Disentangling drivers of the abundance of coral reef fishes in the Western Indian Ocean

Blog written by Melita Samoilys. Click here to read the full article.

Climate change is an increasingly acute pressure on coral reefs and combined with fishing is changing the communities of fishes that live on coral reefs. In order to design conservation and fisheries management action to mitigate these pressures we need to understand clearly how fishes on coral reefs respond to these two over-riding pressures. However, this is not easy because fish communities on coral reefs are notoriously variable, which is not surprising when there are hundreds of species, with different behaviours, sizes and life histories, creating a community of fishes with a myriad of requirements and interactions. Trying to understand how they respond to outside human pressures feels a bit like trying to detect clear patterns in a Jackson Pollock painting!

In this study we tried a two pronged approach, firstly to understand what causes natural differences in fish communities on reefs, and then to look at how fishing and coral mortality affect these fishes. Coral mortality was measured by live coral and is therefore an inverse measure of climate change. We surveyed 11 families of fish (from groupers to surgeonfish) on 53 reefs across four countries in the western Indian Ocean (WIO) – Madagascar, Comoros, Mozambique and Tanzania. This encompassed 12 different trophic groups of species from piscivores to detritivores, and a relatively broad range of natural variables, such as geography, the morphology of the reef, its slope, and exposure to ocean waves. We also included chlorophyll_a which is a good measure of nutrients in the water and can be downloaded from global satellite data.

The results surprised us because the geographic location of the reef within the WIO, and its geomorphology were the strongest and most significant drivers of what sort of fish community was found on the reef, and not fishing or climate change (as measured by live coral). The geomorphology variable described the reef’s structure, position and relative protection from exposure to ocean waves, and so reefs were categorised, for example, as ocean-exposed fringing reefs, or coastal barrier reefs, or within more protected inner seas complexes or within lagoons. These different types of reef geomorphologies supported different communities of fish, with different species predominating and with different levels of fish biomass. Total biomass is the weight of all fish species counted at a site, calculated from visual estimates of fish size. For example, “ocean‐exposed” fringing reefs supporting a high average total biomass of ~1,000 kg/ha, while “lagoon‐exposed fringing” reefs and “inner seas patch complex” reefs yielded much less at ~500kg/ha. Nutrient levels in the water and the slope of the reef and its exposure to the prevailing South-East trade winds, were also significant factors in explaining the fish community on a reef, with typically higher fish biomass associated with higher nutrients and steeply sloping reefs.

Photo credit: M. Samoilys

When we examined patterns at a smaller scale, just taking data from Tanzania and Mozambique on the mainland eastern Africa coast, to remove the large scale geographic differences that sites in Comoros and Madagascar bring to the analyses, fishing pressure became a significant factor explaining differences in the fish community between sites. However, it was not strongly influential, with reef geomorphology being five times more significant, and coral cover was still not a significant factor. This led us to conclude that to better understand the effects of climate change and fishing on reef fish communities we need to work more locally, at smaller spatial scales, so that the other natural reef variables are controlled. This is in fact useful for managers since they tend to work at this local scale.

We also found that out of a total of 123 species that we recorded, 37 species were driving differences in fish communities across reefs, and they were all from lower trophic level categories, that is, they were herbivores and detritivores, such as parrotfishes and surgeonfishes, and not the higher level predators such as fish eating groupers. These significant species may be useful as biodiversity indicators for coral reefs for example, to help track management and conservation impact or to examine future climate change scenarios. The trophic behaviours, responses and interactions of these potential indicator species could be an important avenue for future research to better understand their population dynamics under an increasingly warm ocean. We also suggest that variation in fish assemblages caused by the extent of reef area and coastline is still poorly understood, and the eastern African coastline provides an ideal location for further research, and remains relatively unstudied compared to areas of the Pacific. We also maintain that better measures of fishing pressure are needed urgently if we are to properly understand the influence of anthropogenic effects on coral reef systems. We hope that the results of this study will help managers to balance long‐term sustainability of artisanal reef fisheries with conservation of coral reef systems.

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