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Estimating accurate rope length to minimize wastage in cultch construction for mangrove oyster farming

Ernest Obeng Chuku, Isaac Osei.


The ability to accurately estimate the length of rope needed in the construction of stringed cultch masses for farming oysters without wastage is a major practical challenge. The problem arises mainly due to the illusive “ghost length” subsumed by knotting cultches on strings which potentially has dire financial implications for oyster farmers. Using repeated measures of rope diameter (D) and cultch thickness (T), we derived regression models for estimating subsumed rope length and further deduced an equation for determining the appropriate total rope length (L) for stringing cultches. For 2 mm, 4 mm and 6 mm diameter nylon ropes, the relationship between D and subsumed rope length for a single knot (Sk1) was Sk1 = - 0.34 + 12.25D [R2 = 0.9880; P = < 0.05]. Subsumed rope length for double-knotted cultch (Sk2) increased with increasing cultch thickness with a linear relationship Sk2 = 8.683 + 0.890T [R2 = 0.3871; P = 0.000]. Two equations were derived for estimating total rope length required to string a given number of cultches: 1) L = H + A + Nc(8.683 + 0.890T) for a 4 mm diameter nylon rope, and 2) L = H + A + Nc(24.50D + T - 0.68) for any rope diameter, where H is the desired distance between outermost cultches, A is the total allowance at both ends of the rope and Nc is the number of cultches. The models developed offer oyster farmers the opportunity to determine appropriate rope length for mass cultch preparations with minimal wastage of rope and consequently, save time and labour for readjustments of cultch set-ups.

Key words: Subsumed rope length, Rope estimate, Cultch construction, Oyster farming, Mangrove oyster

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