• What are sea lice? Crustaceans parasites belonging to the copepoda in the family Caligidae. Lepeophtheirus salmonis (salmon louse) and Caligus elongatus causing the greatest problem, especially serious is L. salmonis. C.teres causes serious problems on Atlantic salmon in Chile. 
• (the term "sea lice" has also been applied to ‘seabather’s eruption’ caused by the larval form of Linuche unquiculata or thimble jellyfish, in Florida).
• Geographic Distribution: Lepeophtheirus salmonis (salmon louse) - temperate climates in the Northern Hemisphere; Caligus elongatus - globally; C. orientalis - Japan; C. teres - Chile, C. longicaudatus - New Zealand; C. clemensis and L. cuneifer - Canadian Pacific.

• Sea lice species and host affected:
  L. salmonis :salmonids: Atlantic salmon, Pacific salmon; sea trout, stealhead trout, Arctic char, white-spotted char, brook trout, cut-throat trout
  L. cuneifer: elasmobranch and teleost fish
  L. pectoralis: flounder
  L. hippoglossus: halibut
  C. elongatus: many marine fish species.
  C. curtus: gadids (cod, haddock)
  C. orientalis: rainbow trout
  C. teres: farmed coho and Atlantic salmon
  C. labracis: ballan wrasse

• How to distinguish L. salmonis and C. elongatus

	L. salmonis	C. elongatus
Appr. Size(mm) Male Female	6 11	4 -5 5
Color	light to dark brown	light brown
Morphology	no lunulus	has lunulus (sucker- like organ, anterior-laterally on ventral cephalothorax)
Larvae	3 non-feeding free-living planktonic; 4 attached parasitic stages	3 non-feeding free-living planktonic; 4 attached parasitic stages
Pre-adult stages	2 mobile, parasitic stages	none
Matings	Multiple	Presumable only one
No. of eggs per egg sac	100 - 500	About 100

• Reproduction:
  • Male locate on anterior, dorsal site of fish; fertilized females locate primarily behind adipose and anal fins,
    One mating fertilizes several egg sacs,
  • Egg sacs are chain of fertilized eggs; larvae develop inside the egg sac; even if egg sac is removed; fully developed egg sacs with ready to hatch larvae are dark brown,
  • 6 sets of egg sacs can be produced by one female in 50 days at 14^oC; oviducts are filled with eggs while egg sacs are still attached,
  • no. of eggs per egg sac is related to length of egg sac; egg sac of wild salmon are often longer and contain more eggs, egg sac length increases in late fall, early winter.
• Life cycle:
  • Time from extrusion of egg sacs to eclosion (hatching): 5.5 to 40 days, depending on the temperature (shortest time at warmer temperature); L. salmonis 10-12 days at 11.5^oC; C. elongatus 8 days at 10^oC;
  • 2 nauplii stages; non-feeding, positive phototactic; nauplii I 9 -52 hours; nauplii II 17 - 35.6 hours depending on water T (5 - 15^oC);
  • 1 free-living copepodid stage attaches to fish and becomes parasitic; actively positively phototactic;
  • infection rate of fish kept at 0 - 4 m depth is 40 higher than fish kept at greater depth,
  • attachment of copepodids affected by light, salinity, hydrodynamic factors
  • 4 attached chalimus stages; sex differentiation in chalimus IV
  • generation time (from nauplii I to gravid female): at 9 -12^oC about 40 (males)-52 (females) days.
  • Survival:
  • adult sea lice survive up to 22 days off the host,
  • copepodids remain infective for 4-6 days at 15^oC,
  • life span in freshwater: 21 days; have been seen on migrating salmon 70 km inland;
  • life span on host in seawater 75 days at 9- 10^oC; 191 days at 7.4^oC; C. elongatus survived 260 days on arctic char,
  • nauplii survive best in sea water; copepodids at 15‰; survival of copepodids is higher at 15^oC than at 5^oC;
  • eggs survive and hatch at 15‰ but survival of nauplii is nil; complete development only at ≥30‰
• Pathological effects on salmonids
  • copepodids cause local reaction visible as small black spot; feeding activity of chalimus causes small local erosion; more extensive erosions are caused on the dorsal fin of sea trout,
  • adults graze with an non-selective rasping apparatus; there is no evidence of secretions,
  • pathogenic effect increases with the size, mobility and number of the parasites,
  • L. salmonis causes more damage than C. elongatus,
  • Lesions are can be most severe in head region, with deep erosion occurring,
  • Plasma sodium and cortisol (stress level) increase, hematocrit, lymphocytes and plasma protein levels decrease;
  • Non-specific, innate resistance in coho. Higher resistance is associated with protease activity in mucus (serine and metalloprotease).
  • limited cellular and humoral immune response. Immunity, induced experimentally by antibodies to lice gut antigen, reduced fecundity of females but does not prevent infection.
  • Injection with cortisol increased resistance in smolt,
  • it has been suggested that sea lice can transmit furunculosis, vibriosis, infectious salmon anemia virus, bacterial kidney disease,
  • sea lice infestations increase stress and susceptibility to other diseases; cause starvation, reduce growth, cause mortality, reduce carcass quality, increase secondary infections (cold water sores) and cause expenses for treatments.
• Chemotherpeutants:

• Management strategies:

  

  • Stocking of sites with single year class; copepodids are seen within 3 days of stocking on multiple age sites,
  • Fallowing between year classes for 30 days or longer,
  • Sites are mostly selfinfecting, therefore all cages on a site should be treated within the shortest possible time,
  • Keep fish well-satiated; satiated fish keep in deeper waters where infection rate is lower,
  • Reduce stress, stressed fish are more susceptible. Overcrowding, handling (grading) causes stress,
  • Select for greater resistance to sea lice infection,
  • Coordinated bay-wide treatment to reduce initial infection from external sources
  • Late fall/early winter or early spring treatments,
  • Use of wrasses where possible,
  • Use Integrated Pest Management approach.
• Reference:

  Pike A.W and Wadsworth S.L. Sea Lice on Salmonids: Their Biology and Control.
  Advances in Parasitology. Vol. 44, pp.233-337.1999.