AN EXPERIMENTAL-STUDY ON THE INFLUENCE OF FEEDING VERSUS PREDATION RISK IN THE HABITAT CHOICE OF JUVENILE AND ADULT 2-SPOTTED GOBY GOBIUSCULUS-FLAVESCENS (FABRICIUS)

The foraging behaviour of juvenile and adult male and female gobies Go biusculus flavescens (Fabricius) were compared. The three groups (juve niles, adult males and females) were tested separately and together fo r their distribution between two different habitats in an aquarium: On e safe habitat with a shelter, and one more risky habitat without a sh elter. In half of the trials a predator (cod, Gadus morhua L.) was pre sent. 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. Whe n 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 w ithout a predator, however, the habitat with the highest food level wa s 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 expose d themselves more at the risky side with most food. We conclude that t he juveniles give more weight to feeding opportunities than adult male s (who give more weight to safety) in their decision where to stay. Th is interpretation, however, is not straightforward and is discussed in relation to several experimental constraints.