Estimating Sustainable Live-Coral Harvest at Kamiali Wildlife Management Area, Papua New Guinea

Abstract

Live coral is harvested throughout the Indo-West Pacific to make lime, used in the consumption of the world's fourth-most consumed drug, betel nut. Coral harvesting is an environmental concern; however, because lime-making is one of the few sources of income in some areas of Papua New Guinea (PNG), the practice is unlikely to stop. To better manage coral harvest, we used standard fishery-yield methods to generate sustainable-harvest guidelines for corymbose Acropora species found on the reef flat and crest at Lababia, PNG. We constructed a yield curve (weight-specific net annual-dry-weight production) by: 1) describing the allometric relationship between colony size and dry weight, and using that relationship to estimate the dry weight of Acropora colonies in situ; 2) estimating annual growth of Acropora colonies by estimating in situ, and describing the relationship between, colony dry weight at the beginning and end of one year; and 3) conducting belt-transect surveys to describe weight-frequencies and ultimately to predict annual weight change per square meter for each weight class. Reef habitat covers a total 2,467,550 m2 at Lababia and produces an estimated 248,397 kg/y (dry weight) of corymbose Acropora, of which 203,897 kg is produced on the reef flat/crest. We conservatively estimate that 30,706.6 kg of whole, dry, corymbose, Acropora can be sustainably harvested from the reef flat/crest habitat each year provided each culled colony weighs at least 1805 g when dry (or is at least 46 cm along its major axis). Artisanal lime-makers convert 24.8% of whole-colony weight into marketable lime, thus we estimate 7615.2 g of lime can be sustainably produced annually from corymbose Acropora. This value incorporates several safety margins, and should lead to proper management of live coral harvest. Importantly, the guideline recognizes village rights to exploit its marine resources, is consistent with village needs for income, and balances an equally strong village desire to conserve its marine resources for future generations.

Fig 1. Corals on Lababia Island, PNG harvested for betel-nut lime production.

We found this pile a few hours after a local informant told us the community ceased harvesting coral. We learned that harvest is frequent enough for collectors to know how much lime will be produced every time the boat in the background is filled to the gunwales. Photo: K. Longenecker.

Fig 2. Work flow to estimate sustainable lime production at KWMA.

Parallelograms represent data gathering, rectancles represent mathematical processes, arches represent delays, hexagons represent intermediate estimations or predictions, stadiums represent results presented herein, triangles represent the application of intermediate results to generate subsequent estimates.

Fig 3. Major (L ₁) and minor (L ₂) axes.

These were measured to construct the allomentric relationship and to estimate in situ dry weight of corymbose Acropora colonies at KWMA. Photos: K. Longenecker.

Fig 4. In situ dry-weight predictor for corymbose Acropora colonies at KWMA.

W = 0.2951(L ₁ ∙ L ₂)^1.2040; r² = 0.936. 99% of colonies to which we applied the relationship had area estimates ≤ 4368 cm² (arrow).

Fig 5. Estimated weight of tagged colonies at the beginning (W _t) versus end (W _t+1) of one year.

Reef flat/crest: W _t+1 = 1.67(W _t) − 66.49; r² = 0.549. Fore reef: W _t+1 = 0.990(W _t) + 140.57; r² = 0.668.

Fig 6. Relative frequency plot of estimated weights of corymbose Acropora colonies at KWMA.

70 fore reef specimens, 212 reef flat/crest specimens. Colonies with likely overestimated weights are not shown (8059 and 18,877 g colonies from the reef flat/crest, and a 26,396 g colony from the fore reef).

Fig 7. Net annual dry-weight production, by weight class, of corymbose Acropora on reef flats and crests at KWMA.

Assuming weight and age are positively related, each bar represents a year-class. Putative age is indicated by the number above or in a given bar. Excluded from the plot is the influence of two colonies estimated to be 8059 and 18,877 g. Note that dry weight is plotted on a logarithmic scale.

Fig 8. Harvestable-size estimate.

(L ₁ ∙ L ₂) = 0.9601(L ₁)^1.9062; r ² = 0.970. Colonies with a major-axis length (L ₁) of 41 cm are predicted to have attained harvestable size.