Background levels: Occurrence and effective detection of cod pathogens

Cod farming is considered one of the areas with the highest potential for growth in Norwegian aquaculture. Most cod hatcheries use wild caught broodstock, which represent a danger for the contamination of the farm premises with pathogens (virus, bacteria, parasites); particularly those transmitted vertically (mother-egg-fry). Parallels may be drawn to halibut culture, with problems such as VER. The fish is exposed to further pathogens at cage rearing, from wild fish attracted to the farm. Cod farming may also be blamed as a source of parasites and disease in wild fish, as similar blames have beset salmon farming. Claims have been made that the very high volumes of produced salmon caused elevated levels of parasites such as sea-lice, with devastating effec ts on wild salmonids. A major problem in assessing sea-lice abundance is lack of reference data from before farming. The same problem plagues the understanding of other diseases, e.g. viruses: is the agent native or introduced? Is virulence normal or incr eased? To avoid such problems regarding cod farming it is very important to accomplish a solid base regarding the natural occurrence of parasites and pathogens in cod now, before the production volumes affects the natural populations. This we call "backgr ound levels". Since our knowledge of future disease agents in cod farming is meagre at present, it is also important to retain material for future use in understanding the development of pathogens and diseases. We outline such a screening based on localit ies in areas with cod farming. We deduce the potential for interspecific spread of certain microparasites (e.g. costia, Trichodina, microsporidia) of cod through SSU rDNA sequencing and develop diagnostic assays. The danger of contaminating hatcheries mak es it desirable to screen broodstock for carriers of viral pathogens prior to stocking. We outline experiments to reveal the potential of certain biopsy samples for revealing nodavirus infections in cod.