Sea lice, farmed salmon and the chemicals story., Campbell J. M.

Animal husbandry incorporates disease control whether in human groups, farming practices or wild animal populations.  We are dependent collectively on effective control of disease vectors in our society either through containing the carriers of these disease vectors or protecting ourselves from the carriers of disease or infestation. We owe our very survival, lengthening life expectancy and quality of life in our modern society to biologically active chemicals such as antibiotics and anti-viral compounds that we have either isolated from nature or synthesised in drug company laboratories. There is a wealth of knowledge to show that nature uses a vast array of chemical and biological protective mechanisms to combat predators and invasive species.


We frequently use the word ‘resistance’ to describe our lucky escape from picking up common cold and flu infections, by which we mean that our immune system, our natural defences, are coping, perhaps reinforced by our winter flu jab. The fact that this back-up inoculation needs to be modified each year to address new strains of the virus illustrates that disease vectors are capable of defeating our treatments and the reason for this is largely down to evolution or as we often refer to it – survival of the fittest.  In any population of living organisms there are always going to be survivors who are capable of resisting any onslaught because they possess a genetic make-up that is not found in most of the population.


The smaller and more short-lived the organism and the faster the reproduction the more likely resistance to a particular disease or toxic vector is established over short lifetimes, certainly within years. Examples include myxomatosis, a viral disease affecting rabbits, and malarial parasites carried by mosquitoes. The survivors of myxomatosis, very few rabbits are resistant, live on to breed that resistant strain in the population until the disease vector itself transforms or evolves to take advantage of the next growth cycle in population.


The same applies to the malarial parasite and its capability in population terms to develop resistance to each new drug designed to combat it in the body. This again is simply evolution working with new generations rapidly being selective of the genes of resistance or of defeating resistance.  Those people who visit malarial regions of the World regularly say that with every new drug released the side effects of taking them are becoming harder to bear as the prescriptions require ever more reactive substances to eradicate the evolving parasite.


How is all this relevant to fish populations, fish farming and the marine environment ?  Just before we tackle this question there is one more point of general understanding to take on board. Dog eat dog is how most of us see the natural world functioning, what TV wildlife documentaries refer to as the food chain. The interdependence of nature is to others a complex world, a dynamic ever changing world characterised by unimaginable biodiversity, each organism whether plant or animal dependent on its surroundings and an overall ecological balance governing its ability to survive and thrive. Upset that dynamic balance and survival is either threatened or an opportunity to thrive even better is presented. We see that graphically illustrated in the Firth of Clyde where the higher predator fish have been removed, the balance of marine life distorted by breaking the natural food chain and as a result only prawns doing well because of this breakdown in the natural ecological balance.


We as humans have learnt to thrive by modifying the biodiversity that surrounds us. Intensive farming for example provides us with food resources allowing populations to live in cities of high population density but at increased risk resulting from two factors, both related to concentration and proximity. By living together we increase the possibilities for transfer of disease factors. In the same way by sowing single crops in so-called monocultures or raising animals for food in intensive rearing we increase the vulnerability of that crop or animal herd to invasion by disease. The results are plain to see, the grim history of tuberculosis in late Victorian Glasgow, the spread of bird flu and salmonella food poisoning in battery reared chickens, foot and mouth disease in cattle.


We may have learnt to thrive but as the Black Death of medieval times shows us we are still here because of survival of those few fit enough and appropriately genetically equipped: Ever more so in our modern age through the efforts of our scientific and medical researchers, pharmaceutical companies to keep one step ahead of the ever evolving disease vectors, a frightening thought for some particularly with all the publicity about the battle against antibiotic resistant bacteria in our hospitals.


So it is with the intensive rearing of fish in fish farms. The high population density of fish means that disease is more easily spread than in the more diverse, less populated natural environment. To counter this disadvantage intervention in the form of disease vector containment, prophylaxis in the jargon, and direct treatment are taken. These in turn inevitably lead to selective survival of those organisms that are genetically or environmentally resistant to the counter-measures. These resistant populations then multiply presenting a new threat requiring new countermeasures. This then is the cycle of treatment effectiveness and resistance well known in the sphere of public health.


There are however differences which are frequently fundamental concerning the continuing efficacy of treatment. We know that overprescribing antibiotics hastens the build-up of resistance in attacking organisms. As a result doctors are now much more cautious about prescribing, especially if the purpose is disease prevention rather than treatment of existing infection. In many cases this caution is not seen in terms of farm animal husbandry. Profit has been put before long term sustainability with high density of animals on farms or fish biomass stocking in aquaculture requiring excessive application of biologically active chemicals to contain disease. This in turn accelerates the development of resistance in invading organisms such as sea lice and hence the constant search for more effective prophylactic measures often at ever increasing contingent environmental risk.


So what are these chemical agents that are currently used in fish farms, how effective are they, and what are the environmental risks ?  We can say with absolute certainty that it would be impossible to stock a salmon fish farm without applying chemicals.  And don’t think for one minute you can get away from this dependence by choosing ‘organic’ labelled farmed salmon.


There are essentially two categories of chemicals used


  • Hygiene products for disinfection like most of us use in sinks and toilets
  • Chemotherapeutic agents to treat or prevent disease which are designed to act on the invading organism directly or drugs which address the target invader from inside the fish being reared.


As noted in our previous newsletter sea lice are a particular focus as they can affect the very viability of salmon fish farming. Sea lice as described in our last newsletter are parasitic to salmon and take specific advantage of dense populations of fish reared in aquaculture to invade and multiply. Chemicals are deployed to contain these infestations by destroying the parasite or treating the salmon. This is achieved by either so-called bath treatment akin to adding chlorine to a municipal swimming pool or adulteration of food fed to the fish applying the same principle as adding fluoride to drinking water supplies or antibiotics to animal food. The more targeted the treatment the less chemical is transferred to the surroundings as an environmental pollutant. However by adding chemical agents to food it means they are ingested by the fish constantly whereas adding chemicals to the cage waters means that formulations can be selected simply to attack the sea lice as external parasites. In this way the treatment is transient and excess chemical is very quickly flushed out into the wider surroundings by tidal movement and current flow.


Like treating fruit in an orchard with pesticides there is the issue of residual chemicals in the food being produced, real or perceived by the consuming public. But there is also a huge issue of contamination of the marine environment around aquaculture developments and the impact such biologically active chemicals have on marine life in general. As each year goes by so the invading organisms become more resistant to the chemicals being used and so they have to be replaced by ‘stronger agents of destruction,’ this year’s model compound, as is well illustrated by the constant chase to find new and more efficacious anti-malarial drugs.


What chemicals are commonly used in salmon fish farms today and which ones should we be concerned about ?  The most common are :-

  • Hydrogen peroxide A strong inorganic oxidizer more well known in hair salons
  • Dichlorvos (DDVP) A organophosphate used as a non selective insecticide
  • Azamethiphos An organophosphate like those once used in sheep dips
  • Cypermethrin A synthetic pyrethroid, similar to the active compound in many garden pesticides
  • Teflubenzuron  (Calicide) This substance inhibits or disrupts the hard protective shell growth of sea lice and indeed any crustacean
  • Invermectin (SLICE)
  • Patented premixed fish food containing avermectins A broad spectrum antiparasitic agent used for both human and animal treatment. Traditionally applied to deworming

The list can never be exhaustive and the industry is always on the look out for more effective treatments, the treatment scenario being one of progressive build-up of resistance to known chemicals and their replacement by newer approaches.  Bath treatment by hydrogen peroxide simply dislodges parasites from their host and reinfestation is common.  Dichlorvos bath treatment is now generally considered to be ineffective having been replaced by azamethiphos which is also losing its effectiveness. Similarly pyrethroids such as cypermethrin show the same trend. Integrated feed treatments incorporating invermectin or SLICE, essentially a nerve agent like those capable of being deployed in chemical warfare are most popular at present along with growth inhibitors such as teflubenzuron that interfere with the protective hard shell formation of crustaceans like sea lice in the early stages of development.


If all this sounds a bit scary then you are right to be concerned. You are in good company too !  These compounds are all toxic to a degree. That is of course their function in use.  This is where the toxicological assessment process kicks in with all its investigative techniques, interpretive procedures and reporting protocols. There are human health risks in their use to assess as well as impacts and risks to the natural environment, the latter known as ‘ecotox assessment’.  There is very rarely any simple outcome to these studies and acceptance is a matter of judgement. Just think about arsenic. It is judged to be a deadly poison today but it was not long ago that arsenic was taken internally as a purgative with what were thought to be beneficial results.


The bottom line is that in most cases not enough is known and as a result the precautionary principle is brought into play. Just to take one example, have a look at SEPA’s own guidance note referring to a newish compound called Panacur whose chemical name is fenbendazole. Once in the website you need to search for the fish farm manual and Attachment VIII in the manual. After reading you can make up your own mind whether enough is known about this compound to justify its use even under SEPA’s recommended ‘very precautionary approach’

Then take a stroll through the US-Environmental Protection Agency”s ECOTOX database by examining what is said in each case about the typical treatment chemicals used in fish farming today and you immediately understand the dilemma of the industry regulators.

The outcome of an ecotox risk assessment leads to some form of agreement over an EQS, an Environmental Quality Standard, a maximum concentration in water of acceptable risk, a figure generally reached through a learned science panel or committee agreement.  You can appreciate the workings of this process by example stated in SEPA Policy 17 on the EQS for azamethiphos.

This then enables the regulatory process to come into play with a conceptual perimeter or boundary being drawn around the fish farming operation such that EQS values should not be exceeded throughout the water column ( a vertical line down to the seabed) at that boundary.  The principle of environmental risk management being practised here is one of sufficient dilution and dispersion of chemicals within this notional perimeter to ensure that concentrations of substances at the boundary are at or below maximum permitted levels to achieve no detectable harm. It all sounds very reasonable but we can never know for certain. A risk assessment is only as good as the data available to back up the judgement.  It is well known that sometimes more than one ecotox vector can act synergistically, in other words magnifying the impact well in excess of effects known to be associated with individual elements.


If there is any final word to be said about marine fish farming, sea lice and chemicals it is that the dilute and disperse approach to limiting environmental impact that is tolerated here has long since been abandoned as acceptable in land based situations. In the face of uncertainty on land a near zero discharge would likely be demanded.


So there it is, the story of an ever more demanding battle to contain large populations of progressively evolving sea lice infestation – a fight that the industry and its pharmaceutical researchers and suppliers could ultimately lose. In the meantime we continue to ponder the impact these chemicals are having on the marine environment and indeed on the fish product despite all the regulations governing their use.

We will examine this uncertainty in a third paper on the wider implications of sea lice infestation.

J M Campbell

This is a personal review and analysis presented by the author and is not intended to represent the formal position of COAST

Useful reference : SEPA Report – Regulation and monitoring of marine cage fish farming in Scotland – A procedures manual.