Author: Ryan Benson, Matt Dowsett, Tim Grabski, Paul Geedes

Institution:  Institution:  Curtin University of Technology Western Australia 2001 

Curtin University of Technology, Muresk Institute of Agriculture,

Aquatic Research Unit, Suite 2 Technology Park, Bentley, Western Australia

The addition of shelter materials has been linked to increased growth rates in abalone production systems, but minimal research has been carried out on the preference of one substrate type over another exhibited by H. laevigata. Preliminary studies carried out by Hindrum (1998) found that there was an interaction between the addition of shelters at high stocking densities and improved growth rates. This study compared the growth rates of juvenile H. laevigata over three different substrate types that were introduced into circular grow out tanks. The three shelters were, prefabricated cement Besser® blocks, cut PVC pipes and plastic corrugated sheets.

A growth and survival experiment, evaluating the effects of substrate was conducted at the Curtin University of Technology, Aquatic Research Unit hatchery, Bentley, Western Australia. Juvenile greenlip abalone supplied by GREAT SOUTHERN HATCHERY, Albany, Western Australia, were trialed with three types of shelters and their mean growth rate determined from initial and final measures of dorsal-ventral length and wet weight. Survival percentage was then determined with a transformation of the data using the arcsine statistical method prior to analysis.

The 60-day trial began in July 2001, with a two-week acclimatisation period of 108 abalone (2.53 ± 0.37g in weight and 21.92 ± 1.53 mm in length in a hatchery shade house. The experiment then commenced with the random allocation of 12 abalone to each experimental tank. The nine, 320 litre circular tanks combined with a 720 litre sump, resulted in a recirculating system, having a total water volume of 3.6 tonnes. The tanks were aligned in a row along the northern wall of the shade house. Water entered through a spraybar, exiting via a 400mm in length, 40mm diameter stand pipe, recirculating through a biofilter and protein skimmer. Each tank was covered with shade cloth to reduce light and prevent the abalone from escaping. The abalone were fed artificial food, of a juvenile abalone pellet supplied by ADAM & AMOS ABALONE FOODS, Mount Barker, South Australia, at a rate of 2% of their initial body weight three times weekly. All uneaten food and faecal matter were cleaned from tanks prior to feeding.

Water temperature was maintained at 18 ± 2^oC throughout the experiment with the use of three, 330 watt bar heaters placed into the sump. Salinity was maintained at 35 ± 2ppt with readings taken daily. Fluctuations through evaporation were overcome with the calculated addition of aged freshwater. All other water quality parameters were also measured accordingly.

The three shelter consisted of PVC piping, corrugated plastic and prefabricated Besser® blocks randomly selected in replicates of three in a set line formation of nine tanks. The PVC piping, consisted of two 400mm in length x 90mm diameter pieces of pipe cut in half along their lengths placed inverted to create a shelter situation. The corrugated plastic sheeting was cut into two pieces, 400mm x 600mm each, having a radius of 15mm, placed flat along the tank bottom surface, creating a sheltered situation. Two prefabricated Besser® blocks measuring 400mm in length, 180mm in height and width, with two 180mm square holes evenly distributed along side length were placed along the tank bottom, with the holes to the side. (Figure 1)

The results after the 60 day period show the Besser® block treatment had a mean increase in dorsal-ventral shell length of 4.51 ± 0.14mm The mean dorsal-ventral length in the PVC piping treatment increased 2.8 ± 0.94mm and plastic corrugated sheeting provided a mean growth of 2.61 ± 0.28mm.

An increase in wet weight of 0.63 ± 0.11g was shown in the Besser® block treatment and mean wet weight grew 0.52 ± 0.37g with the PVC piping. Plastic corrugated sheeting provided a mean growth of 0.32 ± 0.10g in the wet weight. (Figure 3)

Survival at the end of the experiment showed that the abalone in the tanks with Besser® blocks had the highest mean survival at 75%, compared to the pipe mean survival at 69.44% and the plastic at 66.67%. (Figure 4)

A one-way ANOVA (P=0.05) was conducted on the final measurement data for the three variables and displayed P=0.13 for length, P=0.88 for weight and P=0.96 for the arcsine transformed survival data. As this shows, no one variable was significant in the results displayed for the shelters.

A previous investigation by Hindrum (1998) found that preliminary growth data indicated that at higher stocking densities, refuges could improve the growth and biomass production of greenlip abalone. Preliminary results in the experiment showed that one shelter was preferred out of three tested. Visual observations and growth data indicated that the greenlip abalone were most successful on the Besser® blocks. The introduction of the blocks provided the abalone with an environment most closely representing its natural conditions including crevices, shade, texture and increasing the surface area to volume ratio.

Juvenile greenlip abalone prefer to orientate towards cracks and crevices of reefs, owing to increased protection and shade, later migrating too outer reef surfaces when older and larger (Bell, 2001 pers com). The greenlip abalone were observed congregating in the corners of the Besser® block wall and under the blocks where a crevice had formed.

In the wild, most species of abalone are cryptic, meaning they hide away in shady areas (Fallu, 1991). The Besser® blocks provided the abalone with a dark environment in which to hide when not foraging at night.

Collection plates are used for the settlement of molluscs and alternately, used as a substratum for micro-algae cultures. Plates are seeded by conditioning them in the ocean and allowing micro-algae to settle and form the basis of a colony (Fallu, 1991). Although no conditioning occurred before the introduction of shelters it could be observed that the rough texture of the Besser® blocks formed by the limestone base provided an ideal substrate for micro-algae growth. The layer of developing micro-algae may have attracted the abalone to the surface of the blocks and provided them with a supplement feed.

Observations on tank design by Hone (1997) found that abalone within tanks were congregating. Covers and shading (shelters) improved the distribution within the tank and made more effective use of the horizontal surface area. The Besser® blocks provided an increased surface area to volume ratio that made more effective use of the tanks water column.

Although half PVC pipe and plastic corrugated sheeting provided the abalone with shade and an increased surface area within the tank in which to occupy, observations indicated that they tended to avoid these shelter types preferring to congregate around the edge of the tank and base of the standpipe. With no conditioning, the texture of the shelter surface was smooth and it was only towards the end of the 60 day trial, when a degree of conditioning occurred, that some attachment of abalone onto these shelters could be observed.

Although the conditioning effect has been a priority in the early stages of abalone culture it has been relatively unexplored in grow-out of juvenile abalone. This trial showed that abalone preferred to the occupy the rough, partially conditioned Besser® blocks rather than the smooth surfaces of PVC piping, corrugated plastic sheeting and internal tank. Their natural behavior to seek out a dark, rough substrate and crevices may have enhanced the well-being of the abalone and any micro-algae growth would have served as an extra source of nutrition. Therefore the Besser® blocks as a shelter or substrate may increase the level of energy utilised for growth by enhancing well-being and nutrition and further investigations into surface type may be warranted.