Fishing, climate change not double trouble for corals

Do fishing and climate change act synergistically on coral reef ecosystems, meaning the combined impact is greater than the sum of each acting individually? Conservation practitioners have expressed this concern, but synergism in ecosystems has been challenging to prove scientifically.

A new study in Kenya reassuringly finds that warming ocean temperatures and fishing have not acted synergistically with respect to a recent large-scale coral bleaching episode. Unfortunately this suggests that marine reserves, which restrict fishing, may not make coral reefs more resilient to climate change.

Emily Darling from the Tropical Marine Ecology Lab at Simon Fraser University and fellow researchers used data from an unusual episode of warm sea temperatures to assess the combined impact of fishing and climatic stress on corals. In general, there are three possible ways that human impacts can interact:

The interaction can be additive (total impact equal to sum of individual impacts), synergistic (total impact greater than sum), or antagonistic (total impact less than sum). 

Some scientists have hypothesized that fishing pressure reduces the resilience of coral reefs to global warming creating a synergistic effect between the two stressors. This could happen for a number of reasons. For example, fishing could reduce herbivore numbers leading to an increase in algae, which in turn could make corals more susceptible to bleaching.

In 1998, the Indian Ocean became unusually warm, causing a massive bleaching episode in which many corals died after expelling their colorful zooxanthellae. Because the warm episode happened on regional scale, it affected all of Kenya’s coral reefs— those subject to fishing and those in no-take reserves.  

The researchers recognized this as a scientific opportunity to compare the effect of climate change on fished and unfished areas. By a stroke of luck scientists happened to be conducting coral surveys in and out of marine reserves in that area from 1987 to 1998. In 1997, one year before the temperature anomaly, unfished areas had approximately 40% coral cover, while fished areas had 20% coral cover.  

One year after the temperature anomaly, coral cover had declined tremendously in both fished and unfished areas. Surprisingly, however, the coral decline was more pronounced in marine reserves, so that in 1999 the fished and unfished areas both had coral cover of approximately 10%.

According to the study authors, “While both stressors decreased coral cover, fishing by 51% and bleaching by 74%, they did not interact synergistically. Instead, their combined effect was antagonistic or weakly additive.”  

Interestingly, another recent study of fishing and climate change in Tasmania came to the opposite conclusion, finding a synergistic impact on kelp beds. The reason for the lack of synergism in the case of the Kenya study is not known, but the authors speculate that it may be due to the fact that bleaching acts as a dominant impact. In addition, fishing pressure may select against corals that are susceptible to bleaching.

The findings in Kenya have immediate implications for conservation and management because some experts have suggested that establishing marine reserves to reduce fishing pressure could help coral reefs withstand a warming climate. But according to Darling and her colleagues,

“[O]ur results…suggest that marine reserves are not enough to protect Kenyan corals in a changing climate. This conclusion challenges the commonly held belief that managing local stressors, such as fishing, will mitigate global stressors, such as climate change.”

Reviewed by Peter Taylor

Darling, E., McClanahan, T., & Côté, I. (2010). Combined effects of two stressors on Kenyan coral reefs are additive or antagonistic, not synergistic Conservation Letters DOI: 10.1111/j.1755-263X.2009.00089.x