Protein content determines the nutritional value of the seaweed Ulva lactuca L for the abalone Haliotis tuberculata L. and H. Discus hannai Ino

The nutritional value to abalone of Ulva lactuca L. with different tissue nitrogen levels was studied. The Seaweed was cultured at two levels of ammonia-N enrichment. Cultures receiving 0.5 g ammonia-N m^-2 d^-1 ('low-N') yielded 164 g m^-2 d^-1 of fresh thalli containing 12% crude protein in dry matter and 12 kJ g^-1 energy, cultures receiving 10 g ammonia-N m^-2 d^-1 ('high-N') produced 105 g of fresh thalli m^-2 d^-1 containing 44% protein and 16 kJ g^-1 energy. High-N and low-N algae and a 'standard' mixed diet of 75% U. lactuca and 25% Gracilaria conferta (w/w) containing 33% protein and 15 kJ g^-1 energy were fed to juvenile (0.7-2.1 g) and adult (6.9-19.6-g) Haliotis tuberculata and H. discus hannai in a 16-week feeding trial. Voluntary feed intake of the high-N and standard diets were significantly lower than the low-N diet in all the cases. Clear differences in performance between treatments were found in the juvenile and adult abalone of both species. Juveniles fed high-N and standard diets grew significantly faster (specific growth rate of H. tuberculata was 1.03% day^-1 on high-N algae as compared to 0.72% on low-N algae; H. discus hannai grew 0.63 and 0.3% day^-1 on high- and low-N algae, respectively) and showed much better food conversion ratios. The nutritional value of Ulva lactuca to abalone is greatly improved by a high protein content, attainable by culturing the seaweed with high supply rates of ammonia.