Study questions cleaner fish efficiency - Part 2

4. DISCUSSION 

4.1. Cleaner fish efficacy 

Experiments that tested efficacy of cleaner fish for sea lice removal were typically unreplicated or had low replication, and our search returned just one study that assessed cleaner fish efficacy at a large commercial scale (lumpfish: Imsland et al. 2018) and 3 comparisons at a small commercial scale (ballan: Treasurer 2013; corkwing: Tully et al. 1996). Insufficient replication precludes the drawing of strong conclusions, as confounding factors may contribute to the observed effects (Quinn & Keough 2002). A lack of studies at a large commercial scale creates a mismatch between the small scale at which proof-ofconcept has been tested and the cage volumes in which cleaner fish are now deployed. The design of experiments must pay attention to scale, as results detected at a small scale often do not match those detected at a large scale (Wiens 1989). Given the industrial use of ~60 million cleaner fishes per year by industry across multiple countries, the lack of well replicated experiments at commercial scale requires redress. 

Most studies testing efficacy have been conducted in small cages with volumes between 100 and 125 m3 (e.g. Bjordal 1991, Imsland et al. 2014a,b), whereas circular commercial cages commonly have a 160 m circumference with a 15−35 m deep net that tapers to a cone-shaped bottom in the last 5 m (cage volume: 20 000−80 000 m3 , e.g. Oppedal et al. 2011a). Therefore, cage volume is approximately 200 to 800 times higher in commercial cages relative to the volumes and sizes used in most efficacy studies. 

Evidence for effective sea lice removal for certain widely used wrasse species is especially sparse. Efficacy of goldsinny wrasse has not been tested at a large commercial scale, while evidence for efficacy of corkwing wrasse is currently limited to 2 technical reports (Skiftesvik et al. 2017, 2018). Given that 5.9 million wild-caught corkwing wrasse and 7.9 million wild-caught goldsinny wrasse were stocked in sea cages in Norway in 2018 (Norwegian Directorate of Fisheries 2019), rigorous experimental assessments of the efficacy of gold - sinny and corkwing wrasse at a commercial scale should be prioritised to justify their ongoing use. In contrast, several studies have assessed the cleaning efficacy of rock cook wrasse, with promising results (Bjordal 1991, Tully et al. 1996), yet their use is negligible at present (Norwegian Directorate of Fisheries 2019). 

The most-used species, lumpfish, has the most robust evidence base among all cleaner fish species deployed, with several studies spanning small to large commercial scales. However, the evidence base for lumpfish requires further de - velopment as it does not span the range of farming conditions across which ~31 million lumpfish are deployed each year in Norway across all 13 production zones (Norwegian Directorate of Fisheries 2019). Numerous studies that have examined cleaner fish stomach contents provide evidence that cleaner fish consume lice at various experimental scales (Deady et al. 1995, Treasurer 2002, Imsland et al. 2014a, 2015, Skiftesvik et al. 2017, Eliasen et al. 2018). However, these studies typically occurred during the warmer months and at relatively sheltered sites. Only one large commercial scale experiment at a single inner fjord site tested the efficacy of lumpfish during winter (Imsland et al. 2018). However, the efficacy and survival of cleaner fish at exposed coastal sites has not yet been investigated. 

Overall, there is a clear mismatch between the current evidence base for the efficacy of cleaner fish and the extent of their use by the industry. The current evidence base is derived from relatively few studies, in a narrow range of environmental settings, and largely in experimental units with small cage volumes and limited numbers of salmon that do not match the scale (volume and depth) of commercial cages, nor the large number of enclosed salmon. The use of cleaner fish in salmon aquaculture essentially trades off the welfare of multiple fish species for that of another more commercially valuable species, and it is therefore important that a robust evidence base justifies their use from ethical, environmental, and economic perspectives. 

4.2. Spatial overlap between cleaner fish and salmon in sea cages 

The spatial overlap between cleaner fish and salmon in sea cages has been sparsely studied, with no research conducted in large commercial scale sea cages. Our systematic review has illustrated that studies mostly focus on cleaner fish behaviour and inter actions with salmon in sea cages; however, there has not been a substantial focus on salmon and cleaner fish swimming depths within sea cages as a measure of the likely interactions between the client and cleaner. As most proof-of-concept scale studies have been done in shallow tanks or cages, this may not have been necessary at a small scale. However, swimming depth preference is an important factor that should be measured at the commercial scale, as salmon and cleaner fish may have different preferences. Larger, deeper cages enable salmon to more readily express such preferences, which could result in fewer encounters between salmon and cleaner fish than expected, which could in turn reduce lice removal efficacy. 

During the day, when cleaner fish are most active (e.g. lumpfish: Powell et al. 2018; ballan: Brooker et al. 2018; goldsinny: Gonzalez & de Boer 2017), sal - mon typically move up into surface waters during feeding times before descending to preferred deeper swimming depths once satiated (Oppedal et al. 2011a). This general pattern may be altered by thermal stratification, with salmon choosing the depth with the warmest water available (up to 16°C). Typically, responses to temperature result in deeper swimming in winter, when surface water is cold, and avoiding surface waters that are too warm in late summer and during transitional periods from spring to autumn (Oppedal et al. 2011a). 

While cleaner fish can control lice under certain conditions, their physiology and morphology are not suited for life in more exposed sea cage environments (Yuen et al. 2019). Wrasses are typically found in coastal rocky reefs and kelp beds, where habitat structure provides the opportunity to shelter from sustained currents and wave surges (Pita & Freire 2011, Villegas-Ríos et al. 2013, Brooker et al. 2018, Leclercq et al. 2018). They are relatively poor swimmers compared to salmon; large ballan wrasse far larger than the size typically used as cleaner fish in aquaculture have a sustained swimming speed of only 27 cm s−1 at 25°C (Yuen et al. 2019), which is considerably lower the sustained swimming speed of post-smolt salmon (75−93 cm s−1 at 3−18°C, respectively; Hvas et al. 2017). Lumpfish also have poor prolonged swimming capacity (25−35 cm s−1 for 300 g fish at 3−15°C and 13% lower in 75 g fish; Hvas et al. 2018), and given that cultured lumpfish are stocked at smaller sizes (Brooker et al. 2018), their welfare, survival, and subsequent delousing performance could be severely compromised at sites with strong currents. Lumpfish are globiform teleosts native to the North Atlantic, where they are found in both pelagic waters and coastal regions (Blacker 1983, Daborn & Gregory 1983). While they cope well at cold temperatures, their mortality rate increases at temperatures above 16°C (e.g. Hvas et al. 2018). In comparison to farmed ballan wrasse and lumpfish, the natural distribution of Atlantic salmon, while partially overlapping, extends much further north than that of wrasse and further south than that of lumpfish (Jensen et al. 2014), with a thermal niche overlapping lumpfish in the north and wrasse in the south. A better understanding of cleaner fish biology is needed to ensure that they are deployed at sites, seasons, and sizes where good welfare and effective delousing is likely. 

To date, cleaner fish and salmon swimming depths have not been mapped simultaneously using non- intrusive technologies. While Tully et al. (1996) re corded cleaner fish and salmon swimming depths simultaneously using SCUBA, evidence from other systems indicates that the presence of divers alters the behaviours of cleaner fish (Titus et al. 2015), and findings of SCUBA observations should be interpreted with caution. Non-intrusive echosounders have been used to record salmon swimming depths in experimental and commercial scale experiments for decades (see review by Oppedal et al. 2011a) and have been used to monitor salmon swimming depths when testing a range of new lice-prevention technologies (Stien et al. 2016, Oppedal et al. 2017, Wright et al. 2017). These data have provided a fundamental understanding of salmon swimming depths and how they can vary with light, temperature, salinity, oxygen, water currents, the entry of feed into cages, and the effects of lice prevention and control measures. In non- stratified conditions typical of coastal sites, salmon typically move to the surface to feed. Throughout the day, they can be found swimming deep within the cage, before ascending to swim in shallow surface layers at nighttime. Depth-based variations in temperature, salinity, and oxygen levels modulate this overall pattern. The consequence is that salmon rarely distribute evenly in a cage volume, instead packing into specific depth layers at swimming densities that exceed their stocking densities (typically 1.5−5× but up to 10−15×; Oppedal et al. 2011a,b). If the swimming depths of salmon do not coincide with the preferred swimming depths of stocked cleaner fish, encounters will not occur, and lice cleaning efficacy will be diminished. This effect is likely to be exacerbated in larger cages.

As yet, echosounders have not been used in conjunction with technologies and techniques that could also monitor cleaner fish depth (e.g. PIT tagging, Nilsson et al. 2013; 3D acoustic tagging, Leclercq et al. 2018) to provide quantitative data on depth distributions of cleaner fish and salmon. Understanding swimming depth preferences of both cleaner fish and salmon when stocked together is key to understanding the likelihood of encounters, as certain environmental conditions (e.g. exposed coastal site or brackish water) and the use of cage manipulations (e.g. control and preventative treatments) may curb cleaner fish efficacy. 

4.3. Recommendations for experimental design and new measures to monitor and improve cleaner fish efficacy 

Our systematic review revealed important gaps in the knowledge base underpinning the use of millions of cleaner fish. Here, we offer several recommendations and highlight areas of research that warrant further investigation to optimise cleaner fish use in commercial settings. 

4.3.1. Replicated studies 

Multiple replicate sea cages are needed to provide a rigorous estimate of cleaner fish efficacy. Regardless of experimental scale, studies should strive to have 3 replicates per treatment as an absolute minimum. More is preferable, as sea lice infestation pressure varies considerably between cages, both within and between farms. Given the scale of cleaner fish use in the salmon aquaculture industry, conducting studies that have enough replicates is essential when testing cleaner fish efficacy. 

4.3.2. Cage volume and depth 

There has been one published study that has documented the efficacy of one species of cleaner fish at a large commercial scale (i.e. lumpfish; Imsland et al. 2018). Cleaner fish stocked in commercial farms are kept in cages that are much larger and deeper than cages commonly used for experimental trials. Working at a commercial scale is expensive and logistically difficult in many cases, which may largely ex - plain why small-scale studies have dominated in the proof-of-concept phase of developing cleaner fish as a biological control. However, the current mass use of cleaner fish by industry requires the promising proof-of-concept data to be benchmarked in experimental units that reflect modern commercial conditions. We recommend that researchers seek opportunities to partner with commercial actors that are al ready using cleaner fish in commercial-scale farms and apply logical, well-replicated experimental de - signs in these settings. Such partnerships are possible and have delivered full production-cycle data on the efficacy of other types of anti-lice technologies (e.g. Geitung et al. 2019). 

4.3.3. Optimum stocking densities at commercial scale 

Imsland et al. (2018) found that pre-adult and adult lice removal efficacy by lumpfish was highest at 8% stocking density. There is not yet any published literature on optimal stocking densities of wrasse species at a commercial scale. Without clear guidelines that recommend stocking densities for wrasses, farmers may be stocking too little or too many, which could in turn compromise lice control within cages or drive unnecessarily high demand for cleaner fish. 

4.3.4. Visual acuity of cleaner fish 

Visual acuity, or the ability to perceive static spatial detail, is highly variable across fish species (Caves et al. 2017, 2018). Light spectrum sensitivity and contrast potential has been researched for ballan, corkwing, and goldsinny wrasse, as well as lumpfish (Skiftes vik et al. 2017), but no research has determined the most relevant visual trait for their role in salmon aquaculture: the distance at which they can identify sea lice on fish. The ability of cleaner fishes to detect attached sea lice at distance, and the role of environmental factors that reduce visibility, will be key factors in their lice removal efficacy. 

4.3.5. Efficacy and encounter rates under various environmental conditions 

Environmental conditions and physiological traits of fish will influence encounter rates between cleaner fish and salmon. For example, lumpfish are slow swimmers with low critical swimming speeds and low thermal thresholds, especially in warm water (15% mortality over 3 wk of acclimation at 18°C; Hvas et al. 2018). Further research is needed to understand how environmental parameters such as current velo - city, salinity, temperature, turbidity (particularly during spring algal blooms), and wave exposure affect cleaner fish performance. 

4.3.6. Acclimation prior to stocking 

As the use of cultured cleaner fish is expanding rapidly, many ‘naive’ fish will be stocked into salmon farms that have never experienced co-habitation with lice-infested salmon. This means that all learning of cleaning behaviour must occur after stocking, and if lice numbers are low and interactions with salmon are few, many cleaner fish may never learn to consume lice. Accelerating this process by acclimating fish prior to stocking may therefore prove useful. Preliminary evidence at a small cage scale suggests that lice cleaning behaviour can be learnt more rapidly if cleaner fish are exposed to lice-infested sal mon or fed live feeds before stocking (Gentry 2018, Imsland et al. 2019); this should be tested further at a commercial scale before implementation.

4.3.7. Interactions with lice-preventative technologies 

Increasing use of methods to prevent sea lice infestations such as skirts and snorkel cages may alter en - counter rates between cleaner fish and salmon. Skirts tend to cause salmon to swim deeper (Gentry 2018), but cleaner fish may prefer the sheltered conditions within the skirt. In a replicated experiment at a commercial scale, Gentry (2018) found that corkwing wrasse ate 10 times fewer lice when used in conjunction with skirts compared to cages without skirts. Increasing use of skirts and other barrier technologies will necessitate a greater understanding of this phenomenon to optimise cleaner fish hide deployments and other depth-related management. 

4.3.8. Interactions with lice control strategies 

Despite stocking cleaner fish, many farms rely on other control strategies to reduce lice numbers when legislated limits are reached. Most cleaner fish are either captured and removed from cages prior to these treatments or held at the opposite side of the cage away from the pumping point, possibly for later re-stocking in the same or nearby cages. Some cleaner fish may go through the crowding, pumping, and treatment processes along with the salmon. While chemotherapeutant-based treatments have de clined in use, they are still important in some regions, and there is some evidence that they can result in cleaner fish mortality. For example, Treasurer & Feledi (2014) recorded a 4% mortality rate of wrasse after a cage-based pyrethroid delousing treatment. Thermal and mechanical delousing measures are now the most common whole cage lice re - moval method applied in Norway (Overton et al. 2019), yet there are no data on their effects on cleaner fish efficacy in the weeks and months posttreatment. This is a clear area for experimental re - search to optimise their reuse and welfare. 

4.3.9. Cleaner fish welfare 

The high reported losses of cleaner fish and high incidence of diseases clearly indicate that the existing handling and treatment methods and environmental conditions do not fulfil the legal demands to secure fish welfare. While some studies have monitored or assessed aspects of cleaner fish welfare (e.g. Sayer & Reader 1996, Treasurer & Feledi 2014, Gentry 2018, Johannesen et al. 2018, Mo & Poppe 2018, Speare 2019), there is not yet a consistent welfare assessment used to document welfare within cages (as is done for salmon using the sal mon welfare index model [SWIM]; Stien et al. 2013). Further, while most cleaner fish experiments monitor cleaner fish mortality, there was no mandatory industry reporting of cleaner fish mortalities until July 2018 in Norway (Norwegian Ministry of Trade, Industry and Fisheries 2008). This is in contrast to the longterm mandatory reporting of salmon mortality in place in many jurisdictions (e.g. Norway, Scotland), which has led to new insights into the outcomes of lice control practices (Overton et al. 2019). The future reporting of cleaner fish mortalities and stocking numbers within farms could lead to similar insights to improve their current management and identify favourable and un favourable stocking conditions. 

Other pathways to improve cleaner fish efficacy are possible, such as selective breeding to improve lice predation behaviour and developing production methodologies to ensure healthy, robust fish that survive well in sea-cage environments, as outlined by Brooker et al. (2018) and Powell et al. (2018).

4.4. CONCLUSIONS 

The widespread use of cleaner fish (~60 million deployed per year) has outpaced the development of a robust evidence base to justify and guide their use. Current evidence, while clearly promising, is patchy in nature, and has been largely gathered in smallvolume experimental units that do not reflect the conditions within commercial cages where cleaner fish are used. Commercial scale experiments to groundtruth the promising results obtained at a small scale are logistically difficult, expensive, and create ethical challenges due to the use of large numbers of experimental animals. However, the level of investment now placed by the industry into cleaner fish demands a more critical assessment of their benefits at a commercial scale, and research efforts should reflect their stature within the industry. De tailed research on a species-by-species basis is required to determine optimal stocking conditions that elicit high encounter rates and cleaning behaviours. The outcomes of this research will enable the industry to use cleaner fish judiciously and strengthen focus on creating cage conditions that optimise cleaner fish welfare and performance.