By Asbjørn Bergheim*
Many reported surveys clearly indicate the close connection between the local or regional fish farming activity and episodic sea lice infestations in wild fish populations. Before farming of salmon or in coastal zones with few cage farms so-called sea lice epizootics rarely happened. According to The Atlantic Salmon Trust, studies in Norway 15 – 20 years ago concluded that the lice egg production in farming areas increased by more than 50 times compared to pre-farming conditions. Such heavy lice infestations of wild sea trout, arctic char and salmon are reported from Scotland, Ireland and Norway, and of wild juvenile pink and chum salmon in British Columbia (BC). In BC, less than 5% of juvenile salmon not exposed to fish farms were suffering from sea lice, while 30 – 40% of the salmon near farms had sea lice (www.fis.com).
At present, the salmon biomass in Norwegian cage farms is several hundred times the wild stock ascending the rivers along the coast. The increased number of salmon parasites along with the dramatically increased biomass of potential hosts is hardly any unexpected consequence. However, the lice infestation level is different from one year to another in the individual fish farms and the potential risk of larval dispersion to local wild salmonid stocks is fluctuating.
Wild sea trout are particularly vulnerable to lice infestation due to the species’ typical migration pattern in the sea close to the river mouth. In some Norwegian fjord systems influenced by cage farming, as much as 50 – 100 lice have been detected per individual and some local stocks of sea trout are actually at risk of elimination. On the contrary, several sea trout strains in rivers and brooks along the coast with little or no fish farms, e.g. on the SE coastline towards the Swedish boarder, seem to recover and even flourish.
Sea lice attack has become a heavy burden and challenge to many cage farmers. Last year, 84% of all Norwegian cage sites performed salmon lice treatment (2013: 69% of all farms), www.kyst.no. Based on collected data from The Norwegian Veterinary Institute, the 679 lice affected farms performed on average five treatments each during 2014. Usage of hydrogen peroxide has exploded over the last year, not least due to the increasing resistance in sea lice against chemotherapeutic drugs, such as chitin inhibitors and pyrethroids, as a result of many years’ repeated usage. In Scotland, the maximum allowed number of adult female lice in winter – spring without implemented treatment is 0.5 per fish. Norwegian authorities made similar demands before 2010, but because of the severely increased infestations, this limit was tightened in spring to 0.1 lice per fish.
The Sea Lice Research Center in Bergen estimates that sea lice cause some EUR 300 million per year in damage to the industry worldwide, primarily to Atlantic salmon producers (www.seafoodsource.com). The major damage and lost income to the fish farmers are attended with reduced fish growth, higher mortality and treatment costs. During delousing, the number of lost fish is occasionally high.
The other lice genera (Caligus) has become a major parasite of concern in salmon farms in southern Chile (www.oceanbites.org). A recent official surveillance program indicated that more than 50% of the cage farms were infested with an average abundance of 11.8 lice per fish.
Synthetic pyrethroids and other drugs invented to control lice and their spread in salmon cages may also be released to the surrounding sea and potentially harm non-target organisms, such as copepods, crustaceans and mussels. It has been demonstrated that the frequently used drug cypermethrin may reach concentrations of 2 ng/L close to treated cages, while the lethal level (LC50) in shrimp is 5 ng cypermethrin/L (www.oceanbites.org).
Despite the massive lice infestations and subsequent chemical treatments, measured residues of drugs in formerly treated salmon are very low or below detection levels. Usage of such chemicals does not represent any health risk to the consumers. However, there have been residues of chitin inhibitors in wild fish (e.g. saithe), molluscs and bristle worms in the vicinity of treated farms (www.nifes.no).
Due to the described problems with drug usage, the industry is seeking alternative and environmentally friendly methods to fight sea lice. Many farms in the North Sea stock wrasses in salmon cages at a density of 2 – 10% (1 per 10-50 salmon) to control the incidence of lice. The Norwegian salmon industry puts out 15 – 20 million wrasses in cages every year and the cleaner fish are occasionally exposed to high mortality caused by epidemical disease outbreaks. Thus, another growing industry is the production of wrasses in hatcheries.
A related species to commonly used wrasses, the “lumpfish” or lumpsucker, has become a promising candidate to bite the sea lice off the salmon. This tiny fish is adapted to lower temperature than ordinary wrasses and seems to graze lice efficiently even at 4 ºC (Project NORDLUS, www.forskning.no). However, great individual differences as regards appetite on lice are observed within the same cage: some “lumpfish” prefer fish pellets, while others demonstrate an enormous appetite, almost breaking their stomachs with sea lice. “Lumpfish” production is also developing rapidly.
Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.