Biswajit Dash

Observation on Forces and Motions of a Mariculture Cage From Model and Prototype Experiments

In India, several experimental coastal aquaculture cages have been installed on sites along the Indian coast. Many of them have been commercially successful, while some have faced technical difficulties. The construction of the Indian coastal aquaculture cage is based on ad hoc designs adopted from other countries. The performance of the coastal aquaculture cage shows that there is a need for development of a cage design with a location-specific mooring system to withstand local wave conditions for a longer period in different coastal zones of the country. This requires that designs of the cages suiting Indian conditions be made based on sound engineering principles. To design the cage, the current- and wave-induced tensions in the mooring chain and net twine and the cage motions need to be estimated. In this study, a model cage was fabricated and tested in a towing tank under different waves and towing conditions. The tension on the mooring chain was measured during model experiments along with the towing speed and wave parameter. A prototype coastal aquaculture cage with single point mooring near the coast of Visakhapatnam in South East India was identified for measurement of forces and motions. On the prototype cage, measurements of cage motions, tensions on mooring chain, and net twine and cage orientation were carried out. Characteristics of the cage response, observed in model and prototype experiments, are presented in this paper. The model cage is not a geometrically scaled down version of the prototype cage. The diameter/depth ratios of the model and the prototype are different. The mooring pattern and net twine of the model cage is the same as that of the prototype. The data presented in this paper could be used as a reference for validating the numerical model for simulating the cage forces and motions under wave and current loads.

Mixed culture of Pacific white shrimp Litopenaeus vannamei (Boone, 1931) and flathead grey mullet Mugil cephalus (Linnaeus, 1758) in floating cages

The study explored the possibility of integrating the grey mullet Mugil cephalus (Linnaeus, 1758) along with Pacific white shrimp Litopenaeus vannamei (Boone, 1931) in floating cage culture in Godavary Estuary, India. Post-larvae (PL 12) of L. vannamei (3 lakhs nos.), were acclimatised and nursed in five hapas for 28 days at a density of 3333 nos. m-2, with survival of 60%. L. vannamei juveniles having mean weight of 0.86 g, stocked at a density of 1060 nos. m-2, were cultured along with and without pre-stocked M. cephalus, in three floating cages each. Six thousand fry of M. cephalus (mean length 4.17 cm; mean weight 1.22 g) were stocked in three cages at uniform density of 23.5 nos. m-3, three months prior to stocking of L. vannamei. Shrimps were fed commercial pellets @ 3-8% of body weight, four times daily and harvested after 68 days. Fishes were fed with pelleted feed and after five months attained mean length of 23.7 cm and mean weight of 274.1 g. Survival was 46.4% and the average production obtained was 250.2 kg. At harvest, L. vannamei in monoculture system attained mean weight of 13.3 g and in the mixed culture system, average weight obtained for the shrimps was 13.5 g. Survival, feed conversion ratio (FCR) and production of L. vannamei from monoculture were 64.7%, 2.0 and 258.9 kg and from mixed culture 76.8%, 1.6 and 311.5 kg respectively. Daily weight increment and specific growth rate (SGR) of L. vannamei was 0.18 g and 4.06 for monoculture and 0.19 g and 4.01 for mixed culture, resepectively. Feed conversion, survival and production of L. vannamei were significantly (p<0.05) better in mixed culture, confirming technical superiority of mixed culture over monoculture.