Although overfishing is usually deemed to be detrimental to coral reefs, an 18-year study of the coral reefs off Kenya has provided concrete evidence to illustrate this point.
Jennifer O'Leary, Timothy McClanahan showed in their paper published in a recent issue of the journal Ecology that more intensively-fished reefs have more sea urchins, which in turn eat the crustose coralline algae (CCA) that aid in building coral reefs.
Conversely, unfished reefs have fewer urchins because of more predatory fishes keeping their numbers in check. As a result, these reefs have more structure and higher growth rates.
The authors assessed abundances of major organisms using three types of surveys (benthic, fish, and sea urchin) over a period of 18 years (with annual surveys being conducted during 15 of these 18 years) at three fished reefs, a reef recently closed to fishing and two reefs that were long-term fisheries closures.
All of the study sites were located along a 450-km reef system running along Kenya’s coastline.
The authors also carried out a series of field experiments in which they manipulated access to sea urchins and fishes on both fished and non-fished sites to examine the effects of their abundances on CCA cover.
The authors found a negative correlation between the abundance of grazing sea urchins and CCA cover (i.e. CCA cover decreases with increasing sea urchin biomass).
This implied that the urchins were eating away the CCA, which produces calcium carbonate and contributes to reef growth.
In the fished sites where the predators of the sea urchins such as triggerfishes and wrasses were largely absent, the authors found sea urchins to proliferate and become the dominant grazers. This led to a significantly slower growth rate of reefs that were fished and had more sea urchins versus unfished reefs with more diverse fish communities.
Even though some grazing fishes were found to negatively affect CCA cover, their abundance affected CCA population less strongly than that of sea urchins.
Furthermore, grazing fishes indirectly aided CCA populations by removing competitors such as fleshy algae.
The authors also found that the negative effect of sea urchin grazing on CCA cover was generally stronger and more ubiquitous than even extreme weather caused by an intense El Niño event.
According to author Tim McClanahan: "This study illustrates the cascading effects of predator loss on a reef system and the importance of maintaining fish populations for coral health."
For more information, see the paper: O'Leary, JK and TR McClanahan (2010) Trophic cascades result in large-scale coralline algae loss through differential grazer effects. Ecology 81, pp. 3584–3597.