Effects of coral bleaching on coral reef fish assemblages

Abstract

Coral reefs have emerged as one of the ecosystems most vulnerable to climate variation and change. While the contribution of climate warming to the loss of live coral cover has been well documented, the associated effects on fish have not. Such information is important as coral reef fish assemblages provide critical contributions to ecosystem function and services. This thesis assesses the medium to long term impacts of coral loss on fish assemblages in the western Indian Ocean. Feeding observations of corallivorous butterflyfish demonstrates that considerable feeding plasticity occurs among habitat types, but strong relationships exist between degree of specialisation and declines in abundance following coral loss. Furthermore, obligate corallivores are lost fairly rapidly following decline in coral cover, whereas facultative corallivores are sustained until the structure of the dead coral begins to erode. Surveys of benthic and fish assemblages in Mauritius spanning 11 years highlight small changes in both benthos and fish through time, but strong spatial trends associated with dredging and inter-specific competition. In Seychelles, although there was little change in biomass of fishery target species above size of first capture, size spectra analysis of the entire assemblage revealed a loss of smaller individuals (<30cm) and an increase in the larger individuals (>45cm). This represents a lag effect where fishery production cannot be assured for the long term. A targeted before (mid-1990s) – after (2005) sampling program of coral reef benthos and fish assemblages in 7 countries across the Indian Ocean demonstrated changes in size structure, diversity and trophic composition of the reef fish community have followed coral declines in both fished and protected areas. The thesis highlights the pivotal role that loss of reef structural complexity plays in the effects of bleaching on fish assemblages and that coral reef management needs to radically adapt to address climate change issues.