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 n itrogen levels was studied. The seaweed was cultured at two levels of ammon ia-N enrichment. Cultures receiving 0.5 g ammonia-N m(-2) d(-1) ("low-N") y ielded 164 g m(-2) d(-1) of fresh thalli containing 12% crude protein in dr y 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% pr otein and 16 kJ g(-1) energy. High-Nand low-N algae and a "standard" mixed diet of 75% U. lactuca and 25% Gracilaria conferta (w/w) containing 33% pro tein 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 tri al. Voluntary feed intake of the high-N and standard diets were significant ly lower than the low-N diet in all the cases. Clear differences in perform ance between treatments were found in the juvenile and adult abalone of bot h species. Juveniles fed high-N and standard diets grew significantly faste r (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% d ay(-1) on high- and low-N algae, respectively) and showed much better food conversion ratios. The nutritional value of Ulva lactuca to abalone is grea tly improved by a high protein content, attainable by culturing the seaweed with high supply rates of ammonia.