Large numbers of sea lice, Caligus chiastos, have been observed on the external surfaces of the southern bluefin tuna, particularly on the head and eyes. The main problem associated with sea lice infection is corresponding blindness of fish.If fish lose their site they will not be able to compete for feed, this may result in mortality from undernourishement.
The initial development of gross eye pathology is thought to be due to direct grazing on the cornea of tuna by sea lice. Anecdotal observations indicate that the irritation caused by the sea lice leads infected SBT to “flash” i.e. rub their body surfaces against solid objects in the water (e.g. the net) in an apparrent attempt to remove parasites from the heads and eyes. It is therefore possible that infected tuna injure thier eyes when flashing. The liklihood of such an injury probably increases when cage netting is fouled with bivalve species having sharp-edged shells.
The overall prevalence of sea lice in tuna ranches is relatively low. Prevalence peaks at 6 weeks of captivity, then decreases again. The deCline may be simply due to decreasing ambient temperatures during the culture period, where lower temperatures reduce the growth and reproductive rate of these sea lice. This infection pattern is analagous to that of blood fluke (see page on blood fluke for more information).
Not all the tuna appear to be susceptible to initial infection. Sea lice at the peak of infection only infected 55% of fish. The reasons why some fish are infected and others aren’t is unknown. It is thought that stress during capture and towing to farm sites is a predisposing factor to infection; however, this hypothesis requires verification.
Assessment of the overall health risk of sea lice to farmed SBT resulted in a low classification. However, monitoring of this species is recommended.