by Adam Yeap, Majbritt Bolton-Warberg, Cecilia Souza Valente, Alex Wan and Simon J Davies, National University of Ireland, Galway, Ireland

 

Wrasses are a group of marine fish (Labridae) typically found on rocky reefs and coastal shorelines. In temperate waters, wrasse have historically been of little interest to local fishermen and have only occasionally been caught as a food source in certain European regions, for example in the west of Ireland (Darwall et al 1992).

These carnivorous grazers are often found feeding on epifauna and temperate European wrasse species have been observed to feed on ectoparasitic copepods, such as sea lice.

Two species of sea lice (Lepeophtheirus salmonis and Caligus elongates) parasitise Atlantic salmon (Salmo salar), with heavy infestations having a major impact on farmed salmon in particular (for example in economic costs, Abolofia et al 2017).

Although historically, chemical therapeutants have been used to manage sea lice on salmon farms, environmental concerns and resistance to treatments have resulted in the development alternative control measures, for example biological control such as wrasse.

Four main species of wrasse are used commercially as cleaner fish in European salmonid farming operations: Ballan (Labrus bergylta), Goldsinny (Ctenolabrus rupestris), Corkwing (Symphodus melops) and Rock Cook (Centrolabrus exoletus) wrasse.

These species are naturally distributed in the North-East Atlantic with Norway being the northernmost limit. They are diurnal, feeding during the day and their activity levels change with the seasons, with fish typically entering a state of decreased metabolism at lower water temperatures around five degrees Celsius (Sayer & Reader 1996) .

Sea lice and treatments

The effects of sea lice can be economically costly amounting to US$436 million as they damage the product and increase mortalities of salmon under production in open sea pens (Abolofia et al 2017).

In addition, traditional chemotherapeutics, such as SLICE, add substantial cost to the salmon production, both in monetary terms and time.

The cost of these treatments is greater than that associated with using cleaner fish, thus wrasse are seen as more economically viable (Liu & Bjelland 2014) to combat this issue.

Considerations in using wrasse for sea lice control

Stocking cleaner fish into salmon sea pens comes with many benefits, however, to achieve the optimum results, the following factors need to be considered:

• The size of wrasse used:Small sized individuals may be eaten by the salmon, or escape through the cage nets. A size range of 10-25 cm is often recommended (Blanco & de Boer 2017).

• The species of wrasse used: Currently, Goldsinny and Rock Cook are preferred during the early-life stages of sea salmon, whereas the larger Corkwing and Ballan are more suitable for salmon weighing between three and five kilograms.

• Temperature: This environmental parameter has the most influence on sea lice cleaning efficacy and wrasse survival. At water temperatures below 10 °C, metabolic activity in these wrasse species decreases along with cleaning behaviour. Mortalities can be high at less than four degrees Celsius

• Stocking density: It is advised for commercial ventures to stock a ratio of 5% wrasse however this is subject to the size (wrasse and salmon) and species of wrasse. This ratio of wrasse to salmon has been seen to maintain the sea lice population to less than one per salmon, when introduced in sea pens with lice free salmon (Skiftesvik et al 2013)

• Size selection: Larger sea lice (e.g. Adult stage 5-18 mm) are actively selected, due to visual limitations of wrasse in spotting smaller lice individuals (e.g. nauplius stage 0.5-0.6 mm).

• Reuse issues: Currently, a cohort of wrasse can only be used for one production cycle. This is because the wrasse may also be affected by sea lice and could become a host.

• Feeding the wrasse: Regulations on sea lice populations in salmon cages requires they are maintained at a certain level (for example 0.5-1 lice per fish) which is too low to sustain the cleaner fish (Aquaculture and Fisheries (Scotland) Act 2007). Therefore, supplementary feeds must be supplied, in the form of pelleted feeds or water-stable agar-based feed blocks. The diet should be around two percent of the weight of the fish every other day (Brooker et al 2018).

Ballan wrasse has the greatest potential

Of the four wrasse species used in salmon farming as a cleaner fish, Ballan is considered to have the greatest potential in large scale application, due to their larger size and robust nature. Therefore, they can delouse larger salmon without the risk of being predated (Blanco & de Boer 2017).

Wild catches do not, however, meet commercial demand (Skiftesvik et al 2013), thus Ballan wrasse are farmed in a number of countries including Norway, Scotland and Wales. From a few thousand in 2009, it is reported that Marine Harvest Norway produced four million fry in 2016 (Treasurer 2018).

Although questions have been raised regarding the efficacy of cultured versus wild Ballan wrasse at sea lice removal, a study by Skiftesvik et al (2013) showed similar efficacies at limiting sea lice without affecting salmon growth.

During the larval rearing stage of ballan wrasse, they are often fed with an enriched diet of copepods and/or artemia to ensure that the best lipid and essential long chain fatty acid profiles meeting the requirements of this marine species.

After 70-90 days, wrasse are weaned onto formulated feeds using dry marine micro diet formulations. There are reports of the development of skeletal deformities and/or caudal fin damage during the weaning process, as a result of nutritional deficits of the feed or the digestive capacities of the agastric ballan wrasse (Treasurer 2018).

This may impact on their capacity to adapt to an open sea pen environment upon transfer to sea and much research has been undertaken to establish optimal nutritional requirements for this species. Once achieving approximately 20g (9-12 months post hatching), the juvenile wrasse can be used in sea cages (Treasurer 2018) and is imperative that fish of the highest condition are employed.

Resolving the wrasse supply problem

For salmon farming to grow in a sustainable manner, there must be viable methods of limiting the effects of sea lice on salmon. It seems that biological methods such as cleaner fish species are currently the most appropriate in terms of cost, efficacy and animal welfare. This is when they are compared to mechanical based techniques such as the use of heat treatment (thermomlice) or advanced laser technologies with inherent risk and costs.

Aquaculture itself could resolve the problem of supplying the salmon industry with sufficient numbers of wrasse. This is becoming even more likely with new diets and greater understanding of their digestive physiology and increased knowledge of their fundamental nutrition and feed preferences, with added emphasis placed on palatability.

Although proprietary diets are available from several manufacturers, these are more generic in nature and possibly do not fully meet the bespoke requirements of wrasse and may not reflect the optimum needs of this species.

It is imperative that more information is also provided concerning the histological level morphometric features of key organs and tissues to identify normal profiles of wild wrasse in comparison to farmed wrasse.

More work is needed to understand broodstock and larval nutrition to provide a more robust fish and development of increased disease resistance through improved genetics and selection.

As RAS land based systems are being further developed it is important to address the adaptation of wrasse to such systems and the effects of water quality and other abiotic environmental conditions on growth rates, feed efficiency and welfare.

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