Control of the herbivorous isopod, Paridotea reticulata, in Gracilaria gracilis tank cultures

The feeding activity of macroalgal grazers such as amphipods and isopods may be responsible for decreased productivity of cultivated macroalgae. This could have potentially disastrous consequences to the well-being of the seaweed cultures, especially in land-based aquaculture systems where the natural predators of the herbivores are absent. Generally, losses due to herbivores grazing on seaweed cultures are small or even beneficial due to their preference for epiphytes on the main seaweed species, but it can be significant when the number of grazers is high. A large population of the herbivorous isopod Paridotea reticulata (approximately 616±116 isopods per 100 g dry seaweed), became established in outdoor Gracilaria gracilis tanks at a commercial abalone farm on the south coast of South Africa. It was found that a freshwater treatment could eliminate the isopods: when G. gracilis cultures containing isopods were placed in freshwater for 3 h, the isopods were killed with no lasting or significant damage to the seaweed resource.