Commercial scallop culture trials at the Pinnace Channel Aquaculture Fisheries Reserve
: Appendix 1 – Observations made by industry on the use of various culture systems for
scallop aquaculture. Pearl, Lantern and Box nets
Neil Hickman, John Mercer and Richard Gasior
Fouling of culture nets occurred year roundwith the heaviest settlement of foulingorganisms in the spring period. Observationsundertaken during the industry trial showedthat by the end of the intermediate and finalgrow-out periods, fouling on all the nets washigh (Figure 13). This was expected given thatthe nets had not been handled or cleaned for aperiod of 9 months. However the scallops in thefouled nets exhibited low mortality andcontinued to have good growth rates. On severaloccasions when the nets remained fouled, thedegree of fouling of the scallop shell remainedlow (Figure 13). This observation requiresfurther confirmation as it has importantmanagement implications for both cleaningregimes and minimising shell fouling.
Over the course of the trial there was a range offouling organisms, which settled on the nets,these included amphipods, hydroids, starfish,fan worms, fairy shell, hard and soft ascidianand spider crabs. Some of these organisms posespecific threats to scallop aquaculture. There arefour ways that fouling organisms can impact onthe culture operation:
- competition for food and space, which couldslow scallop growth and reduce meat yields,
- difficulty of removing some foulingorganisms form the nets at cleaning timewhich would increase labour costs,
- predation of the scallops causing a directloss of production, and
- direct fouling of scallop shells which wouldadversely impact the marketing of wholelive scallops.
The organisms which had the most adverse effect were the European fan worm Sabellaspallanzanii, the “fairy shell” Electroma georgiana.,and a range of hard (Styella clava) and soft (Cionaintestinalis) sea squirts. The fan worm S.spallanzanii can achieve a size of 25cm and onceattached to the nets and scallops they areextremely difficult to remove. Therefore it isimportant that once S. spallanzanii settles it isnot given sufficient time to achieve anysubstantial size before the nets are cleaned.Given its settlement period of winter /spring anet change in early summer appears to achievethis result. The “fairy shell” E georgiana whilstsettling in the thousands on the seed collectorbags and nets was relatively easy to removefrom the equipment and from a managementperspective able to be dealt with. However itsability to smother the nets and compete with scallops for food makes this species an ongoingconcern for any scallop operation. During thecourse of the trial a range of hard and softascidians settled on the nets and were difficult toremove (Figure 13). These ascidians grew intothe nets mesh making removal difficult andoften damaging the mesh in the process. As withthe fan worms it is important that once theascidians settle they are not given sufficient timeto achieve any substantial size before the netsare cleaned.
There were two fouling organisms that couldcause predation concerns. These were the exoticnorth pacific seastar A. amurensis and the spidercrab Leptomithrax gaimardii. The seastar settled inthe nets and by December attained a size of 10-35mm by feeding on net fouling organisms, notthe scallops. However this species has the abilityto grow very rapidly and would have been ableto predate on the scallops within a few months.The net change in December removed thesestarfish and they were not observed in the netsduring the remaining 9 months. The spider crabLeptomithrax gaimardii was found in the box andlantern nets in the latter half of the trial. Thecrab larvae would have settled some time afterDecember 2003 and were able to grow to a largesize over the following 9 months. These crabswhere observed in the nets in August 2004,where they had found sufficient feed to attain acarapace width of approximately 70mm. Thenets containing the crabs were free of significantfouling and only had a slight covering ofhydroids. The scallops in these nets were alsoparticularly clean. However some spider crabsthat had attained a large size had begun topredate on the scallops. These observations raisethe possibility of a biological cleaning optionusing juvenile spider crabs. Given theserendipitous settlement and cleaningundertaken by the crabs during this trial,combined with the abundance of spider crabs inPPB there is sufficient potential to warrantundertaking future research into thedevelopment of a biological control of fouling.
As scallops grow they create shell that is asuitable surface onto which many foulingorganisms are able to settle and grow. Thisfouling occurs to scallops on both the seabedand in culture nets. However fouling is usuallyless on the seabed due the scallops’ ability tocover its upper shell with silt and sand makingthe surface unattractive to fouling organisms.The settlement of these organisms can either killthe scallop or limit its ability to grow. Thesefouling organisms affect the scallops throughbinding the scallop to the net, competing forfood or growing into the shell matrix. Directfouling of the scallop’s shell occurredthroughout the trial. These fouling organismsincluded the fan worms Sabella spallanzanii, thewhite worm Galeolaria caespitosa , the fairy shellElectroma georgiana and a range of hard (Styellasp) and soft (Ciona sp) ascidians. Of theseorganisms, most could be removed from theshell with minimal effort, however removal ofthe white worm was difficult, time consumingand unlikely to be a viable option in acommercial operation. This white worm doesnot affect the quality of the meat or gonad butdoes detract from the scallop’s marketability,particularly if the animal is sold live in the shellor on the half shell. Experience within themussel industry shows that white worm onproduct will reduce its marketability.
Figure 13. Net fouling, a) pearl net used for intermediate grow-out after 9 months, b) box net used forfinal grow-out after 9 months, c) hard and soft ascidians remaining on net after cleaning, d) scallopsfrom fouled pearl net ~60mm, e) scallops from fouled box net ~80mm.
During the course of the trial there were anumber of times when the long-line sunk anumber of metres due to the increasing weightof fouling organisms. This resulted in the lowerpearl nets dragging on the seabed for variousperiods of time. Nets that were on the sea-bedfor a short time showed very limited fouling andin many cases little mortality of the scallops. Ifnets spent a considerable time on the bottom thescallops in the lower nets usually suffered ahigher mortality.
Whilst fouling of nets is regarded as a normalpart of any scallop operation, the techniques to
deal with this issue are limited. One technique,which could not be applied sufficiently in this
trial, but could be used in the future, was thedeployment of nets as deep and as close to the
seabed as possible. This has been shown inmany countries to have a significant effect on
reducing both the settlement and growth rate ofthe fouling organisms with out reducing the
growth rate of the scallop significantly.