An experimental study on the influence of feeding versus predation risk in the habitat choice of juvenile and adult two-spotted goby Gobiusculus flavescens (Fabricius)

Abstract

The foraging behaviour of juvenile and adult male and female gobies Gobiusculus flavescens (Fabricius) were compared. The three groups (juveniles, adult males and females) were tested separately and together for their distribution between two different habitats in an aquarium: One safe habitat with a shelter, and one more risky habitat without a shelter. In half of the trials a predator (cod, Gadus morhua L.) was present. Food was added to the two habitats at four different ratios: 1:1, 1:2, 1:5 and 1:8 with the highest food levels on the risky side. When each group was tested separately in the absence of the predator, the distribution between the two habitats was close to that predicted by the Ideal Free Distribution. When juveniles and adults were together without a predator, however, the habitat with the highest food level was under-utilised because the adults showed a strong preference for the shelter-habitat with the lowest food supply. When juveniles and adult males were together in the presence of predator, the juveniles exposed themselves more at the risky side with most food. We conclude that the juveniles give more weight to feeding opportunities than adult males (who give more weight to safety) in their decision where to stay. This interpretation, however, is not straightforward and is discussed in relation to several experimental constraints.