Species diversity, abundance, biomass, size and trophic structure of fish on coral reefs in relation to shark abundance
Shanta Barley, Jessica Meeuwig | Feb 17, 2017
A common mesopredator on coral reefs, the two spot red snapper Lutjanus bohar.
Photo: Albert Kok.
Barley SC, Meekan MG, Meeuwig, JJ. 2017. Species diversity, abundance, biomass, size and trophic structure of fish on coral reefs in relation to shark abundance. Marine Ecology Progress Series, 565: 163-179.
- Rapid and ongoing loss of sharks from reefs globally may alter the size and trophic structure of fish communities.
- Our study suggests that declines in shark numbers may change coral reef fish communities, with increased biomass of prey and competitors at a range of trophic levels. This is of concern as many of these species are the custodians of reef health – grazing and “cleaning” coral reefs.
- Not all sharks are created equal – the influence of sharks on the reef fish assemblage depends to a degree on their jaw gape. In other words – there are limits on the size of what you can eat and thus your influence on the food web.
- Large-scale unreplicated experiments (LUNES) such as this study from Australia’s northwest are valuable as they provide rare evidence of how whole ecosystems respond to human impacts. As we attempt to slow declines in ocean health, learning from such natural experiments is increasingly important.
Theory predicts that loss of gape-limited sharks should lead to increases in the abundance and biomass of smaller size classes of prey. We used stereo-baited remote underwater video stations (stereo-BRUVS) and stereo diver-operated video systems (stereo-DOVS) to characterise the shark and fish assemblages on 2 remote, atoll-like reef systems in northwestern Australia, the Rowley Shoals and the Scott Reefs. Whereas the Rowley Shoals is a marine protected area, sharks have been removed from the Scott Reefs for over 3 centuries. We found that sharks were significantly more diverse, more abundant, larger in size and greater in biomass in the marine reserve relative to the Scott Reefs. Consistent with a priori hypotheses, bony fishes displayed greater species diversity, abundance and biomass where sharks were common relative to the predator-depleted location. The size and trophic structure of bony fish assemblages also differed between locations. Our results provide large-scale evidence consistent with the hypothesis that reef-associated sharks are gape-limited trophic omnivores that impose top-down effects on medium sized (<50 cm), low- to mid-trophic level fishes. On stereo-BRUVS, for example, prey in the 0 to 29.99 cm size class had 203% more biomass at the predator-depleted reef relative to the location where sharks were abundant. As body size is an important determinant of ecological role and fitness in fishes, these findings suggest that the rapid and ongoing loss of sharks from reefs globally may have important implications for reef management and investigations into the effect of fishing on reef systems.
FUNDING & ACKNOWLEDGEMENTS
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