its full lifecycle 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 ‰. L. salmonis copepodids migrating upwards towards light and salmon smolt moving downwards at daybreak have been hypothesized to facilitate finding a host. Several field and modeling studies on L. salmonis have examined copepodid populations and have shown that planktonic stages can be transported tens of kilometres from their source, including how their behaviour results in their being moved towards the coastline and mouth of estuaries The source of L. salmonis infections when salmon return from fresh water has always been a mystery. Sea lice die and fall off anadromous fish such as salmonids when they return to fresh water. Atlantic salmon return and travel upstream in the fall to reproduce, while the smolts do not return to salt water 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. Sea lice possibly survive on fish that remain in the estuaries or they transfer to an as yet unknown alternate host to spend the winter. Smolt get infected with sea lice larvae, or even possibly adults, when they enter the estuaries in the spring. How sea lice distribute between fish in the wild also is not known. 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, and are regularly found in the plankton. Morphology L. salmonis tends to be about twice the size of most Caligus spp. (e.g. C. elongatus, C. clemensi, etc.). The body consists of four regions: cephalothorax, fourth (leg-bearing) segment, genital complex, and abdomen. The cephalothorax forms a broad shield that includes all of the body segments up to the third leg-bearing segment. It acts like a suction cup in holding the louse on the fish. All species have mouth parts shaped as a siphon or oral cone (characteristic of the Siphonostomatoida). The second antennae and oral appendages are modified to assist in holding the parasite on