The use of cleaner fish has grown rapidly in Norway and the UK over the last few years. Last year, over 25 million cleaner fish were used on Norwegian salmon farms.
However, cleaner fish can be carriers of pathogens and, with high levels of mortality still being reported pre and post deployment, more research is needed into the biology, diagnostics and vaccination of cleaner fish, and, how to reduce mortalities in both cleaner fish and salmon.
In Norway, the main pathogens carried by cleaner fish tend to be Vibrio anguillarum in wrasse and Atypical Aeromonas salmonicida in lumpfish.
According to a study by Lill-Heidi Johansen, Nofima, bacterial diagnostics of cleaner fish in 2015 found that there were 223 diagnostic cases in lumpfish and 69 in wrasse.
Completely different to the Norwegian pathogen findings, a recent study of cleaner fish in the UK showed high findings of A. salmonicida in wrasse and no cases of Vibrio anguillarum.
In lumpfish, no Atypical A. salmonicida was found.
Tim Wallis, Ridgeway Biologicals, discovered these findings during a study of moribund wrasse and lumpfish from UK cleaner fish hatcheries and sea sites, when trying to understand what is causing mortality.
The sampling identified 127 candidate fish pathogens, with the highest pathogen findings being collected from the liver and kidneys.
In terms of site, flow-through hatcheries had higher pathogen findings than RAS. The highest number of pathogens also came from the hatcheries rather than after deployment in sea cages.
The results therefore showed a different pathogen pattern in the UK to that seen in Norway. The results have also highlighted the need for vaccination in the hatchery and pre-deployment, said Mr Wallis.
So How Can We Reduce the Pathogen Risk?
One of the most important ways of reducing the risk of disease is to only use farmed and vaccinated cleaner fish, explained Ms Johansen. Screening the fish for pathogens such as Paramoeba perurans, that causes Amoebic Gill Disease, is also advised.
More effort should now be made into creating pathogen free broodstock, she added.
Ideally, no wild caught cleaner fish should be used but if they are, they should not be moved large distances or between regions, as this can transfer local strains of infections to other areas.
The movement and re-use of cleaner fish on another farm is also not advised as this can increase the chances of introducing new diseases.
If cleaner fish are reused, then this should only take place within the same farm. The health status must also be very good and quarantining should take place before re-use.
Alexander Murray, Marine Scotland Science, added in his presentation, that re-use should only be practiced for a limited time and under strict biosecurity.
He also noted that when moving cleaner fish, strict biosecurity should be practiced.
Selective Breeding Shows Promise
As mentioned previously, another way to improve the health of cleaner fish could be through selective breeding them for resistance to certain pathogens.
A study by M. Breiland, Nofima, challenged 68 lumpfish families with Vibrio ordalii and looked at their survival.
The families were taken from northern and southern parts of Norway and each family contained 30 fish with a mean weight of 35 grams. The fish were housed in 15°C water.
Fifty per cent of the fish were injected with the bacteria and the other 50 per cent were infected through co-habiting in the water.
Mortality started after three days in the injected fish and after 10 days in the cohabiting.
The study showed a large variation in mortality between the families which is an important result for the potential of selective breeding.
From the findings, it can be seen that selective breeding for resistance to Vibrio ordalii is possible, said Ms Breiland. The results were also in line with similar studies in Atlantic cod, she added.
