2010/734 Oyster over-catch: Cold shock treatment

By Bob Cox

The recruitment of fouling or pest organisms to cultured oysters and growing infrastructure imposes a major financial impost for oyster culture throughout Australia and serves as a particular deterrent to industry expansion in certain regions. Oyster farmers have a range of management options such as mechanical cleaning, drying or cooking to control fouling, but each option typically has its limitations. Cold-shock, through immersion in chilled (-12 to -16°C) hypersaline (180 - 200 g l-1 NaCl) baths, is a comparatively new technique that has demonstrated the potential to effectively control a range of pest species without adverse effect on the host oysters. Most notably, hypersaline cold-shock can be used to control subsequent natural oyster settlement known as “over-catch”.

A commercial scale, hypersaline, cold-shock bath, dubbed the “Super Salty Slush Puppy” was constructed to provide proof of concept of cold shock treatment for over-catch control. The cold shock bath was deployed to Port Stephens NSW where it was successfully used for both experimental and commercial scale biofouling treatment

In experimental scale laboratory trials, the cold tolerances of various size classes of both Sydney Rock Oysters (Saccostrea glomerata), Pacific Oysters (Crassostrea gigas) were assessed and tolerance estimates were determined. Overall cold shock tolerance in both species was size-dependent with smaller individuals succumbing faster. Comparatively, S. glomerata of up to commercial size were less tolerant of hypersaline cold-shock than C. gigas. Operating guidelines for cold shock treatment were developed - a period of 75 seconds immersion revealed 100% destruction of fouling over-catch oysters with no detrimental effects on the stock oysters. The effectiveness of cold shock in treating a range of additional pest species (barnacles, hairy mussels, flatworms etc) was confirmed. Cold-shock was found to be particularly destructive to “soft-bodied” pests such as flat worms and smaller organisms such as barnacles.

Based upon simple assumptions, the operating cost of treatment is $1,215 per 100,000 of 60 to 80 mm [top shell measurement] oysters, or, put another way, a cost of $4,200 for 345,600 sale size oysters with a $1m sale value. Even when capital costs are included, and assuming that other mitigation measures see the equipment used only once every four years, the cost per dozen for treatment works out at 3.4 cents per dozen.