The fishparasite Caligus elongatus

Most prevalent on lumpfish and codfishes
In the project, more than 4400 wild fish of 52 different species were examined for C. elongatus. The sampling was carried out in spring and autumn 2002-2004 in a coastal area outside the town of Arendal on the SE coast of Norway. There are no fish farms in the area.
It is known that lumpfish may carry large C. elongatus infections, so free-swimming lumpfish from the North Sea was also sampled. Would it be possible that these open water fish brought the lice into coastal areas during their annual spawning migrations? Could this be a source of lice for fish farms?
Caligus elongatus was found on 15% of the examined fish, and on half of the examined fish species. However, there were clearly more lice on lumpfish, codfishes (cod, saithe, pollock and whiting) and sea trout.
Genetic tracers
To elucidate the infection routes the researchers used genetic analyses. "We found that C. elongatus occurs in two genetically distinct variants, which we have so far called genotype 1 and 2" explains Øivind Øines, who is currently finishing his PhD on the parasite at the National Veterinary Institute.
"Early in the spring mature lumpfish arrive in the kelp forrests along the coasts to spawn. These fish carry only genotype 1 lice, which appear to jump off and subsequently infect local fish, as later in spring there is a strong increase in the proportion of genotype 1 lice on the local fish. Spent lumpfish disperse off into the ocean. During summer there is a genotype shift, whereby genotype 2 lice take over and in autumn there are mostly genotype 2 lice to be found on the coastal fish"
The North Sea lumpfish were the most infected fish in this study, Øines tells. "Many of these lumpfish, which were immature, had larval stages of C. elongatus. This suggests that they had been infected in the ocean, in other words, that the genotype 1 lice may have an oceanic life cycle"
Rapid infections in fish firms
Many fish farmers have reported sudden Caligus elongatus infections, apparently without a prior settling and build-up of a larval population on the fish. Our findings support the hypothesis put forward by several scientists that this is because adult C. elongatus switch from wild to farmed hosts in the adult stage, and that incoming wild fish are the source of these parasites. "This infection pattern is completely different from what we see in the closely related salmon louse Lepeophtheirus salmonis, which only infects in the larval "copepodid" stage, and only has three hosts in Norway, salmon, sea trout and sea charr", says the project head Peter Andreas Heuch at the National Veterinary Institute.
Read more:
The scientific paper describing this work has been published in the journal Diseases of Aquatic Organisms and can be downloaded at:
http://www.int-res.com/articles/dao_oa/d077p149.pdf

Contacts at National Veterinary Institute
Peter Andreas Heuch, Section for parasitology, peter-andreas.heuch@vetinst.no
Øivind Øines, Section for parasitology, oivind.oines@vetinst.no