Polydorid polychaeta as pests in cultivated oysters : a case study of polydorid infestation on a South African oyster farm, including a literature review and proposed best management practices for the reduction of polychaete infestation in the culture of molluscs

Brink, Jacobus Abraham (2006-04)

Assignment (MPhil)--University of Stellenbosch, 2006.

Thesis

ENGLISH ABSTRACT: Reports of a wonnlike "parasitic" infestation of pacific oysters, Crassostrea gi.gas combined with an increased mortality on a land based oyster farm on the west coast of South Africa prompted an investigation as to the identity of the infesting organisms, the extent of the infestation and the impact on the condition of the oysters. As the infesting organism was suspected to be a member of the polydorid complex of the spionid polychaeta, a literature study was conducted and the information gathered, compiled in the form of a review which also serves the purpose of a practical guide for aquaculturists who have to deal with polydorid infestations. The review summarizes information on the systematics, life history, impact and control of the group. The infesting organism was identified as a member of the spionid polychaeta and specifically the genus Polydora, a known pest in bivalve molluscs. It was found that the organism showed varied levels of infestation in the spatially and age segregated oyster population on the farm with the oldest animals the heaviest infected and also the possible source of the infestation as they were brought onto the farm from another oyster farm. The condition of all the animals in the different cohorts on the farm was good compared to international standards (condition indexes (Cl)> 10) ANOV A analysis of data could find no significant difference between the condition of infested and uninfested oysters and therefore it cannot be conclusively proved that the increased mortalities can be ascribed to the polydorid infestation. Although no single cause seemed to be implicated the mortalitie can probably ascribed to a combination of several extrinsic and intrinsic factors. The implementation of quality control systems such as HACCP on aquaculture farms is critical for successful cultivation. A set of Best Management Practices (bmp) for the culture of bivalve molluscs is proposed, with identification and special reference to the critical control points as required by HACCP. The current practices on the farm in question were also investigated and it was concluded that the absolute lack of management on the farm was the primary cause for the · problems experienced.

AFRIKAANSE OPSOMMING: Die aanmelding van 'n wurm-agtige parasitiese infeksie van die Stille Oseaan oester, Crassostrea gigas met gepaardgaande stygings in mortalitiete op 'n oesterplaas op die Weskus van Suid Afrika, het aanleiding gegee tot 'n studie om die identitiet van die organisme, die strekking van die infestasie en die moontlike impak van die infestasie op die kondisie van die oesters te bepaal. Dit is vermoed dat die organisme 'n lid is van die spionid Polychaeta. 'n Literatuur studie van die groep is ondemeem en die versamelde informasie word in die vorm van 'n oorsig aangebied. Die oorsig is so gestruktureer <lat <lit gebruik kan word as 'n praktiese handleiding vir oesterboere wat met die pes moet werk. Die oorsig behandel die sistematiek van die groep, die lewenspatrone, die impak van die organsime op oesters en die beheer van die organisme. Die organisme is geidentifiseer as 'n lid van die spionid polychaeta, en meer spesifiek 'n lid van die genus Polydora, 'n bekende pes van oesters. Daar is gevind <lat die organisme teen verskillende konsentrasies voorkom op die plaas en dat die konsentrasies korrelleer met die ouderdom van die oesters. Die hoogste konsentrasies is gevind in die oudste oesters op die plaas en die oesters is ook heel moontlik die oorsaak van die infestasie aangesien hulle van 'n antler oesterplaas oorgeplaas is. Die kondisie van al die oesters was goed in vergelyking met intemasionale standaarde (kondisie indeks > 10). ANOV A analiese van die data het geen beduidende veskil gevind tussen die kondisie van besmette en onbesmette oesters nie. Dit kan dus nie onteenseglik bewys word dat die mortaliteite toesgeskryf kan word aan die Polydora infestasie nie. Alhoewel geen enkele rede gevind kan word vir die mortaliteite nie is dit heel moonJik toeskryfbaar aan 'n kombinasie van intrinsieke en eksteme faktore. Die implementering van kwaliteitsbeheer sisteme soos HACCP op akwakultuur plase is van kardinale belang vir sukses. 'n Stel Beste Bestuurs Praktyke (BMP) vir die groei van oesters word voorgestel en spesifieke verwysing word gemaak na die kritiese beheer punte soos vereis deur HACCP. Die huidige bestuursisterne van die plaas word ook ondersoek en dit is bepaal <lat die totale gebrek aan effektiewe bestuur die hoof rede is vir probleme wat die plaas ondervind.

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