Effectiveness of different substrates for collecting wild spat of the oyster Crassostrea tulipa along the coast of Ghana
The West African mangrove oyster, Crassostrea tulipa (Lamarck, 1819), has the potential to improve global shellfish food production and is being considered for commercial farming in many countries in West Africa. The current background information to support this venture is, however, inadequate. We assessed the effectiveness of five substrates (coconut shell, oyster shell, nylon mesh, PVC, and ceramic tile) for harvesting C. tulipa spat from the Densu Delta, Narkwa Lagoon, Benya Lagoon and Whin Estuary, along the coast of Ghana from November 2017 to October 2018. Ceramic tile had the highest mean monthly spat settlement in the Narkwa Lagoon (3451 ± 206 spat m−2), Benya Lagoon (1769 ± 145 spat m−2) and Whin Estuary (373.1 ± 52.4 spat m−2). This settlement was not significantly different from settlement on PVC slats (P > 0.05) with the highest settlement in Densu Delta (2880 ± 294 spat m−2). Coconut shell consistently had the least C. tulipa spatfall (P < 0.01). Spat collection with 2 mm nylon mesh was not successful. The under-horizontal surfaces of collectors, [mean (S.E.); 2523.7 ± 66.9 spat m−2] had significantly more C. tulipa spatfall than upper-horizontal surfaces [mean (S.E.); 775.2 ± 33.4 spat m−2] in the main experiment (P < 0.01). In a separate experiment, a change of orientation from “Face down”/0° to “Face up”/180° did not change the observed profuse under-horizontal settlement of C. tulipa spat on the collectors, suggesting that under-horizontal surfaces were more attractive to C. tulipa spat. Larger-sized C. tulipa spat on under-horizontal surfaces, mean (S.E.) 9.88 ± 0.5 mm, compared to upper-horizontal surfaces, mean (S.E.) 5.99 ± 0.5 mm, of the collectors suggest earlier settlement on the undersides. The effectiveness of collectors correlated positively with dissolved oxygen. Ceramic tiles and PVC slats were the most effective materials for C. tulipa spat collection, hence, their use is recommended for large-scale C. tulipafarming.