Hazards of special relevance to capture-based aquaculture

A number of biological (bacteria, viruses, parasites, and biotoxins such as ciguatoxins and scombrotoxins), chemical (heavy metals, pesticides, antibiotics) and physical (foreign bodies) hazards may be associated with capture-based farmed seafood products.

Some of these may derive from naturally present environmental contaminants in which the fish are captured or farmed; others may be due to contamination introduced during processing. Most of these problems have already been described in the previous section of this chapter; the following extra comments draw attention to problems of special relevance to capture-based aquaculture.

Pre-harvest contamination

All of the problems described earlier, in the section on general hazards in wild and farmed seafood, are also applicable to some capture-based aquaculture products; a number have special relevance to this part of the aquaculture sector. For example, in the capture-based aquaculture of eels, 21% of Japanese eel culture ponds were reported to be contaminated with Salmonella in 1989 (Huss, Reilly and Ben Embarek 2000).
In several types of capture-based aquaculture there is a need to develop a suitable practical diet for grow-out production. Trash fish is often used as a principle source of feed in these forms of aquaculture, and can be a source of infection and some diseases involving opportunistic parasites.

For instance, the trematodes, cestodes, nematodes and acanthocephalans recovered from cultured groupers were thought most probably to be transmitted through the trash fish that
was fed to them (Bondad-Reantaso, Kanchanakhan and Chinabut 2001). Parasitism is an ubiquitous phenomenon in the marine environment, and in capture-based aquaculture there is the potential to develop several parasitic infections, which may be zoonotic if the fish is eaten raw. In groupers (Epinephelus suillus), the source of infection may be the fresh trash fish used as feed in the capture-based aquaculture practices applied. In other cases, the correlation between parasitic nematode species and their relative infections and the use of fresh trash fish cannot be ascertained, because their pathogenicity in humans has not yet been established.
Other problems occur in capture-based aquaculture due to the common practice of “seed” importation. In Japan, for example, the capture-based aquaculture of amberjack is based on the importation of fry from Hainan, China and Hong Kong. This trade developed due to the lack of juveniles available in Japanese coastal areas (Ogawa 1996). The increase of fry imports was correlated with an increase of the monogenean infection (Neobenedenia girellae) that suddenly appeared in these fish. It seems that the fry were infected before shipment to Japan. Since this parasite is not host-specific, infection can spread to other domestic fishes (Ogawa 1996), including grouper (Bondad-Reantaso, Kanchanakhan and Chinabut 2001).
The fish families involved in ciguatera poisoning include several capture-based farmed species, including serranids (groupers) and carangids (yellowtails). An example of this type of seafood borne infection occurred in Haiti in 1995, and was due to the consumption of cooked greater amberjack, Seriola dumerili (Poli et al. 1997). This infection is usually limited to wild fish. There is a potential to control such contaminations in capture-based aquaculture, so there could be a possibility of guaranteeing ciguatera-free fish (Sadovy 2000), thus providing a marketing opportunity.
Tunas are recognized as predators able to concentrate large amounts of heavy metals (Voegborlo et al. 1999), depending on their origin. This therefore represents a potential problem for capture based aquaculture. Eels (Anguilla spp.) are good candidates for the investigation of mercury bio accumulation, due to their long life spans in freshwater systems. Studies in Australia, Europe and New Zealand have shown mercury bio-accumulation in eel tissues (Redmayne et al. 2000). However, in general it is wild fish that accumulate high levels of heavy metals. Capture-based aquaculture, being based on the on-growing of wild “seed” in controlled systems where water quality and diets are controlled, should be less susceptible to this problem. In the wild, the larger and older the fish, the greater potential for the accumulation of heavy metals in its tissues exists.
The risk of dioxins and PCBs in fish reared through capture-based aquaculture should be low, and comparable to the levels found in wild fish, when they are fed raw fish (fresh or thawed). In any case, the levels of accumulation in the cultured fish will depend mainly on the levels of dioxins and PCBs in the feed presented, whether it be raw fish or a compounded aquafeed. For instance it is known that the levels of dioxins and PCBs in species of small pelagics fished from the South Pacific off South America are lower than those of similar species in the North and Baltic Seas.
Japan has banned the use of organic tin coatings, the application of a chemical substance to the nets used by the farmers of yellowtails to prevent the growth of fouling organisms, for fear of its accumulation in fish tissues and consequential human health concerns (Ogawa 1996). Initial investigations of human blood and livers have shown enhanced concentration of some organo tin derivatives (Hoch 2001).
Permitted chemotherapeutants, such as the antimicrobials used in capture-based aquaculture to prevent infectious diseases in fish, may lead to the presence of residues in fish flesh and to the development of antibiotic resistance in both humans and fish-pathogens (Schnick 2001); this practice therefore needs control.

Risks of contamination in harvesting and processing

All of the problems described in the earlier section of this report, on the general hazards in wild and farmed seafood products apply to capture-based aquaculture. However, this section mentions some problems that are particularly important for this sector.
Thunnus spp. are known to be easily stressed (Figure 149); if the tuna are over-stressed, the muscle can become “burned”. This is due to the production of lactic acid by the anaerobic glycolysis process. Stress also can create high levels of histidine to be produced, which creates the HFP problems that have been described earlier. Scombrotoxins (biogenic amines) are biotoxins that are produced mainly in scombroid fish species, such as tunas, due to inappropriate handling during harvesting and poor post-mortem conditions, particularly when these fish are kept at elevated temperatures (>5°C). It is important to highlight this hazard, since high levels of histamine in bacterially contaminated fish of these particular species can be toxic to the consumer. It is thus of importance in capture-based aquaculture, particularly to tuna farming. It has been noted that cooked fish have been involved in a higher number of cases of HFP than raw fish in Japan. Given the Japanese preference for raw fish, this may seem surprising but it is probably due to the fact that only the highest quality fish are sold in the raw fish markets there.
Other species that are reared in capture-based aquaculture are also associated with HFP. These include amberjack (Seriola spp.) and yellowtail amberjack (Seriola lalandi). The fish used as feed in some forms of capture-based aquaculture can also be associated with HFP; these include herrings (Clupea spp.), anchovies (Engraulis spp.), and sardines (Sardina pilchardus).

Figure 149. Capture-based farmed tuna; the harvesting time is a delicate phase because tuna are easily stressed (Photo: L. Mittiga)