In most commercial abalone nurseries worldwide, algal biomass grown on vertical plates becomes inadequate once juvenile abalone reach about 5 mm in shell length. At that stage animals need to be moved into a different tank system and weaned onto a formulated feed or alternative algal diets consisting of diatoms or macroalgae that can provide more biomass for the growing juveniles. Two trials were conducted to compare the growth and survival of juvenile greenlip abalone (5.9 ± 0.6 and 7.1 ± 0.1 mm in initial shell length) in two rearing systems: (1) shallower tanks with or without horizontal shelter, feeding a commercial formulated feed on horizontal surfaces; (2) deeper tanks with plate system feeding algal diets. The second experiment included two algal diets and two abalone stocking densities for one of the algal diets. The first experiment revealed that the algal diet consisting of Ulvella lens produced the best growth rates during the first 4 wk of the experiment, when juveniles reached about 9 mm in shell length and seawater temperatures averaged 20.1°C. Growth rates on the Ulvella diet declined rapidly thereafter, whereas growth rates on formulated feeds increased, coinciding with a decrease in seawater temperature to 15.2°C at the end of the experiment. The formulated feed produced high growth rates (over 75 μm day−1) once juveniles reached 7 mm in shell length. In the second experiment, the algal diet that included germlings of the green alga Ulva sp. produced the highest growth rates (105 μm day−1), indicating that Ulva sp. germlings are a suitable additional food source for juvenile greenlip abalone. Growth rates were particularly high towards the end of the experiment when seawater temperatures were above 19°C. Juveniles stocked at higher density (80 juveniles per plate) feeding on one of the algal diets showed reduced growth rates and overall lower average weight gain per individual compared with the juveniles stocked at low density and juveniles in the tank system feeding on formulated diet. In both experiments the survival was higher on the algal diets (77% to 82%) than on the formulated feed (62% to 65%). Culturing and maintaining algae for larger juveniles in the nursery system involved more labor, compared with the tank systems feeding formulated feed, suggesting that in higher labor cost countries, juveniles should be weaned onto a formulated feed as early as possible. We recommend moving animals into a tank system to feed formulated feed once they reach 7 mm in shell length. The weaning process can be delayed (e.g., to 17 mm shell length) by supplementing with macroalgal germlings if the seawater temperature is high and high mortality is expected when weaning onto formulated diets. The algal diet including Ulva sp. germlings produced particularly high growth rates when seawater temperature was above 19°C.
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Vol. 26 • No. 3