Post provided by Ben Whittaker and Hannah L. Harrison

World Fisheries Day is celebrated annually on 21^st November, to reflect on the ever-increasing knowledge about fishing, fishers, coastal communities, and the status of the oceans and fish stocks. This year, the Canadian Wildlife Federation are highlighting their work towards ending the current practice of salmon aquaculture, via the gradual phasing out of open-pen finfish aquaculture (OPFA) to prevent further negative impacts of the practice on wildlife and marine habitats. One of the major impacts of salmon aquaculture on wildlife, is the transmission of sea lice to wild salmonid populations, which has resulted in mass mortalities of wild fish. The most efficient control method to reduce sea-lice in farms is arguably the deployment of cleaner fish, however, there is little information on how this widely used method impacts individual welfare and worldwide sustainability of cleaner fish stocks. In this blog post, Dr Ben Whittaker and Dr Hannah L. Harrison, discuss the current status of cleaner fish fisheries.

Salmon Farms, Sea Lice, and Cleaner Fish

Intensive farming of Atlantic salmon (Salmo salar) is the most commercially successful aquaculture sector worldwide, supplying competitively priced fish to supermarkets year-round. However, ecologically sustainable production remains a challenge due to outbreaks of parasitic sea-lice (Lepeophtheirus salmonis) on salmon farms, which are marine copepods evolved to feed on the skin of salmonid hosts. Sea lice infestation is estimated to cost 9% in annual farm revenue and raises serious animal welfare and ecological concerns, which critics argue justifies banning or significantly limiting intensive salmon farming. Such limitations or bans pose a threat to the booming aquaculture industry, and so efforts are being made to find ecologically friendly solutions of lice management.

Enter cleaner fishes, a collection of species with an appetite for ectoparasites such as sea-lice. In recent years, salmon farming facilities have begun releasing cleaner fish into open-net salmon pens in hope they will feed on sea-lice and protect their valuable salmon commodity. The demand for cleaners is already high and predicted to grow exponentially as the practice escalates. Yet, a reliable supply of captive bred cleaner stocks are not readily available and commercial salmon farmers currently rely on wild harvest of cleaner fishes to supply their farms.

Emerging Cleaner Fisheries

Cleaner fish deployment in salmon farms was first conceived of in the late 1980s, but only in the last six years has largescale deployment become common. The most widely used cleaner is the lumpfish, a rotund species with a pelvic suction cup found across most of the North Atlantic. It’s estimated that 50 million juvenile lumpfish will be deployed into farms this year alone, most of which being grown in hatcheries. But lumpfish are difficult to breed in captivity and so the vast majority of hatchery stocks are the progeny of wild-caught adult lumpfish that are harvested from spawning grounds. Once juveniles reach adequate size, they are shipped internationally to stock salmon farms.

Several species of labrid wrasse are also used commercially as cleaners. Wrasse have proven more challenging and time-consuming to rear in hatcheries than lumpfish, so nearly all the wrasse deployed into salmon farms are wild-caught adults. It’s estimated that over 21 million wild wrasse were used in Norwegian salmon farms during the 2016 season, with the most popular species being ballan, corkwing, and goldsinny wrasses. Farms in western Norway harvest local populations, whereas farms in northern and central Norway rely on imported wrasse from the southern part of the country and Sweden.

High Demand and Data Poor: Current Status of Cleaner Fisheries

While demand and use of cleaner fish is on the rise, data on the impacts of these wild-caught cleaner fisheries is worryingly scarce. There is a recurring theme of “unknown” or “needs updating” when assessing stock status of wild cleaner fish populations, likely because these species received relatively little commercial or research attention prior to the revival of cleaner fish use in commercial aquaculture in 2014.

Historical data on long-term population dynamics is scarce and contemporary surveys on wild populations appear to be uncommon, making informed assessment on the impacts of cleaner harvest a challenging task. Mature female lumpfish are also harvested for their roe, with approximately 3000 to 4000 tonnes sold annually. Roe fisheries are established in Canada, Denmark, Greenland, Iceland, Norway and Sweden, though only Greenland and Iceland fisheries have management plans. Iceland is the only fishery to routinely collect data for detailed stock assessment. Total landings have declined in most fisheries and while this might indicate a reduction in population size, it could also be the result of changing tastes and economic shifts away from lumpfish harvest.

Almost all of commercially used cleaner wrasse are caught in fisheries across Norway. Regulations include seasonal restrictions, minimum fish sizes for each species, and a national fishing quota of 18 million wrasse per year. Cleaner wrasses are also caught in Canada, Ireland, and the UK, though information on these fisheries is lacking.   

Species common name	Species Latin name	Status+	Population Trend+	Resilience#	Vulnerability#
Ballan wrasse	Labris bergylta	Least Concern*	Unknown	Low	High – Very High
Corkwing wrasse	Symphodus melops	Least Concern*	Stable	Medium	Low – Moderate
Cuckoo wrasse	Labrus mixtus	Least Concern*	Stable	Low	High – Very High
Cunner wrasse	Tautogolabrus adspersus	Least Concern*	Unknown	Medium	Moderate
Goldsinny wrasse	Ctenolabrus rupestris	Least Concern*	Stable	Medium	Low – Moderate
Lumpfish	Cyclopterus lumpus	Not Evaluated	Not Evaluated	Low	Moderate – High
Rock Cook wrasse	Centrolabrus exoletus	Least Concern	Unknown	Medium	Low

Cleaning Out the Sea: Possible Impacts of Data-Poor Cleaner Fisheries

Unlike many other fisheries that target fish outside of their spawning seasons or areas, the harvest of cleaner species is directed toward actively spawning adults. In the case of lumpfish this timing intention in order to collect eggs and milt to produce hatchery stocks. In wrasse, the timing is to collect territorial males which typically cope better with the stressful translocation and deployment process. Although these cleaner fisheries may still be too young to detect any significant impact of harvest on wild populations, data available from older roe fisheries have shown declining male to female sex ratios, likely as a result of female-focused harvests. Altering natural population dynamics like this can increase the vulnerability of wild stocks, especially for long-lived species with complex life-cycles.

While there are concerns about the sustainability of wild-caught cleaner fisheries and the welfare of the cleaner fish themselves, the impacts of commercially imported cleaner fish on local ecosystems is also worth noting. Escape events in which farmed fish escape their enclosures and disperse into the local wild environment are common within the salmon aquaculture industry. Perhaps less understood is whether these escape events provide opportunity for cleaner fish to also swim free of their enclosure. If so, this raises concern of whether non-native cleaners, which have been imported over large distances, might interact and interbreed with native cleaner fish populations. Wild populations of both wrasse and lumpfish show genetic differences across geographic regions, and hybridisation between farm and wild wrasse has already been documented in Norway. While similar hybridization phenomenon of farmed and wild fish has resulted in deleterious effects on wild salmon populations, the effect of this hybridisation on the fitness of wild cleaner fish stocks is yet unknown.    

What Can We Do: Methods for Developing Sustainable Cleaner Fisheries

More data is needed to evaluate the status of cleaner fisheries and inform fisheries management plans for these species. A first step could be to implement catch reporting standards for cleaner fish fisheries similar to those already used in Iceland (e.g. recording numbers harvested, length, weight, and sex) to fisheries elsewhere across the North Atlantic. Additionally, stock assessments and genetic studies could monitor shifts in population structure where cleaner fish harvest is increasing. These datasets can then be used to develop cleaner-specific guidelines, such as setting sustainable or optimum harvest limits, advancing quota-based harvest systems, or establishing protected areas for spawning cleaner fishes to ensure the viability of future generations. Finally, encouraging closer collaboration between roe and cleaner fisheries for lumpfish could improve efficiency and minimize wasteful harvest.  

Within the commercial aquaculture landscape, developing new methods for efficient cleaner deployment in farms would help alleviate harvest pressure on wild fish. For example, by identifying and selecting individual cleaner fish with high sea-lice consumption rates, or reusing cleaners at the end of the farming cycle rather than the current practise of harvesting alongside salmon as a waste product. Similarly, investing in cultivation research to close the life-cycle of cleaners in captivity would also reduce dependence on wild-caught fisheries, perhaps enabling local stocks to supply neighbouring farms and ending translocation practises.   

Summary

The rapid commercialisation of cleaner fish is testament to the pragmatism of salmon farming, though the limited information available for these species suggests wild populations show low resilience and high vulnerability to exploitation. A collective effort from multiple countries and industries is now needed to ensure cleaner fish are harvested sustainably. The first steps are education and awareness, perhaps starting by promoting the story of cleaner fishes on World Fisheries day.