Abstract
Despite increasing threats to Tonga’s coral reefs from stressors that are both local (e.g. overfishing and pollution) and global (e.g. climate change), there is yet to be a systematic assessment of the status of the country’s coral reef ecosystem and reef fish fishery stocks. Here, we provide a national ecological assessment of Tonga’s coral reefs and reef fish fishery using ecological survey data from 375 sites throughout Tonga’s three main island groups (Ha’apai, Tongatapu and Vava’u), represented by seven key metrics of reef health and fish resource status. Boosted regression tree analysis was used to assess and describe the relative importance of 11 socio-environmental variables associated with these key metrics of reef condition. Mean live coral cover across Tonga was 18%, and showed a strong increase from north to south correlated with declining sea surface temperature, as well as with increasing distance from each provincial capital. Tongatapu, the southernmost island group, had 2.5 times greater coral cover than the northernmost group, Vava’u (24.9% and 10.4% respectively). Reef fish species richness and density were comparable throughout Tongatapu and the middle island group, Ha’apai (~35 species/transect and ~2500 fish/km2), but were significantly lower in Vava’u (~24 species/transect and ~1700 fish/km2). Spatial patterns in the reef fish assemblage were primarily influenced by habitat-associated variables (slope, structural complexity, and hard coral cover). The biomass of target reef fish was greatest in Ha’apai (~820 kg/ha) and lowest in Vava’u (~340 kg/ha), and was negatively associated with higher human influence and fishing activity. Overall mean reef fish biomass values suggest that Tonga’s reef fish fishery can be classified as moderately to heavily exploited, with 64% of sites having less than 500 kg/ha. This study provides critical baseline ecological information for Tonga’s coral reefs that will: (1) facilitate ongoing management and research; and (2) enable accurate reporting on conservation targets locally and internationally.
Highlights
Coral reefs are increasingly threatened by cumulative human-induced disturbances [1, 2]
Mean hard coral cover in Vava’u was less than half that of the other island groups (Fig 2; S2 Table in S1 File), and this pattern was similar for soft coral
Many sites in Vava’u had 0% live coral cover, around the inner islands where turfing algae and bare matrix were dominant, and coral cover generally increased towards the outer islands
Summary
Coral reefs are increasingly threatened by cumulative human-induced disturbances [1, 2]. These range from large-scale global impacts such as climate-driven coral bleaching [3, 4], to more local stressors such as overfishing, destructive fishing practices, and pollution [5]. Many of these impacts are increasing both in frequency and severity [6]. Managing the multiple threats facing coral reef ecosystems requires accurate data on both the status of the ecosystems and the dominant influences on reef condition
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