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Limiting Parasite Pressure in Farmed Fish

The presence of parasites can result in delayed performance, the emergence of opportunistic infections and even death in farmed fish. Experience-sharing among aquafarmers and increasing research in feed additives are among the new ways to cope with this issue. How can parasite pressure be controlled in farms in order to allow for satisfactory growth performance and profitability?


Parasites are pathogenic organisms that live off their host’s resources. Fish parasites are often classified according to their lifestyle, as follows:

  • ectoparasites (external parasites) develop on the skin or gills of animals.
  • endoparasites (internal parasites), develop in the gut lumen or in other organs of farmed fish. They are therefore more difficult to detect, especially at an early stage.
     

How can the level of infestation in a farm be measured?

Sea bream gills The parasites that contaminate farmed fish belong to very different species: protozoa, crustaceans, flatworm, etc. Their pathogenicity depends on many variables such as the age of infested fish, their health and their living conditions.
Various indicators can help measure the level of infestation in a fish population. Prevalence (i.e. the percentage of animals infected) can be assessed from the number of fish examined. The intensity of infestation is calculated from the average number of parasites in an infested host. In almost all cases, determining these variables means that some fish are sacrificed in order to count the number of parasites they contain.

What threats do parasites pose in aquaculture?

Open systems, earthen ponds or sea cages are much more conducive to contacts between fish and parasites than recirculated systems, the latter being much more circumscribed. Yet recirculated facilities are still subject to exterior contamination: once parasites start filtering through them, it can become difficult to get rid of them.
Symptoms of infestations vary substantially from one fish species to the other and from one strain of parasite to the other. Depending on the parasite species and the health of animals, the prevalence and intensity of infestation, parasite pressure in a fish population can have different impacts:

  • decline in animal performance: the parasites develop by feeding on some of their hosts’ resources, thereby weakening them;
  • deterioration of the integrity of tissues onto which the parasites hang: these wounds constitute potential entryways for other pathogens;
  • death occurs in the most critical cases.
     

How to control the decline in performance resulting from parasite presence?

Favourable conditions for the development of parasites include:

  • high densities of fish: this context favors the transfer of parasites from one fish to the other;
  • the biofouling of farming systems due to the accumulation of organic wastes or fouling nets;
  • contacts with wild animals.

Since many systems use water that has potentially already been contaminated by parasites, filtration systems are frequently used. While these can prevent a fish population from being contaminated by large pathogens, they are inefficient in blocking the smaller parasites.
Managing parasitical pressure is complex, as shown by the recent difficulties experienced by the more mature aquaculture salmon industry in attempting to contain sea lice invasion.
Many products that work against parasites can end up being toxic to farmed fish and / or the fish farmers who use them. The environmental impact of these products also pose serious problems, as do their potential effects on those who consume them when the withdrawal date has not been respected.
In addition, parasites are pathogens whose life cycles are often complex. A product that is efficient at a given stage of development can be completely powerless if used against the same parasite at another stage. When that happens, treatments can only temporarily decrease the pressure.
It is therefore often necessary to apply more or less prolonged bathing treatments. While these actions can be difficult in above-ground ponds they are often impossible to undertake on sea cages.
 
Feed additives are another promising avenue for managing parasite virulence. Some of them have in fact demonstrated their ability to reduce parasite pressure and / or delays in performance. The possibility of incorporating treatments in the feed can allow avoiding manipulation of toxic products.
The sharing and pooling of experiences among farmers can be beneficial, as it provides them the opportunity to learn from each other. Exchanges between producers, technicians and experts involved in production chains can be successful as parasite problems often have regional impacts that have already been experienced by other farmers.
 

The TECHNA Group assists many farmers in their search for efficient methods of controlling parasite pressure. Our position at the upstream stage of the production process allows us to take a global approach to these problems. For more information please contact our experts!

 

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