The Production of Scallop Spat (Pecten Maximus) in Floating Ponds
(Mesocosms)
Aquaflow Technical Leaflet 2002-39
European Network for the Dissemination of Aquaculture
RTD Information (Q5CA-2000-30105) and previously FAIR-3837, URL: http://www.aquaflow.org/
The annual landings of scallops in Ireland dropped
from 1,587 tonnes in 1990 to less than 500 tonnes (value: IR£900,000) in
1995. Scallop farming and the restocking of wild scallop beds, offer hope to
increase production, but one major obstacle is the reliable production of
scallop spat. The only significant Irish spat collection site, Mulroy Bay,
has declined in recent years and failed completely in 1996. Commercial
hatcheries have failed to produce spat consistently due to a number of
biological and commercial factors. Aims: To develop a low - tech methodology
for the reliable and cost effective production of scallop (Pecten maximus)
spat using a floating bag (or mesocosm) system.
Methodology: An array of eight floating bags (each 6 x
6 x 6 metres) were moored in Rabbit Island Sound, off Union Hall, West Cork.
They were constructed of either butyl rubber, plastic or cloth and filled
with 250,000 litres of seawater filtered to 50 microns. Conditioned adult
scallops were stimulated to spawn in the bags and larval concentrations were
adjusted to about 1 per 10ml by pumping between bags. Nutrients were added
in an attempt to maintain chlorophyll a levels at a maximum of 3 ug/l and/or
6 million microflagellate cells per litre. Water quality and larval
development were intensively monitored throughout the culture cycle.
Results: The plastic and butyl rubber bags proved
expensive and difficult to deploy whereas the cloth bags were not only
easier to handle, but also allowed a limited exchange of nutrients,
dissolved oxygen and phytoplankton with the outside water. Productivity was
improved by continually pumping new seawater into the bag via a suitable
filter and also by creating top to bottom circulation within the bag. Spat
settlement was achieved in three of the plastic bags - one of them was
significant at 500,000 spat. Spat settled on artificial collectors were
floated out of the bags and deployed on longlines in good current flow. This
reduced the usual loss of spat which occurs on transfer from hatchery to
nursery. Floating pond production of scallop spat will help reduce the
bottleneck caused by a reliance on unpredictable natural settlement and/or
inconsistent hatchery performance. The advantages of a cloth bag system are
that it is easy to deploy and, because it is semi-permeable (15 micron
mesh), it will fill itself with filtered seawater if suitably weighted at
each bottom corner. In areas where natural spawnings are unpredictable
and/or unsynchronised, the cloth bag system could be used to produce large
numbers of bivalve larvae. These would be released in situ immediately after
fertilisation to enhance natural settlement. By repeating this technique at
regular intervals throughout the spawning season the chances of hitting a
good window for larval survival and settlement will be increased.
* This project was supported under the Marine Research
Measure of the Operational Programme for Fisheries (1994-1999), administered
by the Marine Institute and part funded by the ERDF.
For information:
Dr Gavin Burnell
Aquaculture Development Centre
Dept of Zoology and Animal Ecology National University of Ireland Cork Lee
Maltings Prospect Row Cork, Ireland
Ireland
Tel: +353 21 4904745
Fax : +353 21 4270562
E-mail : g.burnell@ucc.ie