consultancy on routes for exposure of aquatic animals to aquatic animal products intended for ...

229 Pages·1999·0.56 MB·English

by  Farleigh Richard

FINAL REPORT - AQIS CONSULTANCY “ROUTES FOR EXPOSURE OF AQUATIC ANIMALS TO AQUATIC ANIMAL
PRODUCTS INTENDED FOR HUMAN CONSUMPTION”
FINAL REPORT
CONSULTANCY ON ROUTES FOR EXPOSURE OF AQUATIC
ANIMALS TO AQUATIC ANIMAL PRODUCTS INTENDED
FOR HUMAN CONSUMPTION
MAY 1999
Prepared for the Australian Quarantine and Inspection Service
by
Aquaculture Development and Veterinary Services Pty. Ltd.
329 Allens Rivulet Road, Allens Rivulet, TASMANIA 7150
in association with:
Food Factotum
Food Microbiology Research Group, University of Tasmania
Australian Water Technologies Pty. Ltd.
Enviromental Biotek Pty. Ltd.
Water Ecoscience Pty. Ltd.
River and Sea Pty. Ltd.
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FINAL REPORT - AQIS CONSULTANCY “ROUTES FOR EXPOSURE OF AQUATIC ANIMALS TO AQUATIC ANIMAL
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AusVet Animal Health Services Pty. Ltd.
Dr. Richard Callinan
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TABLE OF CONTENTS
1. EXECUTIVE SUMMARY..................................................................................................................................................8
2. INTRODUCTION............................................................................................................................................................13
2.1 BACKGROUND.............................................................................................................................................................13
2.2 TERMS OF REFERENCE..............................................................................................................................................17
2.3 PROJECT PERSONNEL................................................................................................................................................17
2.4 METHODOLOGY.........................................................................................................................................................18
2.5 GLOSSARY OF TERMS.................................................................................................................................................20
2.6 ABBREVIATIONS........................................................................................................................................................22
2.7 ACKNOWLEDGMENTS...............................................................................................................................................24
3. PATTERNS OF DISTRIBUTION, USAGE AND CONSUMPTION OF SEAFOOD IN AUSTRALIA.............25
3.1 GENERAL.....................................................................................................................................................................25
3.1.1 Australian Seafood Production/Exports/Imports......................................................................................25
3.1.2 Seafood Consumption in Australia..............................................................................................................25
3.2 MAJOR AUSTRALIAN FISHERIES.............................................................................................................................27
3.2.1 Rock Lobster.....................................................................................................................................................28
3.2.2 Abalone.............................................................................................................................................................28
3.2.3 Tuna...................................................................................................................................................................30
3.2.4 Scallops.............................................................................................................................................................32
3.2.5 Pilchards...........................................................................................................................................................32
3.2.6 Shark..................................................................................................................................................................33
3.2.7 Crabs..................................................................................................................................................................33
3.2.8 South East Trawl.............................................................................................................................................33
3.2.9 Other Fisheries.................................................................................................................................................34
3.3 FURTHER SEAFOOD PROCESSING............................................................................................................................34
3.3.1 Canning.............................................................................................................................................................34
3.3.2 Coated Fish Products.....................................................................................................................................34
3.3.3 Surimi.................................................................................................................................................................35
3.3.4 Smoked Fish, Marinades and Other Preserves..........................................................................................35
3.4 IMPORTED SEAFOOD................................................................................................................................................37
3.5 SEASONALITY OF SEAFOOD CONSUMPTION.......................................................................................................37
3.6 BY-PRODUCT UTILISATION.....................................................................................................................................38
3.7 SEAFOOD PACKAGING...............................................................................................................................................40
3.7.1 Bulk Fish Packaging......................................................................................................................................40
3.7.2 Pre-packaged seafood....................................................................................................................................41
4. COMMERCIAL AND RECREATIONAL BAIT.........................................................................................................42
4.1 INTRODUCTION..........................................................................................................................................................42
4.1.1 Recreational Angling.....................................................................................................................................42
4.1.2 Commercial Fishing........................................................................................................................................43
4.1.3 Bait and Berley................................................................................................................................................43
4.1.4 Trends................................................................................................................................................................45
4.2 THE ROLE OF BAIT AND BERLEY IN THE POTENTIAL EXPOSURE OF AQUATIC ANIMALS TO AQUATIC
ANIMAL PRODUCTS INTENDED FOR HUMAN CONSUMPTION................................................................................45
4.2.1 General..............................................................................................................................................................45
4.2.2 Seafood Products Used as Bait.....................................................................................................................47
4.3 VOLUMES AND PATHWAYS......................................................................................................................................50
5. WASTE DISPOSAL PATHWAYS..............................................................................................................................55
5.1 INTRODUCTION..........................................................................................................................................................55
5.2 MUNICIPAL WASTE WATER...................................................................................................................................55
5.2.1 Introduction......................................................................................................................................................55
5.2.2 Primary Treatment...........................................................................................................................................58
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5.2.3 Secondary Treatment......................................................................................................................................58
5.2.4 Tertiary Treatment...........................................................................................................................................60
5.2.5 Sludge Stabilisation and Beneficial Reuse................................................................................................62
5.2.6 Effluent Reuse...................................................................................................................................................64
5.2.7 Wet Weather Flows Affecting Treatment and Disinfection.......................................................................65
5.3 INDUSTRIAL AND COMMERCIAL WASTEWATER TREATMENT........................................................................67
5.4 SOLID WASTE MANAGEMENT................................................................................................................................69
5.4.1 General..............................................................................................................................................................69
5.4.2 Landfill..............................................................................................................................................................70
5.4.3 Pathogens in Landfill Leachate....................................................................................................................71
5.4.4 Composting and Beneficial Reuse................................................................................................................72
5.5 DISPOSAL OF WASTE OUTSIDE OF TRADITIONAL DISPOSAL ROUTES............................................................72
6. THE ROLE OF BIRDS IN THE TRANSMISSION OF PATHOGENS FROM SEAFOOD WASTE TO
AQUATIC ANIMALS.........................................................................................................................................................74
6.1 INTRODUCTION..........................................................................................................................................................74
6.2 HOW READILY DO BIRDS BECOME CONTAMINATED WITH DISEASE AGENTS................................................74
6.3 THE EFFECT OF BIRD MOVEMENT ON DISEASE TRANSMISSION........................................................................76
6.4 THE VIABILITY OF DISEASE PATHOGENS DURING TRANSIT.............................................................................77
7. FACTORS AFFECTING PATHOGEN SURVIVAL THROUGH PROCESSING, HANDLING AND
CONSUMPTION PATHWAYS........................................................................................................................................79
7.1 INTRODUCTION..........................................................................................................................................................79
7.2 THE ECOLOGY AND PHYSIOLOGY OF MICROBES IN FOODS...............................................................................81
7.2.1 Introduction......................................................................................................................................................81
7.2.2 Microbial Death..............................................................................................................................................81
7.2.3 Microbial Stability and Mechanisms of Inactivation...............................................................................83
7.2.3.1 Mechanisms of Inactivation...........................................................................................................................84
7.2.3.2 Temperature...................................................................................................................................................84
7.2.3.3 Freezing/Drying/Reduced Water Activity/Increased Osmotic Potential.......................................................86
7.2.3.4 pH and organic acids......................................................................................................................................87
7.2.3.5 Phosphates.....................................................................................................................................................88
7.2.3.6 Sulphites........................................................................................................................................................88
7.2.4 Relative Effects and Combinations of Factors...........................................................................................89
7.2.5 Microbial survival and limits to growth.....................................................................................................91
7.2.6 Microbial Growth............................................................................................................................................92
7.2.7 Rate of growth..................................................................................................................................................94
7.2.7.1 Temperature...................................................................................................................................................94
7.2.7.2 Water Activity...............................................................................................................................................95
7.2.7.3 pH and Organic Acids....................................................................................................................................97
7.2.7.4 Combined Effects on Growth Rate................................................................................................................98
7.3 SITES OF INFECTION, UNAPPARENT INFECTIONS, AND POTENTIAL FOR CROSS-CONTAMINATION........99
7.3.1 Sites of Infection...............................................................................................................................................99
7.3.2 Unapparent Infection....................................................................................................................................100
7.3.3 Cross-contamination, Transient Carriage...............................................................................................100
7.4 THE PHYSICO-CHEMICAL CHARACTERISTICS OF FOOD PROCESSES...............................................................101
7.4.1 Temperature....................................................................................................................................................101
7.4.1.1 Cooking........................................................................................................................................................101
7.4.1.2 Canning........................................................................................................................................................102
7.4.1.3 Chilling and Freezing....................................................................................................................................102
7.4.2 Brining and Salting......................................................................................................................................103
7.4.3 Smoking...........................................................................................................................................................103
7.4.5 Fermented Fish, Fish sauces etc................................................................................................................106
7.4.6 Surimi, Sushi and Minced Fish Products..................................................................................................106
7.4.7 Fish Protein Concentrates...........................................................................................................................106
7.4.8 Caviar..............................................................................................................................................................106
7.4.9 Modified Atmosphere or Vacuum Packaging..........................................................................................107
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7.5 TOLERANCE OF AQUATIC ANIMAL PATHOGENS TO FOOD HANDLING AND PROCESSING CONDITIONS
..........................................................................................................................................................................................108
7.5.1 Rates of Inactivation.....................................................................................................................................108
7.5.2 General Physiological Factors and Extent of Resistance.....................................................................108
7.5.3 Viruses.............................................................................................................................................................109
7.5.3.1 Temperature.................................................................................................................................................109
7.5.3.2 pH................................................................................................................................................................111
7.5.3.3 Chemical and Other Disinfectants...............................................................................................................111
7.5.4 Bacteria...........................................................................................................................................................112
7.5.4.1 Temperature.................................................................................................................................................112
7.5.4.2 pH................................................................................................................................................................114
7.5.4.3 Chemical and Other Disinfectants and Processes........................................................................................115
7.5.5 Other Aquatic Animal Pathogens...............................................................................................................115
7.5.5.1 Salt...............................................................................................................................................................116
7.5.5.2 Freezing........................................................................................................................................................116
7.6 PRODUCTS THAT ARE EATEN RAW, AND EFFECTS OF PASSAGE THROUGH THE HUMAN
GASTROINTESTINAL TRACT........................................................................................................................................117
7.6.1 The human gastro-intestinal tract..............................................................................................................117
7.6.2 Survival of microorganisms through the GIT...........................................................................................118
7.6.3 Potential for increased risk by passage through the gastro-intestinal tract.....................................119
7.7 SPOILAGE AND DISPOSAL OF SPOILED PRODUCT AND WASTE.....................................................................120
8. FACTORS AFFECTING SURVIVAL OF PATHOGENS IN THE AQUATIC ENVIRONMENT................123
8.1 INTRODUCTION........................................................................................................................................................123
8.2 GROWTH....................................................................................................................................................................123
8.2.1 Bacteria...........................................................................................................................................................123
8.2.2 Viruses.............................................................................................................................................................124
8.3 SURVIVAL MECHANISMS........................................................................................................................................124
8.3.1 Starvation-survival.......................................................................................................................................124
8.3.2 Viable But Non-Culturable Phenomenon (VBNC).................................................................................125
8.3.3 Attachment to Surfaces and Biofilms.........................................................................................................126
8.3.4 Ingestion by Protozoans and Crustaceans...............................................................................................126
8.3.5 Protozoans: Intermediate Hosts..................................................................................................................127
8.4 DEATH:— CAUSES...................................................................................................................................................127
8.4.1 Bacteria...........................................................................................................................................................127
8.5 GENE TRANSFER.......................................................................................................................................................128
9. ASSESSMENT OF THE RELATIVE RISKS POSED BY VARIOUS POTENTIAL PATHWAYS WHEREBY
IMPORTED PRODUCT INTENDED FOR HUMAN CONSUMPTON MAY ENTER THE AQUATIC
ENVIRONMENT................................................................................................................................................................130
10. CASE STUDIES...........................................................................................................................................................139
10.1 INTRODUCTION......................................................................................................................................................139
10.2 CASE STUDY 1 - THE USE OF FRESH, FROZEN OR CHILLED ATLANTIC SALMON IN THE HUON &
DERWENT ESTUARINE REGION OF TASMANIA........................................................................................................140
10.2.1 Salmonid Production in Australia...........................................................................................................140
10.2.2 Imported Salmonids....................................................................................................................................140
10.2.3 Location of Case Study..............................................................................................................................141
10.2.4 Patterns of Distribution, Usage and Consumption of Salmonid Products in the Derwent/Huon
Estuarine Region of Tasmania..............................................................................................................................141
10.2.4.1 Commercial Salmonid Fishery...................................................................................................................141
10.2.4.2 Salmonid Aquaculture................................................................................................................................141
10.2.4.3 Processing and Distribution.......................................................................................................................142
10.2.4.4 Packaging....................................................................................................................................................144
10.2.4.5 Use of Salmonid Product as Bait...............................................................................................................146
10.2.5 Recreation and Tourism.............................................................................................................................147
10.2.6 Wild salmonid populations.......................................................................................................................147
10.2.7 Waste Disposal Pathways..........................................................................................................................147
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10.2.7.1 Wastewater Treatment...............................................................................................................................147
10.2.7.2 Municipal Wastewater Treatment.............................................................................................................147
10.2.7.3 Industrial and Commercial Wastewater Treatment....................................................................................148
10.2.7.4 Solid Waste Management...........................................................................................................................149
10.2.8 Examination of the Potential for Exotic Pathogens in Imported Salmonid Products to Enter the
Aquatic Environment in the Derwent/Huon Estuary Region..........................................................................158
10.3 CASE STUDY 2 - THE USE OF GREEN PRAWNS IN THE MORETON BAY REGION OF QUEENSLAND.........161
10.3.1 Prawn Production in Australia...............................................................................................................161
10.3.2 Imported Prawns........................................................................................................................................162
10.3.3 Distribution Patterns in Australia..........................................................................................................163
10.3.4 Location of Case Study..............................................................................................................................166
10.3.5 Patterns of Distribution, Usage and Consumption of Prawn Products in the Moreton Bay Region
of South East Queensland......................................................................................................................................166
10.3.5.1 Commercial Prawn Fishery in Moreton Bay............................................................................................166
10.3.5.2 Prawn Aquaculture...................................................................................................................................168
10.3.5.3 Processing and Distribution......................................................................................................................169
10.3.5.4 Packaging....................................................................................................................................................170
10.3.5.5 Use of Prawns as Bait................................................................................................................................171
10.3.6 Recreation and Tourism.............................................................................................................................171
10.3.7 Wild Prawn Populations............................................................................................................................172
10.3.8 Waste Disposal Pathways..........................................................................................................................172
10.3.8.1 Wastewater Treatment...............................................................................................................................172
10.3.8.2 Municipal Wastewater Treatment.............................................................................................................172
10.3.8.3 Industrial and Commercial Wastewater Treatment....................................................................................173
10.3.8.4 Solid Waste Management...........................................................................................................................173
10.3.9 Examination of the Potential for Exotic Pathogens in Imported Prawn Products to Enter the
Aquatic Environment in the Moreton Bay Region............................................................................................182
10.4 CASE STUDY 3 – THE USE OF OYSTERS IN THE SHELL IN THE GEORGES RIVER REGION OF SYDNEY..185
10.4.1 Oyster Production in Australia................................................................................................................185
10.4.2 Imported Oysters.........................................................................................................................................185
10.4.3 Distribution Patterns in Australia...........................................................................................................186
10.4.4 Location of Case Study..............................................................................................................................188
10.4.5 Distribution, Usage and Consumption of Oysters in the Shell in the Georges River Region of
Sydney........................................................................................................................................................................188
10.4.5.1 Commercial Wild Oyster Fishery..............................................................................................................188
10.4.5.2 Oyster Aquaculture...................................................................................................................................188
10.4.5.3 Processing and Distribution......................................................................................................................189
10.4.5.4 Packaging....................................................................................................................................................190
10.4.5.5 Use of Oysters as Bait...............................................................................................................................191
10.4.6 Recreation and Tourism.............................................................................................................................191
10.4.7 Wild Oyster Populations............................................................................................................................191
10.4.8 Waste Disposal Pathways..........................................................................................................................191
10.4.8.1 Wastewater Treatment...............................................................................................................................191
10.4.8.2 Municipal Wastewater Treatment.............................................................................................................191
10.4.8.3 Industrial and Commercial Wastewater Treatment....................................................................................192
10.4.8.4 Solid Waste Management...........................................................................................................................192
10.4.9 Examination of the Potential for Exotic Pathogens in Imported Oysters In The Shell to Enter the
Aquatic Environment in the Georges River Region..........................................................................................199
10.5 CASE STUDY 4 - THE USE OF SALMONID PRODUCTS IN THE LAKE EILDON REGION OF VICTORIA.......201
10.5.1 Location of Case Study..............................................................................................................................201
10.5.2 Patterns of Distribution, Usage and Consumption of Salmonid Products in the Lake Eildon
Region of Victoria...................................................................................................................................................201
10.5.2.1 Commercial Salmonid Fishery...................................................................................................................201
10.5.2.2 Salmonid Aquaculture................................................................................................................................201
10.5.2.3 Processing and Distribution.......................................................................................................................202
10.5.2.4 Packaging....................................................................................................................................................203
10.5.2.5 Use of Salmonids as Bait...........................................................................................................................203
10.5.3 Recreation & Tourism................................................................................................................................204
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10.5.4 Wild Salmonid Populations......................................................................................................................204
10.5.5 Waste Disposal Pathways..........................................................................................................................204
10.5.5.1 Wastewater Treatment...............................................................................................................................204
10.5.5.2 Municipal Wastewater Treatment.............................................................................................................205
10.5.5.3 Commercial and Recreational Wastewater Treatment...............................................................................206
10.5.5.4 Solid Waste Management...........................................................................................................................207
10.5.6 Examination of the Potential for Exotic Pathogens in Imported Salmonid Products to Enter the
Aquatic Environment in the Lake Eildon Region of Victoria.........................................................................211
11. REFERENCES..............................................................................................................................................................213
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1. EXECUTIVE SUMMARY
The Report of the National Task Force on Imported Fish Products (1996) identified potential
pathways whereby exotic aquatic animal pathogens might be introduced into the Australian aquatic
environment via imported seafood products intended for human consumption. As no relative risk
was ascribed to these pathways, a more detailed investigation of potential pathways and
assessment of the relative risks* posed by them has been undertaken.
* Unless otherwise stated the term “risk” in the context of this report is deemed to mean – the probability
that an aquatic animal pathogen in a seafood import intended for human consumption or its by-products
will enter the Australian aquatic environment while still viable.
The mere existence of a pathway, or potential for viable exotic pathogens to enter an aquatic
environment does not by itself mean an existing or proposed import should be prohibited. The
information presented in this report forms only one component of a number of issues relevant to
quarantine risk analysis. Some of the other factors considered, include:
• characteristics of the pathogen of concern (e.g. heat tolerance, spore formation etc.)
• initial degree of product contamination with pathogens
• import volume
• probability of pathogen establishment in Australian aquatic species (e.g. infective dose,
pathogen survival in the aquatic environment etc.)
• consequences of disease establishment
Imported and locally produced aquatic animal products cover a diverse range of aquatic animal
species and product forms. Woolworths supermarkets alone report over 1,000 lines of seafood
sold in their stores. As with processing of other animal products, processing of aquatic animals can
generate significant quantities of wastes which may be commercially utilised or disposed. The
edible portion of most aquatic animals comprises less than 50% by weight of the whole animal.
An FRDC study in 1990/91 found that 94.6% of Australian householders eat seafood, with per
capita seafood consumption being 12.06 kg (edible weight) per annum. Australian fisheries’
production (wild and farmed) in 1996/97 was 210,000 tonnes (whole fish) of which 51,000
tonnes was exported (ie. 159,000 tonnes used locally). Seafood imports totalled 121,000 tonnes,
a proportion of which was already partly processed. Once processing losses are accounted for,
these figures show that as much, or more imported seafood is consumed in Australia than locally
produced seafood. Seafood imports include fish fillets, whole fish, canned fish, prawns, “calamari,
squid and octopus”, mussels and oysters.
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It is recognised that the volumes and types of seafood imported can be dynamic, for example due
to changing market forces or quarantine regulations, and during periods of high demand such as
Christmas and Lent.
Close to 5 million Australians currently fish recreationally at least once each year, while in 1990,
some 14,000 people were employed full-time by the catching sector of the commercial fishing
industry. A significant quantity of the bait and berley used by fishermen is derived from products
primarily intended for human consumption.
Recreational anglers, and to a lesser extent commercial fishers will sometimes purchase bait and/or
berley supplies directly from sources intended for human consumption, particularly when they are
seeking premium or specialist baits not normally available from traditional bait suppliers (e.g. large,
fresh or frozen green prawns, fresh squid and octopi, cuttlefish, various shellfish, whole and
cut/processed fin fish). In other cases, aquatic animal products (or parts thereof) intended for
human consumption are used as bait because they are no longer deemed suitable for human
consumption (e.g. processing waste and spoiled products).
At all stages during distribution, handling and processing of seafood products, waste is generated
which may contain viable pathogens. This waste may comprise liquid (e.g. wash water) and/or
solid components (e.g. heads, viscera, skin, trimmings, shell, packaging etc.).
Wastewater treatment is categorised primary, secondary and tertiary. Primary treatment essentially
removes particulate materials and microorganisms attached to them. In general this results in a
90% reduction of bacterial numbers, but has less impact on the number of viruses and protozoan
parasites. Significantly, the high organic matter content of primary treated waste often attracts
aquatic animals to points of discharge (e.g. ocean outfalls). Secondary wastewater treatment
reduces the biochemical oxygen demand (BOD) and suspended solids loading by about 90%.
5
Tertiary treatment further reduces suspended solids. Predictably, the level of pathogen loading in
wastewater decreases with each treatment step. The probability of tertiary treated wastewater
containing viable pathogens is very low. However such assurity is not possible with lesser
treatment regimes.
Most solid wastes result from seafood processing (e.g. frames, gills, guts or shells) or from
wastewater treatment processes. Where this material is moved to properly designed and
controlled sanitary landfills the risk posed is extremely low. In other cases, however, the risks are
more significant.
Potential pathways for seafood waste to enter the aquatic environment, usually directly, include use
as bait and disposal from tourist operations (e.g. houseboats, boat cruises, yachts) where seafood
is often consumed on board. Commonly, feeding of fish or birds with seafood wastes is a feature
of the tourist attraction. Similar pathways exist from shoreline activities such as picnics and
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barbecues. Organic wastes are also sometimes released offshore from seaside resorts (e.g. Barrier
Reef island resorts). Some bird species are common scavengers at recreational sites (e.g. picnic
and camping grounds) and at dumping or landfill sites, and therefore have potential to act as
mechanical vectors of some aquatic animal pathogens.
For establishment of disease to occur, viable exotic pathogens must reach a waterway. Their
capacity to do this in imported seafood products depends on the numbers initially present, the
potential for increase or inactivation, and the subsequent handling, treatment and pathway of
disposal. The probability of pathogen release from seafoods processed, prepared and consumed
as expected is very low as they would be required to survive any processing treatments as well as
the gastrointestinal tract and then some effluent treatment process before reaching aquatic
environments. Only the most resistant organisms are likely to survive these routes. Rather, it is the
volume of inadequately cooked or unprocessed product that bypasses these pathways that
presents a greater risk and represents the greatest probability of introduction of exotic aquatic
animal pathogens by the food for human consumption route.
If aquatic animal pathogens in seafood product reach an aquatic environment they may grow (in
number and biomass), simply survive, or die in that environment. Obligate pathogens require a
suitable host for multiplication. The major factors governing growth of opportunistic pathogens are
nutrient availability and temperature, but also include appropriate pH, oxygen concentrations,
salinity and trace elements. Some aquatic bacteria have survival mechanisms such as “starvation-
survival” or “viable but not culturable” (VBNC) which are activated in suboptimal conditions and
allow survival until a more favourable environment is re-established. Protozoan cyst and spore life
cycle stages are also protective and provide a survival mechanism if they are released into the
environment. Crustaceans and protozoa can also act as non-host vectors for certain pathogens,
thereby increasing the opportunity for them to find new hosts.
Recently there has been a great increase in knowledge of gene transfer mechanisms, whereby
pathogens themselves may not survive and establish in a new environment, but they may transfer
genes for virulence to new host microbes making them pathogenic.
Case studies on the potential pathways for exotic pathogen release from salmonid products in the
Huon and Derwent estuarine region of Tasmania and Lake Eildon region of Victoria, oysters in the
shell in the Georges River region of Sydney and green prawns in the Moreton Bay region of
Queensland revealed many common themes in terms of risk. However, the case studies also
demonstrated important differences between the seafood products studied. For example, prawns,
which are a favoured bait, are much more likely to be diverted for use as bait than oysters which
are rarely used as bait. Oysters (ie. apart from the shell, but including viscera) are usually
consumed whole, while usually only the meat of salmonids and prawns is eaten. In contrast,
prawns and salmonid are more likely to undergo some form of heat treatment then oysters.
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