Improvement of rearing conditions for juvenile lobsters ( Homarus gammarus) by co-culturing with juvenile isopods ( Idotea emarginata)

Growth conditions of the juvenile lobsters, Homarus gammarus, were optimized in view of a restocking project of the lobster population at Helgoland (German Bight, North Sea) aimed to produce more than ten thousand juvenile lobsters per year. Growth and survival rates of juvenile lobsters depend on diet, temperature and water quality. In the present study, diet at optimum temperature was considered, but special emphasis was placed on the optimization of cleaning and feeding methods from both an economical and ecological point of view. Six dietary treatments of juvenile lobsters (each n = 99) were tested in individual compartments in a semi-closed re-circulation system at around 20 °C. Lobsters were fed with combinations of two diets, newly hatched Artemia sp. nauplii and minced crabs Cancer pagurus (whole carcasses), every 2 or 4 days until a carapace length of 10 mm was reached. During the experiment (max. 105 d), juvenile isopods, Idotea emarginata, were constantly present in the lobster boxes. More frequent feeding significantly increased growth rates of the juvenile lobsters while different feeding combinations had no effect. The highest growth rate (0.091 ± 0.02 mm CL day − 1 ) was at a feeding frequency of every 2 days for each diet. At this rate the carapace length of 10 mm was reached in 68–71 days. The survival rate of the juvenile lobsters ranged from 90–97%. The diet consisting of C. pagurus was most cost-efficient and was obtained as discards from the crab fishery at Helgoland. The co-culture of juvenile lobsters with juvenile isopods I. emarginata as “cleaning organisms” was ideally suited for the rearing of lobsters and reduced the maintenance time by 50%. The isopods also served as supplementary diet.