A Marine Park's Visitor Capacity and Reef Damage
A marine park receives 210,000 visitors per year at a single reef site. Dive and snorkel operators run 12 boats, each making two trips per day on 300 days per year, with an average of 18 passengers per trip. Surveys show that 6 percent of snorkellers contact the reef per trip, and each contact breaks an average of 4 coral fragments. Coral cover at the site fell from 42 to 31 percent over five years, while a control site 40 km away with similar depth and exposure fell from 40 to 36 percent. A 2019 bleaching event caused 8 percent coral mortality at both sites. The park is considering a daily visitor cap of 500 people at the site, plus mooring buoys and a compulsory briefing. Operators argue the cap will destroy their business; they currently carry about 1,300 passengers per day at the site.
1. Estimate the annual coral breakage from snorkeller contact and assess whether the proposed cap is justified as a management response.
Read the worked answer
Estimate.
- Trips per year: 12 boats x 2 trips per day x 300 days = 7,200 trips.
- Passengers per year: 7,200 x 18 = 129,600 snorkellers (close to the reported 210,000 visitors, the difference being other access modes).
- Contacts per year: 129,600 x 0.06 = 7,776 contacts.
- Fragments broken per year: 7,776 x 4 = 31,104 fragments.
Is the cap justified?
- Supporting evidence: the impacted site lost 11 percentage points of coral cover versus 4 at the control, a difference of 7 points beyond the shared bleaching effect of 8 percent. This is consistent with a local, visitor-related pressure. Breakage of about 31,000 fragments per year is a substantial, chronic mechanical disturbance.
- Caveats: the estimate assumes contacts and fragments per contact are constant across the year and all visitors; it does not account for diver contacts, anchoring or storm damage. Two sites cannot fully separate visitor pressure from other local differences such as currents, fishing or runoff.
- Would the cap work? A cap of 500 per day versus about 1,300 now would cut snorkeller numbers by roughly 60 percent, reducing breakage proportionally to about 12,000 fragments per year if contact rates hold. Moorings remove anchor damage and briefings can reduce contact rates below 6 percent, so the combined package targets both the number of contacts and the rate.
Judgement. The cap is defensible as part of a package, but the evidence is correlational; the briefing and moorings may deliver much of the benefit at lower economic cost. A phased cap with monitoring would be more defensible than an immediate hard limit.
2. Design an adaptive management approach that balances conservation and operator livelihoods, specifying indicators and decision triggers.
Read the worked answer
Design features.
- Phased implementation: introduce moorings and compulsory briefings first, then a moderate cap (for example 1,000 per day), tightening only if indicators fail. This tests which measure delivers benefit and protects livelihoods during learning.
- Co-management: include operators in setting and reviewing the cap, which improves compliance and data sharing.
- Economic instruments: differential pricing, off-peak promotion and rotating site closures to spread pressure across nearby sites.
Indicators.
- Coral cover and recent breakage scars at the site and control, surveyed annually.
- Observed contact rate per snorkeller (target below 3 percent after briefing).
- Visitor numbers and operator revenue.
- Compliance with mooring use and briefing attendance.
- Water quality and bleaching severity as covariates so visitor effects are not confounded with climate impacts.
Decision triggers.
- If contact rate stays above 4 percent for two consecutive seasons, strengthen briefing enforcement or reduce the cap.
- If coral cover declines more than 2 percentage points beyond the control in a year, cut the cap by 20 percent.
- If indicators are stable for three years, allow a modest cap increase.
- If a bleaching event occurs, impose a temporary moratorium on the most sensitive areas.
Judgement. Adaptive management with explicit triggers is more defensible than a fixed cap because it treats the cap as a testable hypothesis, protects the reef if evidence worsens, and gives operators predictability.