Sea Louse - Wild Fish

Wild Fish

Most of our understanding of the biology of sea lice, other than the early morphological studies, is based on laboratory studies designed to understand issues associated with sea lice infecting fish on salmon farms. Information on sea lice biology and interactions with wild fish is unfortunately sparse in most areas with a long-term history of open net-cage development, since understanding background levels of sea lice and transfer mechanisms have rarely been a condition of tenure license for farm operators.

Many sea louse species are specific with regards to host genera, for example L. salmonis which has high specificity for salmonids, including the widely farmed Atlantic salmon (Salmo salar). Lepeophtheirus salmonis can parasitize other salmonids to varying degrees, including brown trout (sea trout: Salmo trutta), arctic char (Salvelinus alpinus), and all species of Pacific salmon. In the case of Pacific salmon, coho, chum, and pink salmon (O. kisutch, O. keta, and O. gorbuscha, respectively) mount strong tissue responses to attaching L. salmonis, which lead to rejection within the first week of infection. Pacific L. salmonis can also develop, but not complete, its full life cycle on the three-spined stickleback (Gasterosteus aculeatus). This has not been observed with Atlantic L. salmonis.

How planktonic stages of sea lice disperse and find new hosts is still not completely known. Temperature, light and currents are major factors and survival depends on salinity above 25 ‰. It has been hypothesized that L. salmonis copepodids migrating upwards towards light and salmon smolt moving downwards at daybreak facilitate in finding a host. Several field and modeling studies have examined copepodid populations in intertidal zones and have shown that planktonic stages can be transported tens of kilometres from their source.

The source of sea lice infections when salmon return from freshwater has always been a mystery. Sea lice die and fall off anadromous fish such as salmonids when they return to freshwater. Atlantic salmon return and travel upstream in the fall to reproduce, while the smolts do not return to saltwater until the next spring. Pacific salmon return to the marine nearshore starting in June, and finish as late as December, dependent upon species and run timing; whereas the smolts typically outmigrate starting in April, and ending in late August, dependent upon species and run timing.

It is possible that sea lice survive on fish that remain in the estuaries or that they transfer to an as yet unknown alternate host to spend the winter. Nonetheless, smolt get infected with sea lice larvae, or even possibly adults, when they enter the estuaries in the spring. It is also not known how sea lice distribute between fish in the wild. Adult stages of Lepeophtheirus spp. can transfer under laboratory conditions, but the frequency is low. Caligus spp. transfer quite readily and between different species of fish.