Summary

Objectives

1. To extend hatchery and nursery rearing techniques and equipment developed in an earlier grant (FRDC 91/53) to the production of triploid P. fumatus.

2. To compare the relative quality and production of diploid and triploid scallops in hanging culture and for reseeding enhancement of the Jervis Bay fishery.

3. To evaluate alternative methods of rearing scallops to a harvestable size using:

• Direct adhesion to tapes etc.
• Intermediate rearing in pearl cages and lantern cages and thence to bottom seeding, ear-hanging or second stage lantern cages.
• Intermediate bottom rearing of seeded scallops in predator proof cages.

Non Technical Summary:

This is a report on the second stage of a two part program investigating hatchery production, farming and seeding of the commercial scallop, Pecten fumatus. A total of more than four million scallop spat were produced for farming trials and over 250 000 scallops ranging in size from 20-60 mm were released into Jervis Bay.

Hatchery techniques were broadened to include procedures for the production of triploid P. fumatus. In general, the survival of triploid scallops was considerably lower than that of normal (diploid) scallops, particularly, immediately after treatment and during early larval rearing. No differences were observed between triploid and diploid spat, however, juvenile triploid scallops tended to be larger and heavier with significantly larger muscle tissues. The significant reduction in embryo development percentages (>80%), the relatively low percentage triploidy achieved initially (41%) followed by consistent reductions in percentage triploidy, mean that further research is required before it would be applied to the commercial sector.

Although our previous research had, for the first time, demonstrated reliable large scale rearing of P. fumatus larvae was possible, several major constraints to hatchery production were noted. Previously, larvae were either retained in the hatchery until they were large enough to be transferred to land based upweller systems, or larvae were settled on mesh in collector bags and transferred directly to the field. A new technique was developed in which larvae were settled on mesh screens held in the hatchery for about a fortnight before being placed in mesh bags at known densities and moved to the field for culture. This technique allows control of spat stocking density, and has produced threefold increases in spat yields over previous bag settlement techniques while significantly reducing maintenance costs.

The bulk of hatchery produced scallop spat were reared in Jervis Bay using pearl and lantern cage techniques similar to those used overseas. While excellent growth has ensued, extensive trials were undertaken to evaluate alternative techniques for culture. In one of the most promising modifications, scallops were glued to plastic mesh disks which produced growth and survival equal to or greater than that of similar sized scallops cultured using the alternative techniques. Disk spacing was found to affect levels of predation, while the valve by which the scallops were glued was found to affect survival and meat weights, especially in the gonad and adductor muscle. Disk culture was also found to affect mudworm infestation of the shell when scallops were glued in particular orientations.

Preliminary seeding trials were undertaken to establish protocols for subsequent seeding attempts. Notably, several aspects of diver survey accuracy were assessed. The ability of P. fumatus to avoid detection was greater than had been expected, with the best of the trialed survey techniques (10 m long, 1 m wide transects) underestimating scallop densities by approximately 17.5%. Diver experience in scallop collection was not a factor in survey accuracy, however, experienced divers did perform surveys significantly faster. Subsamples of the seeded scallops were placed in cages and deployed at the time of seeding, which proved useful in estimating handling mortality and subsequent predation of the seeded scallops.

Attempts to seed scallops in Jervis Bay using thousands to tens of thousands of hatchery produced juvenile scallops were largely unsuccessful due mainly to predation. On one occasion, major apparent losses were attributable to dispersion from the seeded area over a period of 11 weeks, although more commonly all scallops were depleted within 6 weeks. There were no consistent affects from bottom type on scallop survival and seeding areas which had existing populations of wild scallops offered no apparent advantage. In these cases, populations of larger wild scallops remained unaffected while naive seeded scallops were depleted rapidly. Size dependent losses were evident in most seedings with larger scallops (35-65 mm) often showing longer survival times.

This report describes the various predators and parasites encountered at several sites within three estuaries in NSW. While crabs, rays starfish and octopus were the among the most important predatory species, the impacts of other species are discussed. An ancillary finding of this study is that deployment of small naive scallops under protective plastic mesh developed by the horticulture industry limited losses to less than 1% per week. Research could, therefore, be extended using equivalent biodegradable mesh canopies to protect seed scallops until they lose their naivety and attain a size of about 65 mm, beyond which they are vulnerable to few predators.

The potential ecological impact of aquaculture and the paucity of information with respect to farming practices in NSW prompted an attempt to measure the accumulation of organic material beneath experimental longlines in Jervis Bay. Although the line increased the amount of organic material in the area, this increase appeared to remain within the assimilative capacity of the environment as no significant increases in organic material could be found in sediment samples taken in or around the site. Subsequent sampling, six months after the removal of the longline, found significant increases in organic loads in the sediment compared to earlier work. While this increase was a product natural processes within the bay, it gives some perspective to the scale of disturbance induced by experimental farming procedures.

Natural settlement of P. fumatus spat in Jervis Bay was monitored, which supported previous work (Fuentes et al., 1989) that despite several spawning events, significant recruitment occurred only in late winter to early spring. The intensity of this recruitment was also similar to that reported earlier and remained far too low for commercial farming purposes. This confirmed the need for a commercial hatchery if P. fumatus farming is to occur in NSW.

The research presented in this report has contributed to a continual process of improvement to the hatchery techniques developed in a previous FRDC grant (91/053). Specifically, triploidy induction, chemical induction of settlement, settlement substrates, spat transport and handling techniques have all been improved. These production techniques have been documented in a separate manual to be published in conjunction with the FRDC concurrent with this report (Project 94/084, Vol II).