NURSERY PRODUCTION TECHNOLOGY FOR PACIFIC THREADFIN (POLYDACTYLUS-SEXFILIS)

The effect of various rearing tank designs and diet on the occurrence of cannibalism and overall survival and growth of postlarval Pacific t hreadfin (Polydactylus sexfilis) was examined in a series of large-sca le rearing trials at The Oceanic Institute (OI) and the State of Hawai i's Anuenue Fisheries Research Center (AFRC),Oahu, Hawaii. Twenty-six day old (D26) postlarval threadfin were stocked in either 5 m(3) or 9. 5 m(3) round tanks, 8.5 m(3) doughnut-shaped circular tanks, or 1.2 m( 3) shallow, oval raceways at densities from 0.5-4.4 fish l(-1). The fi sh were fed either commercial (RAN, MCM) or experimental (RNB, OIM) fe ed to satiation until D40 (Nursery I). Nursery II (D40 - D61) trials w ere conducted at AFRC in either 9.5 or 17.0 m(3) round or 8.5 or 15.0 m(3) doughnut tanks using RNB, OIM or MCM diets. Nursery I survival ra tes ranged from 40-97%, and were highly dependent on diet and rearing system. Cannibalism rates were inversely correlated with survival rate s. Overall survival of fish was highest when fed OIM (88 +/- 7%), comp ared to those fed MCM (74 +/- 12%), RAN (63 +/- 15%), or RNB (57 +/- 5 %). Survival was also best when fish were raised in oval raceways (86 +/- 8%), rather than in doughnut (70 +/- 4%) or round (65 +/- 13%) tan ks. Use of oval raceways also minimized the effects of poor diet perfo rmance on both survival and growth. Overall growth and feed conversion ratio (FCR) was best on the OIM feed. In contrast, survival during Nu rsery II was independent of rearing system and diet. Overall survival averaged 91 +/- 5%, and cannibalism was commensurately low (6 +/- 5%). Fish grew an average 10.6 +/- 0.5% body weight daily, at an FCR of 0. 9 +/- 0.1. Shortened and hared operculae were common among postlarval threadfin. Deformities increased from Nursery I to Nursery II. However , there was no correlation with diet during either stage. Results indi cated that mortalities due to cannibalism are most prevalent during th e Nursery I stage, but can be greatly reduced with the proper diet and rearing system. Given optimum conditions, survival rates of 90% or gr eater can be achieved in Nursery I and II stages, Growth rates in exce ss of 20% body weight daily (bwd) in Nursery I, and 10% bwd in Nursery II can be obtained at FCR approximating unity on pelleted feeds. Larg e-scale, intensive production of juvenile threadfin can be achieved.