In the last decade of mariculture, new parasitic diseases have emerged that afflict seafood products. Among these diseases we can mention the one caused by sea lice in salmonids.

The life cycle of the sea lice, like that of all crustaceans, develops with a series of moults. In its first stage of life, that is, when it is nauplius, the sea louse swims freely and feeds through its yolk reserves, as it does not have a well-developed digestive system. In the intermediate stage of the copepod, the lice use their hooked antennae and front filaments to attach themselves to the fish; at this stage they are still too small to cause any real severity to their host.�� As the lice move through the more mobile pre-adult and adult stages, they attach themselves to the fish by suction and can become lethal. Lice can move across the entire body surface of the host fish, preferring the head, back and anal fins. As an adult crustacean, lice prefer to feed on the fish’s dermal mucus, blood and skin.

There are two species of sea lice that can be found in salmon, Caligus elongates and Lepeophtheirus salmonis. The first of these affects many species of marine fish, while L. salmonis is only found in salmon and related species.

Sea lice are common in adult salmon and fall off when the salmon return to freshwater streams to spawn, as the physical-chemical conditions of freshwater are not favourable for this crustacean. This, in turn, ensures that no sea lice survive when the young salmon migrate to the sea in spring.

How sea lice affect to salmon production?

Sea lice have existed for as long as salmon, but only recently have they become a problem, as the proximity of salmon farms to the natural range of wild salmon facilitates the passage of the copepod from wild fish to farmed fish, whose crowded conditions provide an ideal breeding ground for sea lice.

In the wild, young salmon and adult salmon rarely mix. Fish farming, as currently practiced, allows adult salmon to be raised in static mesh pens near the migratory route of juvenile wild salmon. Wild salmon near production ponds are 73 times more likely to have lethal sea lice than juveniles not adjacent to production sites. Sea lice can survive for 3 weeks without their host, making it possible to transfer farmed salmon to wild salmon.

Although an adult salmon may be relatively unaffected by a sea lice infestation, the small size and thin skin of young salmon make them very vulnerable to a sea lice infestation. In addition, pink and young salmon lack scales, making them even more vulnerable to the effects of sea lice than their other scaly salmonids. Open lesions compromise a small fish’s ability to maintain its balance in salt water.

Sea lice prevention

Sea lice resistance to various chemotherapeutics has been consistently reported over the last decade so the aquaculture and pharmaceutical industry has worked to develop an integrated management system for the treatment of sea lice, in order to avoid resistance to chemotherapeutics by this crustacean. This approach considers good management practices, rotation of chemotherapeutics and the use of complementary medicine.

Such comprehensive management may consider the following practices:

• Planning: An agreed plan for the management of areas, treatments and personnel who will be responsible for each of the tasks.
• Monitoring: Monitoring is critical to measuring treatment success in order to make informed decisions.
• Synchronized rest of an area: A rest period of sufficient length in an area can eliminate a significant lice population through the absence of guests.
• Maximum biomass per area: Establishing the maximum biomass in a pond can be very helpful in controlling lice.
• Appropriate location of cultivation sites.
• Synchronized coordinated treatments: An initial coordinated treatment can be followed by a second strategic coordinated treatment several days or weeks after the first one in order to treat any relocation of lice. Different therapeutics are used as treatment of salmon lice, however, legislation on the use of this chemicals depends on the legislation of each country. The main therapeutics used against sea lice are described below:
  • Synthetic pyrethroids: they cause paralysis of the sea lice and later their death. Common examples are cypermethrin and deltamethrin.
  • Organophosphates: such as azamethiphos and dichlorvos, which act on the enzyme acetylcholinesterase and consequently interfere with the transmission of nerve impulses, this will generate uncoordinated movements, tremors, convulsions and/or paralysis in the louse.
  • Macrocyclic lactones: have been shown to be up to 95% effective against all stages of sea lice.
  • Growth regulators: lufenuron, diflubenzuron and teflubenzuron are compounds used as inhibitors of chitin synthesis, which cause a weak exoskeleton and interrupt the molting process and the development of the louse.
  • Oxidising agents: hydrogen peroxide is commonly used however, this type of treatment releases the parasite from the fish but does not kill the louse.

In addition to therapeutic measures, numerous physical methods have been developed to prevent, as far as possible, the entry of the parasite into aquaculture facilities and its effect on animal health. These physical methods try to minimize the entry of the parasite into the cages, based on the premise that the parasite habits the first meters of the water column. In this way different models of cages have been developed, such as closed cages, cages with skirts or cages with snorkels, which minimize contact between animals and the parasite.

Figure 1. Cages with skirt and cages with snorkel to control the sea lice.

At the same time there are physical methods that allow the parasite to be removed from affected animals, such as heat treatments, to pass the animals through waters at different temperatures, causing the parasite to be separated from the animal or by using jets of pressure water.

Figure 2. Heat treatments and pressure water treatments to eliminate sea lice from animals surface.

Images: https://globalsalmoninitiative.org/en/what-is-the-gsi-working-on/biosecurity/non-medicinal-approaches-to-sea-lice-management/