Application of ammonium to enhance the growth of giant clams (Tridacna maxima) in the land-based nursery: effects of size class, stocking density andnutrient concentration

This study investigated the effects of ammonium (NH,I) enrichment and stock ing density on the growth of juvenile Tridacna maxima throughout the land-b ased nursery stage. In four sequential experiments, clams of 5, 11, 16 and 18 mm shell length (SL) were maintained at low and high stocking densities in 60-1 tanks with unfiltered seawater (8 l h(-1)) for 25 days. Throughout this period, clams received daily spikes of dissolved ammonium sulfate at c oncentrations of < 1 (control), 20, 35 and 50 mu M NH4+ for the three small est size classes of clams, and < 1, 35, 50 and 80 mu M NH4+ for the largest size class. Our experiments simulated the conditions used to mass-produce clams of this species during the nursery phase. All individuals used in the four experiments were from the same cohort. The growth responses of T. max ima to ammonium enrichment were dependent on the size of the clams. For cla ms of 5 mm SL, both increase in mean wet weight and SL were significantly l ower at 35 and 50 mu M NH4+ compared to control and 20 mu M NH4+ spikes. In contrast, increases in mean wet weight and SL of giant clams of 11, 16 and 18 mm SL were all significantly greater when ammonium was added, with the greatest increases in mean weight and SL occurring at the highest ammonium levels for the two largest size classes. The abundance of zooxanthellae per clam increased in response to addition of ammonium for clams of 11, 16 and 18 mm SL, but not for individuals of 5 mm SL. Stocking density had a varia ble effect on changes in mean weight, shell length and abundance of zooxant hellae. Values of all variables were significantly greater at low stocking densities for clams of 11 mm SL, but only marginally significant for most v ariables for clams of 16 and 18 mm SL. This study shows that addition of am monium does not enhance growth of T. maxima during the early stages of rear ing clams in the land-based nursery, but that relatively high levels of amm onium should be applied in the latter part of the nursery phase. This highl ights the importance of identifying changes in nutrient requirements as juv enile giant clams grow. (C) 1999 Elsevier Science B.V. All rights reserved.