Difficulties in controlling settlement densities of hatchery-produced Pecten fumatus larvae on mesh-filled collector bags prompted an evaluation of two alternative nursery-rearing protocols. In both cases, larvae were initially settled and on-grown on mesh downweller screens in the hatchery. In the first protocol, spat were retained on conventional downweller screens until it is large enough to be transferred to a field nursery system comprising tiered stacks of mesh screens located in upwelling systems. Mean growth rates were greatest for spat held on the first (most upstream) of eight screens within each stack and exhibited a progressive decline in successive downstream screens at all densities tested. Daily rotation of screen position within stacks overcame differential spat growth and significantly increased overall mean biomass gains. Stocking density per unit surface area of screen is critical in determining growth rate of P. fumatus spat in field upwellers. Irrespective of other growth-limiting factors such as food, the upper limit of stocking rate at which maximum growth rate is maintained is approximately 70% screen coverage. Although only marginal reduction in growth occurs at stocking rates of up to 100%, screen stocking rates above 70% coverage are not recommended because of the problem of death and damage that occurs through inadvertent clamping at higher densities. Maximum growth rates recorded for P. fumatus spat held in a single tier upweller screen coincided with high flow (1 min-1) to biomass (kg) ratios of 20 to 30:1. Reuse of seawater via tiered upweller stacks enabled in the case of an eight-tiered stack up to a doubling of yield per unit of seawater flow. Optimised production efficiency would appear to coincide with use of about six to eight tiers per stack and daily alternation of screens from the top to bottom locations within the stacks. In the alternative nursery-rearing protocol, P. fumatus were settled on mesh screens using inexpensive nylon curtain material and retained in the hatchery for periods of 1-5 weeks post-settlement. The mesh from each screen was removed and was cut into sections with the desired number of spat attached. Each section was placed in a spat collector bag stuffed with conventional coarse plastic netting and was then deployed to the field. Subsequent survival of scallops after 30 days was found to depend on spat size at the time of deployment and upon handling methods used during deployment. The percentage of 500-750 μm spat (2-3 weeks post-settlement) recovered at a size of > 5.0 mm, suitable for transfer to grow-out facilities, was 25-30% and comparable with that achieved in tiered upweller nurseries. However, screen to collector bag transfer requires less capital outlay and is less labour-intensive than tiered upweller systems.
- Pecten fumatus