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7. Market Surveys

Coconut crab sales from the weekly Friday market in Alofi were monitored over a 3 month period from 7-9-90 to 14-12-90. Coconut crabs presented for sale at the market were counted and where possible their TL measured. If a TL measurement was impossible due to the manner in which the crabs were bound, a weight was taken and then converted to a thoracic measurement using the following formulas:

Males: TL = 24.628 + 0.02170 × Weight

Females: TL = 18.815 + 0.0462 × Weight

(Formulas generated by conducting a regression analysis on population survey thorax: weight data.)

During the survey period 12 market days were monitored and a total of 104 individual crabs and 9 bags of crabs were offered for sale. The majority of crabs available at the market were male, accounting for 93% of the crabs for which an accurate sex determination was possible. In fact female crabs were only available at the one market, held on 7-9-90. This corresponds to the final stages of the coconut crab moulting period and so coconut crab density in the forests would still be low. Females are generally smaller than males and therefore would fetch lower sale prices. It is probable that females are only present at the market during the winter months when insufficient males are available in the forest to meet demand.

The mean TL of market crabs, males in particular, was much higher than that for crabs recorded during the population surveys. Market male coconut crabs had a mean TL of 48.1 mm (n=97) whereas males from the population surveys had a mean TL of 32.6 mm (n=150). Obviously there is pronounced size-selection associated with crabs offered for sale at the market. The extent to which the market size-selection process reflects on actual hunting size-selection is impossible to determine. While size-selection is prominent amongst hunters, small crabs are not ignored and will be taken provided the effort in catching them is no greater than that required for large crabs.

During this author's visit to Niue in July 1991 a survey was conducted of crabs offered for sale from the Friday (19-7-91) Alofi market. A total of 54 crabs, all males, were recorded with a mean TL of 43.9 mm. This value is significantly less (T-test, p=0.001) than the mean TL recorded for market sales of male crabs during a similar month in 1990. This suggests that there has been a reduction in availability and/or size of the large male crabs in the coconut crab population on Niue; the result of high and size-selective hunting pressure.

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