IV Aquaculture

IV Aquaculture

Though wild finfish and shellfish still make up the bulk of the seafood in commercial distribution, the most rapidly growing segment of the fisheries industry is aquaculture products. Aquaculture is estimated currently to account for 25% of the total world seafood supply, including a wide range of aquatic, estuarine, and marine species of finfish, molluscs, and crustaceans (FAO, 1998). This includes popular items such as catfish, salmon, trout, shrimp, crayfish, and oysters. Aquaculture is international in scope, with catfish being produced in Southeastern United States; salmon in Northern Europe, Tasmania, and the Maine and Pacific Northwest coasts of the United States; and shrimp and prawns from Asia and South America (IOM, 1991). Future gains in seafood availability are likely to be the result of increased use of aquaculture; harvesting of wild seafood appears to be reaching its maximum sustainable level (Reilly et al., 1992).

A Initial microflora The initial microbial populations on freshly harvested, properly handled pond-reared finfish will be

similar in composition to wild caught fish and consist of a diverse mixture of Gram-negative and Gram-positive genera including Acinetobacter, Aeromonas, Citrobacter, Enterobacter, Escherichia, Flavobacterium , Micrococcus, Moraxella, Pseudomonas, Staphylococcus, Streptococcus, and Vibrio. Pond-reared animals, particularly those from tropical areas, often have significantly higher incidence of Enterobacteriaceae than equivalent marine products (Reilly et al., 1986). However, this appears to

be a function of locality and use of manure for fertilizing ponds (Christopher et al., 1978; Reilly et al., 1992; Dalsgaard et al., 1995).

B Spoilage The icing of fish raised in warm-water ponds tends to produce a decline in initial populations, favoring

the survival of Gram-positive species. However, upon extended refrigerated storage, the Gram-negative

209 species, particularly Pseudomonas and Acinetobacter are selected (Acuff et al., 1984; Wempe and

FISH AND FISH PRODUCTS

Davidson, 1992; Nedoluha and Westhoff, 1993) and the products spoil similarly to wild caught fish.

C Pathogens The presence of pathogenic species in aquaculture products is dependent on a number of factors including

the source of nutrients used to enrich ponds, the extent of feeding, the population density in the ponds, and the methods used to harvest, process, and distribute the product. Pond-reared trout have been reported to have a high incidence of Cl. botulinum (Huss et al., 1974a; Cann et al., 1975), which may

be aggravated by excessive feed levels. It has been suggested that the use of “wet fish” as trout feed, led to the very high levels of spores found in Danish trout ponds (Huss, 1980). This is no longer allowed as feed and extruded dry feed is not likely to carry high levels of bacteria.

The use of animal and human excreta to enrich ponds, as well as run-off from adjacent agricultural lands, can significantly increase the incidence of enteric pathogens in both the growing waters and the harvested animals (Ward, 1989; Reilly et al., 1992; Twiddy, 1995). This practice has been associated with certain food-borne parasitic infections, particularly trematodes (flukes). A high incidence of classic enteric pathogens such as Salmonella and Shigella is common in water receiving manure or sewage run-off (Wyatt et al., 1979; Saheki et al., 1989; Reilly et al., 1992; Twiddy, 1995), and is likely to be reflected in an increased incidence of enteric pathogens on raw aquaculture products and in the processing environment (Iter and Varma, 1990; Reilly et al., 1992; Reilly and Twiddy, 1992; Ward, 1989). However, others did not find that chicken manure increased the isolation of salmonellae (Dalsgaard et al., 1995). Even in the absence of overt faecal contamination, a low incidence of salmonellae in ponds and pond- reared aquaculture products is not uncommon. Some of the factors thought to affect their presence in pond water include water temperature, organic content, salinity, pH, stocking level, and fish size. Fish feeds, as well as amphibia and aquatic birds, have been implicated as potential sources. Also important in this context may be findings that Salmonella and E. coli survive well in warm, tropical waters (Jim´enez et al., 1989). A high percentage of salmonellae isolated from pond-reared finfish and crustaceans were resistant to multiple antibiotics (Hatha and Lakshmanaperumalsamy, 1995; Twiddy, 1995).

A high incidence of pathogenic Vibrio spp., including V. cholerae non-01, V. parahaemolyticus, and

V. vulnificus can also occur in both the growing waters and raw products from rearing ponds in tropical regions (Christopher et al., 1978; Leangphibul et al., 1986; Varma et al., 1989; Nair et al., 1991; Reilly and Twiddy, 1992; Wong et al., 1992; Dalsgaard et al., 1995). Elevated incidence of V. cholerae non-O1,

V. parahaemolyticus and V. vulnificus has been reported in aquaculture products from sewage-enriched waters (Varma et al., 1989; Nair et al., 1991). The presence of V. cholerae non-O1 in tropical shrimp culture areas could, however, not be correlated to water salinity, temperature, dissolved oxygen, or pH (Dalsgaard et al., 1995).

Listeria spp. appear to be common in the freshwater aquaculture environment (Jemmi and Keusch, 1994) but is not typically isolated from marine units in open waters (Huss et al., 1995). If net pens are close to land where heavy rainfall results in run-off, the incidence of Listeria spp. may be very high.

In addition to the aquaculture-specific concerns listed above, care must be exercised to control the pathogens normally associated with the wild finfish and shellfish. It has been suggested that special care

be exercised when finfish and shellfish derived from wastewater aquaculture are destined for human consumption (Hejkal et al., 1983), particularly those that will be consumed raw. However, there appears to be little indication that aquaculture products that are subsequently cooked have any inherent increased risk in relation to food-borne disease compared with other raw meat or poultry (Reilly et al., 1992). Considering the increased potential for control, aquaculture products reared using sound management practices would be expected to have increased microbiological safety and quality.

MICROORGANISMS IN FOODS 6

One of the major constraints of aquaculture is disease. Whilst good management practices and vaccines are important measures to control disease, treatment with antibiotics remains the method of choice in several places. Antibiotic residues are seldom found because fish must be held for specific periods of time without antibiotic treatment before slaughter. Use of antibiotics will cause development of resistance in the bacterial population (Spanggaard et al., 1993; DePaola et al., 1995). Thus, unlimited use of antibiotics (as prophylactics) has led to collapse of fish farms because antibiotic-resistant fish pathogens could no longer be eliminated (Karunasagar et al., 1994). Of more concern to the consumer, is the potential spread of such antibiotic resistance to human pathogenic bacteria (Twiddy, 1995). Of 187 Salmonella isolated from imorted foods in the US in 2000, 15 were resistant to one or several antibiotics and 10 of these 15 were derived from seafoods (Zhao et al., 2001). Whilst this could be caused by excessive use of antibiotics in agriculture, or clinics filtering into the growing/catching waters, it could also be caused by use of antibiotics in the ponds or netpins. To our knowledge, the use of antibiotics in aquaculture has not been directly linked to treatment failures in humans. However, a Salmonella enterica DT104 out-break in Denmark in 1998 involved 25 people and two deaths and the quinolone-resistant type was traced to swine herds (Mølbak et al., 1999).