Episodic Disturbance from Boat Anchoring Is a Major Contributor to, but Does Not Alter the Trajectory of, Long-Term Coral Reef Decline

Isolating the relative effects of episodic disturbances and chronic stressors on long-term community change is challenging. We assessed the impact of an episodic disturbance associated with human visitation (boat anchoring) relative to other drivers of long-term change on coral reefs. A one-time anchoring event at Crab Cove, British Virgin Islands, in 2004 caused rapid losses of coral and reef structural complexity that were equal to the cumulative decline over 23 years observed at an adjacent site. The abundance of small site-attached reef fishes dropped by approximately one quarter after the anchoring event, but this drop was not immediate and only fully apparent two years after the anchoring event. There was no obvious recovery from the impact, and no evidence that this episodic impact accelerated or retarded subsequent declines from other causes. This apparent lack of synergism between the effect of this episodic human impact and other chronic stressors is consistent with the few other long-term studies of episodic impacts, and suggests that action to mitigate anchor damage should yield predictable benefits

Anchor damage data

Data collected in the field in Crab Cove, British Virgin Islands

A map of the Crab Cove study site near Guana Island, British Virgin Islands.

<p>The image of Crab Cove shows the portion of the site damaged by the one-off anchoring event in 2004, and the three zones within the site that are used to account for the offshore gradient in reef structure.</p

Three potential trajectories of community change after an episodic disturbance (Impacted site), against a backdrop of gradual decline due in response to chronic stressors (Control site) [24].

<p>An antagonistic effect of the episodic impact is indicated by recovery before eventual resumption of the pre-impact decline (ant), an additive effect is indicated by an almost immediate continuation of decline at the pre-impact rate (add), and synergism is revealed by resumption of community decline but at an accelerated rate (syn).</p

Mean adult fish density over time at Crab Cove, showing changes inside and outside the area damaged by the 2004 anchoring event.

<p>The dotted vertical line indicates the timing of the 2004 anchoring event.</p

Mean reef structural complexity over time at Crab Cove, showing changes inside and outside the area damaged by the 2004 anchoring event.

<p>The dotted vertical line indicates the timing of the 2004 anchoring event.</p