Culture of Panulirus ornatus lobster fed fish by-catch or co-cultured Perna viridis mussel in sea cages in Vietnam

Abstract

In order to evaluate the benefit of combining culture of mussel and lobster on lobster farming, a study was conducted at Van Phong Bay (near Xuan Tu village, Van Ninh district, Khanh Hoa province). Four culture sites, two offshore sites (sites 2 and 3) and two nearshore sites (sites 1 and 4), were selected for the study. At sites 2 and 4, strings of Perna viridis (green mussel) were hung around the lobster cages, whereas no mussels were cultured at sites 1 and 3. The lobsters were fed by-catch at all four sites, but three of the lobster cages at sites 3 and 4 were additionally fed mussel. The growth and survival of the lobsters were measured monthly. Lobsters were harvested after 15 months of culture at site 4 because they had attained an acceptable average harvest weight of about 900 g. At the other three sites, the experiment was terminated after 17 months of culture irrespective of the size of the lobsters so as to avoid further losses due to disease. At the end of the experiment, the average weight of the cultured lobsters fed only by-catch was 772 g or higher at sites 2, 3 and 4 and highest at site 4 (864 g); while the average weight of the lobsters at site 1 was 557 g and significantly lower than lobsters at all other sites (P<0.05). Carapace length (CL) of lobsters at site 1 was also significantly smaller than those at other sites. There were no significant differences in either average weight or CL for lobsters that were fed only by-catch at sites 2 and 3 (P>0.05). Comparing lobsters that were fed mussels as well as by-catch (sites 3 and 4), the mean weight of lobsters fed mussels at site 3 was significantly higher (P<0.05) than those fed only by-catch at that site, although no significant difference was seen for mean CL. Survival of lobsters was very high for the first 4 months, with no deaths being observed at any of the sites. However, after this time, lobster deaths occurred, initially due to black gill disease, and subsequently due to 'milky disease'. At site 4, survival was very high for lobsters fed mussels (99%) while cages with lobsters fed only by-catch exhibited losses as early as 9 months, with a total survival of 85% at the end of the trial at 15 months. After 17 months at the three other sites, mean percentage survival was 75, 73 and 79% for sites 1, 2 and 3, respectively, and not significantly different (P>0.05). Similar to site 4, lobsters at site 3 that were fed mussels and by-catch had a higher (P<0.05) survival rate (91%) than those fed only by-catch (67%). Despite the disease problems, the experiment demonstrated that lobsters fed mussels as a supplement to the fish by-catch grew and survived better than those fed only by-catch. There was a tendency for lobster performance to be better at offshore compared to nearshore sites.