The rise and fall of cleaner fish

Editor/Mohamed shihab

Cleaner fish were widely adopted as a “natural” solution to sea lice in salmon aquaculture, but large-scale evidence now shows inconsistent efficacy, poor welfare, and high mortality. Our recently published review argues that cleaner fish use represents a case of premature technological adoption at industrial scale. 

by Dr Kathy OvertonPostdoctoral research fellow, Deakin University

Tim DempsterProfessor of marine biology and aquaculture, Deakin University

Dr Luke T BarrettPostdoctoral research fellow, Deakin University

More than 500 million fish have been deployed in Norway, often with limited lice control benefits and substantial ethical and economic costs. With new anti-lice technologies available and regulatory pressure increasing, the industry should phase out lumpfish and wild wrasse by 2029, rigorously test ballan wrasse, and prioritise scalable, welfare-aligned lice management strategies.

A solution that arrived too quickly

Atlantic salmon farming is one of the world’s most advanced and valuable aquaculture sectors, producing more than 2.7 million tonnes annually and operating at industrial scale across multiple regions. Despite this success, the sector continues to grapple with a persistent and costly constraint: the management of parasitic sea lice. These ectoparasites affect fish health, welfare, and productivity, and additionally carry environmental risks for wild salmonid populations. Given these impacts, controlling sea lice is imperative for industry sustainability and environmental stewardship.

Cleaner fish emerged as a compelling solution to this challenge. Species such as wrasse and lumpfish, which naturally graze ectoparasites, were promoted as an environmentally sound alternative to chemical treatments. Early trials suggested strong potential, aligning neatly with industry goals to reduce chemical use and improve environmental performance. However, as reviewed in Overton et al. (2026), the subsequent trajectory of cleaner fish reveals a more complex story, and highlights the risks of scaling innovations before their performance has been rigorously tested under commercial conditions.

The rise of cleaner fish: convergence of opportunity and necessity

The rapid expansion of cleaner fish use from the late 2000s onward was not accidental, but rather the result of several converging pressures. Increasing resistance to chemical treatments reduced the effectiveness of traditional lice control strategies, creating an urgent need for alternatives. Simultaneously, the collapse of cod aquaculture in Norway left substantial hatchery infrastructure that was searching for new production opportunities, and many of these facilities transitioned to producing cleaner fish species such as lumpfish and ballan wrasse.

These factors combined to create the conditions for rapid adoption. Cleaner fish numbers increased exponentially from 2008, reaching a peak of more than 61 million fish stocked annually in 2019. Cleaner fish became embedded as a core component of lice management strategies. With more than half a billion cleaner fish deployed in Norwegian salmon aquaculture over the past few decades, their use was supported by a narrative that they offered continuous, non-chemical control. 

However, this rapid uptake occurred before the industry had established whether cleaner fish could reliably deliver lice control at the scale and complexity of commercial farming operations. The assumption that results from small-scale trials would translate directly to industrial systems proved flawed.

The evidence gap: efficacy at commercial scale

We previously reviewed how effective cleaner fish are, and found that the evidence base supporting cleaner fish efficacy at commercial scale remains limited and inconsistent. An analysis of industry-reported lice counts also found that farms stocking cleaner fish only slightly delayed lice treatments, effects were highly variable and weak when viewed at a national scale. In some regions, particularly those relying primarily on lumpfish, no measurable reduction in lice levels could be detected. These findings suggested that cleaner fish performance is highly context-dependent and cannot be relied upon as a consistent control measure.

While numerous experimental studies show that cleaner fish consume lice, these studies were typically conducted in small tanks or cages, over short durations, and under controlled conditions that do not reflect the variability and complexity of commercial farms. However, these effects are not as predictable when cleaner fish are deployed in large sea-cage systems. 

Further insight comes from stomach content analyses, which provide an indication of how many cleaner fish are actively feeding on lice. These studies consistently show that a minority of individuals engage in lice grazing at any given time. In the Faroe Islands, 13.5 percent of lumpfish collected across farms had lice inside them, whereas a Norwegian study found 8.7 percent of lumpfish had lice inside them. In the largest study on lumpfish to date (24 693 lumpfish from 80 Norwegian farms), only 3.1 percent of lumpfish contained lice in their stomachs, indicating that most individuals do not remove the parasite. Even where cleaner fish appear to contribute to lice control, their effectiveness is influenced by factors such as fish size, environmental conditions, and species-specific behaviour. For example, lumpfish have a limited period of effective lice grazing that declines as they grow, further constraining their use.

Taken together, this body of evidence suggests that cleaner fish cannot provide the reliable, scalable lice control that is required for modern aquaculture operations.

Welfare and mortality: the defining constraint

While questions around efficacy are significant, the issue that has most strongly shaped the decline of cleaner fish use is their welfare. Cleaner fish are physiologically and behaviourally distinct from salmon, yet they are placed into farming environments designed for salmon production. This mismatch exposes them to conditions that are often suboptimal or stressful, including strong currents, fluctuating temperatures, and high stocking densities.

As a result, cleaner fish frequently experience a range of health and welfare challenges. These include infectious diseases, physical injuries associated with handling and delousing operations, and chronic issues such as emaciation. Environmental conditions further compound these challenges, as cleaner fish often have lower swimming capacity and different habitat requirements compared to salmon. The cumulative effect is a system in which cleaner fish struggle to adapt to life in sea-cages.

Mortality rates reflect these challenges. Although precise estimates are difficult due to limitations in reporting and the difficulty of tracking losses, our review of the available evidence indicates that mortality is consistently high. Commercial-scale studies have reported substantial losses over production cycles, and broader analyses suggest that cumulative mortality may reach levels as high as 80 percent or more. In many cases, farmers report that no cleaner fish remain in cages by the time salmon are harvested, implying near-total losses over the production period.

Despite significant effort by farmers to improve conditions through measures such as providing artificial shelters, supplementary feeding, and enhanced monitoring, there is little evidence these interventions substantially reduce mortality. This raises fundamental questions about the suitability of cleaner fish as a lice control strategy, particularly given that they are vertebrate animals used to manage parasites on other vertebrates.

The fall: declining use in a changing industry

Cleaner fish use has declined rapidly since its peak in 2019, falling to approximately 22.5 million fish in 2024. This decline reflects a combination of scientific, technological, and regulatory factors.

Uncertainty around efficacy has reduced confidence among farmers and fish health professionals, many of whom now question whether cleaner fish provide meaningful benefits. At the same time, growing awareness of welfare issues has increased scrutiny from regulators, the public, and within the industry itself. Media coverage and public perception have shifted as evidence of high mortality and poor welfare outcomes is more widely known.

Technological developments have also played a critical role. The industry now has access to a broader suite of lice management tools, including barrier technologies, depth-based farming strategies, mechanical and thermal treatments, functional feeds, and laser-based systems. These approaches offer more controllable and predictable outcomes, even if they come with their own challenges.

Regulatory changes have further accelerated the decline. In Norway, stricter welfare requirements now mandate practices such as removing cleaner fish from cages prior to delousing treatments and enforcing more rigorous biosecurity and reporting standards. These requirements have increased operational complexity and costs, while also exposing producers to financial penalties for non-compliance. As a result, several major salmon companies have phased out some (e.g., Lerøy) or all (e.g., SalMar, Bolaks, CERMAQ) cleaner fish entirely, citing an inability to meet welfare expectations.

An industry-scale experiment: lessons learned

The trajectory of cleaner fish use can be understood as a large-scale experiment in aquaculture innovation. We argue that it represents a case in which commercialisation outpaced the development of a robust evidence base. The widespread adoption of cleaner fish occurred before their efficacy was demonstrated at commercial scale and before their welfare requirements were fully understood.

This experience offers important lessons for the aquaculture industry. It highlights the need to test new technologies under realistic production conditions, rather than relying on small-scale trials. It also underscores the importance of integrating animal welfare considerations into the design and evaluation of new systems from the outset. Finally, it reinforces the value of independent, transparent research that can provide objective assessments of performance across different environments and operational contexts. As emphasised in broader discussions around innovation, such research must be conducted rigorously and shared openly to ensure that industry decisions are grounded in reliable evidence.

Recommendations for a transition pathway

Current evidence points toward a clear transition away from widespread cleaner fish use. Lumpfish and wild-caught wrasse, which have the weakest evidence base for efficacy and face significant welfare and biosecurity challenges, should be phased out over the coming years. Farmed ballan wrasse may retain a limited role, but only if their use can be supported by strong evidence demonstrating both effective lice removal at commercial scale and substantially improved survival outcomes. Achieving mortality rates in line with broader industry and government targets, will be essential if their use is to continue.

At the same time, the industry is moving toward integrated lice management systems that combine multiple approaches. Preventative technologies, spatial and depth-based farming strategies, and targeted treatments are increasingly being combined to manage lice pressure. Within such systems, any future role for cleaner fish needs to be carefully defined, rigorously tested, and continuously evaluated against alternative approaches.

Looking forward: toward evidence-led innovation

The rise and fall of cleaner fish marks an important moment in the evolution of salmon aquaculture. It reflects a transition from a phase of rapid expansion and experimentation toward a more mature, evidence-driven approach to innovation. As new technologies continue to emerge, the industry faces the challenge of ensuring that they are adopted in ways that are both effective and responsible.

This will require a stronger emphasis on commercial-scale trials, long-term monitoring, and transparent reporting. It will also require a commitment to aligning production practices with evolving expectations around animal welfare and environmental sustainability. By applying these principles, the industry can avoid repeating the mistakes associated with cleaner fish and instead build more robust and resilient production systems.