Marine Atlas: Maximizing Benefits for Tonga

REEFS AT RISK: REEF RISK LEVEL

Tonga’s reefs are at risk and the direct and indirect impacts of climate change are exacerbating a system already un- der threat, jeopardizing marine values worth billions of dollars.

As seen in the previous maps, coral bleaching is the silent reef killer, caused by rising sea levels and ocean acidification. A large coral bleaching event was recorded in Tonga in 2000 (Lovell and Palaki, 2003), with significant coral bleaching around Ton- gatapu and the Ha’apai group. This same bleaching event also affected coral reefs in Fiji. During this event, certain species of coral exhibited between 80 and 100 per cent bleaching. Outbreaks of the crown- of-thorns starfish (Acanthaster planci) were also re- corded in the 1970s and 1980s, with moderate num- bers in 1992. These starfish can destructively graze corals. Predicted rise in SST and sea level, coupled with increased nutrients from land-based activities, will add further stress to the corals of Tonga. The cumulative impact of climate change and local human activities on Tonga’s reefs will increase over time. The risk of these threats is shown on the map of Tonga’s reefs, classified by estimat- ed present threat from local human activities, according to the Reefs at Risk integrated local threat index (Burke et al., 2011). Threats consid- ered in the index include coastal development, including coastal engineering, landfilling, run-off from coastal construction, sewage discharge (see also chapter “The dose makes the poison”) and impacts from unsustainable tourism (see also chapter “Beyond the beach”); watershed-based pollution, focusing on erosion and nutrient fertilizer run-off from agriculture entering coastal waters via rivers; marine-based pollution and damage, including solid waste, nutrients, toxins from oil and gas installations and shipping, and physical damage from anchors and ship groundings (see also chapter “Full speed ahead”); and overfishing and destructive fishing, including unsustainable harvesting of fish or invertebrates, and damaging fishing practices such as the use of explosives or poisons (see also chapters “Fishing in the dark” and “Small fish, big importance”).

Acropora coral field in Tonga was exposed to multiple impacts, including a crown-of-thorns outbreak and cyclone damage.

This multitude of man-made threats leaves Ton- ga’s reefs at risk. Analysis of the threat index indi- cates that 65.7 per cent of the reef area is classi- fied as facing low risk, 25.4 per cent medium risk, 7.7 per cent high risk and 1.2 per cent very high risk. The areas of high and very high risk (orange and red) are concentrated around urban centres such as Tongatapu, the Ha’apai group and the Vava’u group. Wilkinson (2008) identified the ma- jor human disturbances as overfishing, pollution, sedimentation, eutrophication and coastal de- velopment. The reefs are important to the econ- omies of local communities, especially through tourism. They are also important for subsistence

and coastal protection. According to the Reefs at Risk Assessment (Burke et al., 2011), there is a very high social and economic dependence on coral reefs in Tonga. Luckily, there are many initiatives aiming to facili- tate the necessary changes. However, an integrat- ed approach to coral reef conservation needs to include acknowledgements of land–sea connec- tions and requires an understanding of how and where terrestrial conservation actions influence reefs. Klein et al. (2012) examined the impact and cost effectiveness of protecting forests as a reef conservation measure. They found that relative coral reef condition could be improved by 8–58 per cent if all remnant forest was protected rather than deforested. It seems that rainforests on land support Tonga’s “underwater rainforests”. The Pacific Ridge to Reef project is working to build these connections into management decisions and planning in Pacific Island countries.

Crown-of-thorns starfish damage Tonga’s reefs. Outbreaks often occur when their natural predators are overfished.

MAXIMIZING BENEFITS FOR TONGA

CLIMATE CHANGE THREATS

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