Alternative means of nursery
culture for commercial scallop (Pecten fumatus Reeve) spat
M.P. Heasman, W.A. O'Connor, A.W. Frazer, Y. Languet,
S.J. O'Connor-2002
Aquaculture, 213(1-4): 323-338
Abstract:
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 (l 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.
(NSW Fisheries, Port Stephens Fisheries Centre,
Taylors Beach, NSW 2316, Australia, e-mail: heasmanh@fisheries.nsw.gov.au)