Effects of resource availability on the competitive behaviour of butterflyfishes (Chaetodontidae)

Coral reefs throughout the world are highly degraded and subject to an increasing prevalence of disturbances. Degradation of coral reef habitats is likely to lead to a decline in resource availability for many reef fishes. To assess whether coral-feeding butterflyfishes (Chaetodontidae) would demonstrate increased competition for reduced prey following habitat degradation, the competitive behaviours of several species of butterflyfishes were quantified at Lizard Island, Great Barrier Reef, Australia. We compared the aggressive interactions of butterflyfishes in two reef habitats with very different resource availability during timed visual observations. Home range sizes of two species (Chaetodon baronessa and C. lunulatus) were measured at four sites with varying coral availability. The dominant butterflyfishes at Lizard Island apprear to be C. baronessa and C. trifascialis, which show aggression towards many other species of butterflyfishes. At exposed locations where its preferred prey was highly abundant, C. baronessa aggressively defended small territories. In back-reef locations where food was more limited, C. baronessa had larger territories and was less aggressive towards other butterflyfish. Territory size varied little between habitats for C. lunulatus, and it rarely showed aggression towards other butterflyfish. As its territory size was a decreasing function of the abundance of high-quality preferred prey resources, C. baronessa appears to be an optimal forager. Meanwhile, C. lunulatus appears to be a subordinate competitor, restricted from accessing high-quality coral prey, and consequently it utilises a different optimal foraging strategy.