Comment
BY DR MARTIN JAFFA
The truth about lice Campaigners who suggest that the open sea is infested with lice do not understand the parasite’s life cycle
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f you had to urgently find a person who would help save your life and knew nothing about them, then what you might do is to go out into the street and stop everyone who passed by in the hope of bumping into them. However, you could be looking for ever. It might be be�er trying to narrow the field, by for example, if the person you sought was a doctor and then you might reduce your search area by targe�ng the local hospital. You might further reduce the search by finding out whether the doctor you need works on the day or night shi�, so you could be there at the right �me. In much the same way, it makes no sense that sea lice larvae would be carried on the sea’s currents and �des in the hope that a
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poten�al host in the form of a salmon smolt will swim by. The seas around Scotland cover a huge area and sea lice larva are microscopic. Yet, campaigners argue that salmon farms pump out millions of sea lice larvae into the open sea and these infest passing salmon. They say that these infested fish will then die, and this is why wild salmon stocks on the west coast have declined so rapidly. It seems so unlikely that a parasite like sea lice has developed a life strategy that depends on a chance mee�ng with a host. Most parasites have developed very complex life cycles to maximise their prospects. One of the more interes�ng fish parasites is Cymothoa exigua, which targets a variety of fish species, but its main target is the rose snapper that lives in the Gulf of California. The parasi�c isopod infiltrates the gills of the fish and latches on to its tongue. It then proceeds to consume the organ, which it replaces with its own body, and it then acts as the tongue for the host fish. The ini�al parasite is a male, but if the fish is infested by a second male, the first one turns into a female and the two breed. Once the eggs develop and are released then the female lets go of the host and without a tongue the fish eventually dies and having successfully bred, so does the female parasite. Sea lice are not as complex as Cymothoa, but they too must find their host. In the wild, the most likely place for sea lice to meet a host fish is at a river mouth where freshwater meets the sea. At this point, the area through which the fish must pass is �ny compared to the open sea. Migra�ng smolts also slow down as they leave the river in order to adjust to the changing salinity, so they become easier targets. Sea lice are phototaxic – a�racted to the light – and are drawn to the surface layers through which fish are most likely to pass. There is also likely to be some sense of smell or movement involved, as found in freshwater fish lice, but this is less clear in the marine species. Unfortunately, most research on sea lice has been directed on the interac�ons with salmon farming. The full life cycle of the lice was only discovered as late as 2013. There is
The lice larvae appeared to be “ aware that a ready food source was on their doorstep ”
Above: Smolt on their way out to sea Left: Sea lice on salmon Right: Sea lice
www.fishfarmermagazine.com
09/08/2021 14:33:25
