If 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 better 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 targeting the local hospital. You might further reduce the search by finding out whether the doctor you need works on the day or night shift, so you could be there at the right time.

In much the same way, it makes no sense that sea lice larvae would be carried on the sea’s currents and tides in the hope that a potential 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 meeting with a host. Most parasites have developed very complex life cycles to maximise their prospects.

One of the more interesting 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 parasitic 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 initial 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 tiny compared to the open sea. Migrating 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 – attracted 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 interactions with salmon farming. The full life cycle of the lice was only discovered as late as 2013. There is clearly a lot more to discover, including a better understanding of how the lice locate their host in the wild.

It is also thought that larval sea lice arrive at river mouths and estuaries when adult salmon return to freshwater to spawn. Sea lice do not survive the journey into freshwater and drop off as the salinity decreases so effectively completing the natural marine life cycle.

The question is how does the sea lice’s natural life cycle translate to salmon farming? Industry critics have concluded that there is an alternative scenario in which larval sea lice drift the seas in the hope of finding a moving host. Yet, there is probably a better chance that such critics will win the lottery. A paper by Emily Nelson and others in 2017 found concentrations of larval sea lice in the sea dropped away sharply in a very short distance from salmon pens. The lice larvae appeared to be aware that a ready food source was on their doorstep and stayed around the pens rather than drift away. With so many potential hosts in the salmon pen, it makes little sense to move away.

This leaves the question as to how salmon pens become infested, and the likelihood is that lice on passing salmon or sea trout recognise a new host is nearby and transfer to it. It is known that some sea lice stages move from one fish to another, as some fish recaught in sweep netting sampling have a lower lice count than when initially caught. As the fish in salmon pens or experimental sentinel cages are effectively tethered in one place, the chance of infestation increases compared with those hosts that are free swimming.

We still don’t know that much about the biology of sea lice, but it seems that those whose narrative suggests that the seas around Scotland have become a soup of infective sea lice seem to know even less.

 

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