Nursery culture of the abalone Haliotis laevigata: larval settlement and juvenile production using cultured algae or formulated feed

This study was conducted to investigate the settlement, growth, survival, and size variability of the abalone Haliotis laevigata on commercial scale. At settlement, two plate conditioning times for the green alga Ulvella lens (conditioned for 6 or 8 wk) were evaluated and compared with plates colonized by different diatom species (Navicula cf. jeffreyi and Cocconeis sp.). In a choice experiment an overall settlement rate of 87% was estimated 3 days after larval release. The majority of the larvae chose to settle on U. lens (61% on 8-wk-old and 14% on 6-wk-old U. lens, with average algal cover of 97% and 82% respectively). Larvae showed a clear preference for older than for younger U. lens, with similar percentage cover, indicating that the developmental stage of the alga and not percentage cover per se is important in settlement induction. Only 7% and 5% respectively of the settled larvae were found on the plates colonized by the diatom species N. cf. jeffreyi and Cocconeis sp. Results clearly demonstrate that U. lens provides a suitable substrate to improve the settlement of Haliotis laevigata larvae on commercial scale. Juvenile growth ([micro]m [day.sup.-1]) on a mixed diet of U. lens and N. cf. jeffreyi was not constant and increased with size to 70-80 [micro]m [day.sup.-1]. Both growth rate and size variability increased over time until juveniles reached approximately 5 mm in shell length. The survival declined to ca. 30% at 44 days after settlement but stabilized thereafter. Distinct increases in size variability were observed between 3 and 16 and between 30 and 44 days after settlement, followed by elevated growth rates and a change from monomodal to multimodal distribution at day 44. The latter coincided with a shift in diet from a diatom-dominated diet to a macroalgal diet of U. lens at day 44. The increase in growth rate between 3 and 16 days is believed to reflect a shift in nutrition with postlarvae switching from absorbing yolk reserves to efficient exogenous feeding. The growth, survival, and size variability of juveniles was assessed when feeding on algal diets as well as a formulated diet. Juveniles grew faster on U. lens than on a formulated feed or plates colonized with the diatom N. cf. jeffreyi. Results indicate that growth of juveniles may be more variable when feeding on natural feed (Navicula cf. jeffreyi., U. lens) than on a formulated teed. However, until a feed is formulated that can match the mean growth rates achieved on U. lens, we suggest keeping juveniles on plates colonized with U. lens as long as possible. Juvenile H. laevigata should not be weaned onto formulated feed until they reach at least 5 mm in shell length.

KEY WORDS: abalone, algae, diatoms, growth, size variability, settlement, Ulvella lens


INTRODUCTION

Australia has one of the world's largest abalone fisheries and its abalone aquaculture industry is expanding (Fleming 2000). With wild abalone fisheries declining in other countries, the interest in abalone aquaculture has increased substantially.

Since the inception of abalone aquaculture in Australia, research has focused on achieving the best growth rates possible in growout systems. This is because any increase in growth rates reduces production time and will usually have large cost benefits for an abalone farm. Recently it has been shown that differences in early growth of the abalone Haliotis rubra Leach persisted and were amplified by the end of a 4-too growth trial (Daume et al. 2004), suggesting that early growth is important in determining later performance. Despite this result and the consequent benefits to the abalone aquaculturist, very little emphasis has been placed on improving growth rates of juvenile abalone.

Laboratory experiments have shown that the macroalgae Ulvella lens Crouch is a suitable settlement inducer for larvae of the abalone Haliotis rubra Leach and H. laevigata Donovan (Daume et al. 2000). These findings were recently verified in the nursery for H. rubra (Daume et al. 2004). However, the growth rates of postlarvae (up to 3 mm in shell length) feeding on U. lens have been less promising and significantly better growth was achieved with benthic diatom species (Daume et al. 2000).

Formulated feeds have long been recognized to have beneficial effects for the growth and survival of juvenile abalone (Fleming et al. 1996). Experiments conducted in Australia showed that juvenile abalone can achieve maximum growth rates of up to 53 [micro]m [day.sup.-1], and 90 [micro]m [day.sup.-1] for animals ranging between 3 and 18 mm and 7 and 20 mm in shell lengths respectively when feeding on an unspecified formulated diet (Table 6 in Fleming et al. 1996). There are several studies on growth and survival of abalone juveniles when feeding on different algal species and growth rates of 60-100 [micro]m [day.sup.-1] can be expected for juveniles at 3-4 mm in shell length (see review by Kawamura et al. 1998a). Some studies compared formulated diets to an algal diet. For example higher growth-rates were achieved with juvenile abalone (H. fulgens), of approximately 13 mm in shell length, feeding on a formulated diet compared with juveniles feeding on macroalgae (Viana et al. 1993). Corazani and Illanes (1998) showed a similar trend with larger abalone of H. rufescens (~21 mm in shell length). Knauer et al. (1996) found no significant difference in growth rates of Haliotis midae juveniles (3-11 mm in shell length) when comparing a diatom diet (50 [micro]m [day.sup.-1]) and a formulated diet (59 [micro]m [day.sup.-1]). However the species composition and density of diatom species was not given in this study. Results of these studies indicate that formulated diets may support similar or better growth in larger juveniles, however abalone farmers still have to rely on algae as a settlement inducer and as a food source for recently settled postlarvae. Significantly better growth rates on an algal diet are expected for postlarvae and young juveniles.

by Sabine Daume, Stephen Ryan
Refer: http://findarticles.com/p/articles/mi_m0QPU/is_4_23/ai_n13682081