experienced. The population was sampled in 1982, 1987, and 1991. Variation of plate morph frequencies was studied, and a prediction model for the change in frequencies is proposed. An increasing frequency of specimens with four dorsal spines is observed. This may be related to high summer temperature. Changes in morphology were recorded and compared with those in the marine population from which the isolated population originated. Discriminant analysis was used to reveal differentiation in morphometric characters. The main morphometric change recorded was that the isolated population had developed a more compact body, which is probably an adaptation to a less active, more benthic way of living.