Effects of habitat complexity and group size on perceived predation risk in goldfish (Carassius auratus)

Abstract

Habitat structural complexity can be an important factor in animal decisions such as habitat choice, patch use, and even mate choice. We investigate how the structural complexity of cover and group size affect perceived predation risk in goldfish (Carassius auratus) by measuring behaviors indicative of vigilance. Goldfish were videotaped in an aquarium while foraging on a saturated food patch near structurally complex protective cover (artificial plants) or structurally simple obstructive cover (solid blocks) in varying group sizes (three, six, or nine individuals). We measured the amount of time individuals spent foraging and their body position within the patch and their orientation (toward or away from cover). When foraging in the presence of obstructive cover, individuals from all group sizes spent more time on the side of the patch farthest from cover than close to it and oriented toward the cover. The presence of protective cover had no effect on the position or orientation of fish on the food patch for any group size. There was a nonsignificant trend for the proportion of time spent foraging increasing with group size for the obstructive cover treatment but it was independent of group size when fish fed near protective cover. These results indicate that goldfish do alter both their use of space and their orientation behaviors to offset the increased perceived predation risk produced by obstructive cover and that there is a significant effect of habitat structural complexity on the behavior of these fish.

Key Words:

• foraging
• predation risk
• cover
• habitat complexity
• vigilance
• group size
• goldfish
• Carrasius auratus

ACKNOWLEDGEMENTS

We greatly appreciate the contributions of Tom Valone, who was instrumental in developing the design and analysis of the experiment and whose numerous reviews and criticisms significantly improved this manuscript. We also thank B. Nordell and E. Nordell for support throughout the project. The office of Graduate Research and International Programs at California State University, Northridge, and the National Science Foundation, for a grant to S.E.N. (IBN-9707349), provided financial assistance for the completion of this work.