Ten different stages have been described in the life cycle of the salmon louse. Each stage is separated by a moult (two naupliar stages, one copepodite, four chalimus, 2 pre-adult and one adult stage) (Johannessen 1978; Johnsen & Albright 1991a, b; Schram 1993).

We can further divide the louse’s life cycle into two phases, a free-swimming planktonic phase and a parasitic phase (Tully, O. et al. 2002).The two naupliar stages and the copepodite stage swim freely in the water, thus spreading the parasite. The fish is infested by the copepodite, which attaches itself with two pairs of antennae and then with a chitin thread, before it moults and becomes a chalimus 1 (Heuch P.A. et al., 1999). During the four chalimus stages the parasite clings tightly to the fish with the aid of the chitin thread. The pre-adult and adult stages can move freely on the fish. A female can make at least eleven pairs of egg strings, each with several hundred eggs, after a fertilisation. In experiments, females were seen living for 140 days after reaching the adult stage (at 7oC), and everything indicates that they overwinter on the host and cause new infestations in the spring (Heuch & Schram, 1999). In female L. salmonis researchers have observed that the egg strings were replaced already 24 hours after having released the previous brood of naupliar larvae (Johannesen, 1978).