Fish Health

Like any livestock in commercial production, fish are sometimes exposed to bacteria and viruses that, combined with stress, can lead to disease in individual fish or in disease outbreaks among a number of fish. Since fish on a farm or in a hatchery are in close proximity to other fish, some diseases can easily spread from one fish to another.

The health of fish can also be compromised by the presence of harmful algae or contaminants in water, as well as by pollution from a variety of sources.

It is in the best interest of the fish farmer to do whatever can be done to ensure that the fish on a farm are healthy, and that the conditions within the farm are as good for the fish as possible. For example, the fish farmer must know how many fish he can keep in an individual pen without stressing the fish excessively. Fish don’t like to be alone, but prefer to swim in schools. On the other hand, the fish don’t like to bee too crowded either, so experience and knowledge of the farmer is very important. Some fish are more tolerant to high densities than other, and some salmon species need more room than other species. Chinook salmon in B.C. farms for example, need twice the space in their pens compared to Atlantic salmon. And Atlantic salmon need three times the space compared to Arctic Char.

Salmon farmers in B.C. spend a lot of time and effort in preventing disease outbreaks on their farms. This begins with the fish used for brood stock. All brood fish are sampled for the presence of bacteria and viruses. When the eggs are taken from a female salmon, they are kept separate from the eggs from other brood fish, until the result of the sampling comes back from the veterinary laboratory that examined the samples. If bacteria or viruses are found in the sample, the eggs from that fish are discarded. The fish eggs are also put in a bath of disinfectant before being placed in the hatchery. This ensures that the young fish in the hatchery are free of disease pathogens. The process of taking eggs, milt and sampling of brood fish must be done in a way that eliminates contamination, and is therefore done by highly trained staff, using specialized equipment.

Most hatcheries use sterile ground water for their operation, and the hatcheries that use surface water normally disinfect their water before it goes into the fish tanks.

Before the fish leave the hatchery for the salmon farm in the ocean, the fish are vaccinated against the most common bacteria found in salmon in B.C. Recently, new vaccines have been developed against the IHN (Infectious Hematopoetic Necrosis) virus also, and they are being tested for the first time this year. Some of them have shown very good protection of Atlantic salmon against IHN in laboratory tests.

In the ocean environment there are many pathogens present, and usually the farm fish can be exposed to normal levels of bacteria and viruses, without getting sick from this exposure. The same goes for wild fish- sometimes they can be carrying bacteria or viruses, without showing signs of disease. Both farmed fish and wild fish can sometimes shed some of these bacteria and viruses into the water, and they can then be picked up by other fish. The bacteria and viruses usually can not live very long outside a host, however. Some species of salmon are more tolerant to pathogens than other species. .For example, Coho salmon is very tolerant to the exposure to most viruses, while Atlantic salmon is very sensitive to exposure to IHN virus.

If a bacteria causes a disease outbreak on a farm, it can be treated with antibiotics. Any treatment must be done after a veterinarian issues a prescription for the use of antibiotics. Before a prescription is issued, samples must be submitted to the veterinarian in order to ensure that the type of antibiotic used will be effective in controlling the particular bacteria on that farm. The antibiotic is then mixed with the feed given to the sick fish for a prescribed period of time, usually 10-20 days. The veterinarian also prescribes a period of time that must go by after the last day of treatment before the fish can be harvested for human consumption. This will ensure that there is no antibiotics left in the fish before it is slaughtered. Disease outbreaks don’t happen very often- usually the health of the fish on farms is good. Less than 3% of all fish food used in B.C. contains any kind of medication.

Antibiotics do not work on viruses, so there is no treatment other than practicing general good husbandry methods against viruses.

Parasites can also affect the health of the salmon in the wild or in farms. In salmon aquaculture, there are 2 parasites that have played an important role- the salmon louse (Lepeoptheirus Salmonis) and the gill parasite Gyrodactylus salaries. The latter has not been a problem in B.C., but it has attracted a lot of attention in Norway, where the government has used drastic measures in an attempt to eradicate the parasite from important salmon-producing rivers. Gyrodactylus was imported to Norway from neighbouring Sweden through the importation of live fish, and possibly also by the movement of sport fishing boats between Sweden and Norway. Further spread took place by moving enhancement fish from infected facilities to various watersheds. Infected fish from private hatcheries also escaped and caused infection in down-stream rivers. The eradication attempts were done when the rivers were as empty of salmon as possible, and then Rotenone was added to the river, essentially killing all parasite-bearing fish remaining there. This procedure has been successful in eradicating the parasite from some watersheds, but not in others. Recently, experiments have been carried out with less intrusive treatment methods, where only the parasite is killed, but not the fish.

The salmon louse is a common parasite in B.C. waters, and it can live on all species of salmon. Everyone involved in the capture of salmon in B.C. know that most fish caught in the wild have a few sea lice on them, usually located near the vent under the tail section of the fish. The female louse develops 2 long egg strings when it is mature, and these egg strings then shed the eggs into the water, where they drift with the currents and hatch to become small larvae. These larvae go through different life stages, until they are old enough to attach itself to a fish.

A Japanese scientist (Nagasawa) studied the presence of adult female salmon lice on Pacific salmon caught in the Pacific Ocean from the Bering Sea southwards, using long lines as a catch method. He found an average of : 5.35adult female lice on pink salmon, 0.08 on sockeye, 1.02 on chum, 1.38 on Coho, 2.3 on Chinook and 3.6 adult female lice on steelhead trout. This and other studies show that it is normal for healthy Pacific salmon to have a few lice attached to them.

Studies are now underway to look at number of sea lice on wild and farmed salmon in coastal B.C.. Information about lice on wild fish can be found on DFO’s web site:
http://www-sci.pac.dfo-mpo.gc.ca/mehsd/sea_lice/2004/2004_intro_e.htm

Monitoring results of sea lice levels on farmed salmon can be obtained from the Ministry of Agriculture, Food and Fisheries at : http://www.agf.gov.bc.ca/fisheries/health/sealice_monitoring_results.htm

Since the eggs and larvae from the adult, female lice drift in the ocean before attaching to a passing salmon, lice can be transmitted from wild fish to farmed fish and vice versa. The farm fish don’t have lice on them when they come from the hatchery, as lice doesn’t live in fresh water. There is little that can be done to control the amount of lice that you find on wild fish, and a few reports have shown high mortality in returning salmon in B.C. due to high sea lice numbers. Farmers can treat their fish for lice, if it becomes a problem. This is done by mixing a lice medicine with the fish food, just like antibiotics are mixed with the fish food to treat bacterial diseases. The most commonly used medicine is Emamectin Benzoate, sold under the trade name Slice®. It is very effective, and studies have shown that it is not harmful to other marine life when used as prescribed. A more persistent medicine, Ivermectin, was used in the past, but it is very seldom, if ever, administered to fish in B.C. now.

There is also another species of lice that is sometimes found on salmon, and it is called Caligus Elongatus. It usually doesn’t bother the fish, and it is more mobile than the Lepeoptheirus- it can easily move from one fish to another. It is often found on fish species other than salmon. A large study in the Broughton Archipelago in 2003 by DFO showed this species of lice to be the dominant species of lice on juvenile outmigrating Pacific salmon that year. Many of this type lice were found on other species of fish at the same time, especially on herring and sticklebacks. This year’s studies show a majority of Lepeoptheirus on these fish. Most of these studies are designed to get a better understanding of the role lice play in the survival of juvenile, out-migrating salmon, and also to establish the mechanisms around transfer of lice from farmed fish to wild fish, and vice versa.

The two species of lice are difficult to tell apart, especially during the younger life stages.