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While our rivers, lakes and canals are susceptible to invasive species, they are equally vulnerable to a range of harmful diseases. It therefore makes sense for anglers to keep watch for both threats whilst simultaneously ensuring that we do not accidentally make the problem worse, which we could easily do if we were irresponsible in regard to our equipment.
Fish Diseases and Caring for Fish
Unless we take precautions, fishing tackle can help to spread both invasive species and diseases by transporting, unbeknownst to ourselves, spores, bacteria, larvae and eggs from one habitat to another. It is therefore essential that we thoroughly clean off all equipment that has come into contact with any body of fresh water, especially, but not limited to, those where disease, parasites or invasive species are known to be established. This includes boats, float tubes, flippers, waders, landing nets, carp mats, weigh slings, carp cradles, keep nets, wellington boots and any other equipment that routinely comes into contact with water, including both lures and the legs and sides of seat boxes.

It is also best to err on the side of caution. Just because a body of water is not known to harbour any particular threats, it is by no means certain that it is clear of them. It may well be that there is something wrong but that the problem has not yet been identified. To my mind, if we treat all waters as suspect and look after our equipment accordingly, then there is less chance of problems spreading from where they currently reside. Clean waters will continue to be clean and there will be less chance of threats becoming established elsewhere.

It is important to remember that both diseases and invasive species can be spread by damp angling equipment. Chemical dips - as long as they are using approved chemicals correctly – can help but one of the most effective controls is to make sure that all landing nets etc are thoroughly dried before being used again. Direct and prolonged exposure to sunlight is highly recommended with several sources quoting five days between use an ideal length of time to ensure harmful organisms are killed.

In regard to chemical dips, if they are not maintained correctly, they can become part of the problem. Excess dilution, e.g. from rainwater, reduces their effectiveness and, left untended, they can become breeding grounds for the very organisms that they are intended to eradicate.

As the water temperature rises in response to climate change, it is likely that the threat posed by some diseases will lessen but, unfortunately, the danger posed by others will increase. The risk of viral haemorrhagic septicaemia (VHSV) and infectious haematopoietic necrosis virus (IHNV) in particular may decline since these infections are usually only severe when water temperatures are less than 14°C. Spring viraemia of carp virus (SVCV) can survive up to 17°C but, as our water temperatures soar, this too may be affected.

Unfortunately, the flip side of this coin is that a number of salmonid diseases (e.g. enteric red mouth, furunculosis, proliferative kidney disease and white spot) will become more prevalent and difficult to control as water temperatures increase. Outbreaks of koi herpesvirus in carp fisheries are also likely to occur over a longer period each summer.

Diseases to watch out for include:

  1. The Rosette Agent (Sphaerothecum destruens)
A fungus-like parasite associated with the invasive species topmouth gudgeon, which can tolerate it. Mortality, when it spreads to other species, is high and estimated at half of infected stocks.

Species Affected: Salmon, Brown Trout, Carp, Bream, Roach

Symptoms: Once infected, the parasite spreads to multiple organs including the liver, kidney, gonads, intestine, and gills. The spores spread to other fish via urine and seminal fluid.
Topmouth gudgeons can tolerate the rosette agent but unfortunately can also spread it quickly to other fish species.
  1. Bacterial kidney disease (BKD)
This is a chronic bacterial disease first reported in wild Atlantic salmon populations in the rivers Spey and Dee. (A long time ago – 1933.) Outbreaks can occur throughout the year, but generally accompany rising water temperatures in the spring.

Species Affected: Salmon

Fish may sometimes exhibit protruding eyes (known as exophthalmia), darkening of the skin and haemorrhage at the base of the fins. The gills may appear pale and anaemic and internally there may be fluid in the abdominal cavity while the kidney may also be enlarged. Cream or grey coloured nodules are sometimes found on the kidney, spleen, liver and heart.

  1. Infectious haematopoietic necrosis virus (IHNV, causative agent of IHN)
This is an infectious viral disease of salmon and trout. Officially, Britain is ‘declared free’ of it. However it is one that we should be aware of since juvenile fry sometimes survive it and go on to become carriers in new populations elsewhere.

Species Affected: Salmon, Rainbow Trout, Most Salmonid Species

Symptoms: Signs of this disease include lethargic fish showing occasional bouts of unusually frenzied activity before death. The fish may have protruding eyes with pale gills and haemorrhaging at the base of the fins. The abdomen is often swollen, may be dark and have a length of trailing mucus from the vent. Inside, the fish will appear pretty anaemic with a mucus-like fluid found in the digestive tract rather than food.

  1. Koi herpesvirus disease (KHV)
This is a serious viral disease that is listed in the United Kingdom. It affects all varieties of common and ornamental carp species (Cyprinus carpio) and can result in serious numbers of fatalities in infected waters. The infection thrives in water temperatures between 16 and 28 degrees.

Species Affected: All varieties of common and ornamental carp species and carp hybrids.

Symptoms: Signs include necrotic (white or brown) patches on the gills, rough patches on the skin and shedding mucus. The fish may also have sunken eyes.

  1. Viral haemorrhagic septicaemia (VHSV)
This is a highly contagious virus causing high mortality in a wide range of both marine and freshwater fish species. It is most often associated with rainbow trout but has spread and has even been recorded in farmed turbot. It has also been recorded in silverfish, pike, grayling and many other species (82 in total). Like our own recent experience of Covid, it is a disease that adapts to its surroundings and produces variants in the process. It is believed to be spread via urine released into the water by infected fish.

Species affected: Rainbow trout, brown trout, pike, grayling, cod, silverfish, sea bass, turbot etc (82 species in total recorded so far). Bass and turbot are particularly susceptible to it with up to 90% mortality recorded in experiments linking the infection to waters where the sea temperature is less than 15 degrees.

Symptoms: External signs include darkening of the body, pale gills, protruding eyes – exophthalmia - hyperactivity, erratic swimming and haemorrhages in the skin and gills. Internal signs include swelling and discolouration of the liver and kidneys.

  1. Spring viraemia of carp virus
This is an infection capable of inflicting an acute haemorrhagic (leading to haemorrhages) and contagious viraemia (virus present in and spreading throughout the blood stream) in several carp species.

Species affected: Common carp, koi carp, grass carp, crucian carp, goldfish and tench.

Symptoms: In carp, the most common symptoms include swelling of the abdomen, protruding eyes, inflammation or swelling of the vent (caused by fluid and is often accompanied by trailing faecal matter covered in mucus). Tiny, round spots that appear on the skin as a result of bleeding (haemorrhages) are present on or in the skin, gills and eyes. The body is also often darkened while the gills are pale.

There are a number of other diseases that I could mention but their status seems to fluctuate. For many, our waters are declared free of them but an outbreak could occur as a result of recent, asymptomatic fish imports or even illegal fish stocking which, unfortunately, does happen from time to time. One that is worth mentioning is Fish Tank - sometimes referred to as Swimming Pool - Granuloma (Mycobacterium marinum) which, if the temperature rises, may become a problem for cyprinids such as the common carp (Cyprinus carpio) and goldfish (Carassius auratus) in ponds or elsewhere. Other species are also likely to become infected since this bacteria has the capacity to jump species and may even infect humans, where the infection appears as lesions on the skin that take a long time to heal. It appears to favour stagnant water but we cannot be complacent since it has also been found in two Austrian rivers.

What can we, as anglers, do about fish disease?

In addition to maintaining the cleanliness and infection-free status of our equipment, there are a couple of common-sense things that we can do, the first of which involves the capture of an infected fish or the netting of a clearly diseased fish corpse from the water. Take a photograph of it on your mobile or camera and check it at home to see if the disease that killed it is identifiable and potentially dangerous. If it is, then report it and advise the EA – and, if applicable, the fishery owner – of the location where it was found.

If the fish is dead, the EA will probably want you to take it home and freeze it until they can collect it. In my case - and I am sure in many others - this would probably lead to a short and possibly rather loud interruption to domestic tranquillity. Not everybody is comfortable with a dead, diseased fish in the fridge. Personally, I would probably use a knife and open the stomach cavity to photograph the internal organs, all the while maintaining proper hygiene for my own protection (I carry a pair of disposable gloves with me, just in case). It is not very nice but sometimes the internal organs can tell you more about the disease than the outside of the fish. You will also need to dispose of the corpse as far away from water as possible. You don’t want to run the risk of contaminated fluids re-entering the water to infect the rest of the shoal.

If the fish is diseased but alive, telephone the fishery owner and ask him or her what to do with it. You will obviously need to keep it alive in a container of some sort while they decide what to do but they could decide to retain the fish for dissection and autopsy, quarantine it or even kill it to reduce the chance of the disease spreading. There is also the chance that the disease is a parasite that got a hold on its host through injury rather than infection. Check with the owner first before you do anything drastic.

The second thing that we can do relates to potential injury. As anglers, we can minimise the chances of the fish that we catch becoming infected by treating any wounds that they gather in the fight. Barbless hooks are easy to slip free, which is good, but we could also treat the site of the injury with a fish antiseptic spray, a number of which are now coming onto the market. We also need to minimise the chances of the fish injuring themselves by flapping around on the bank after capture. Unsupported by dense water, they run the risk of breaking ribs or other bones, which can lead to internal bleeding and infection.

A carp unhooking mat reduces this possibility while a carp cradle does it even better. For myself, if I am not going to record the weight of the fish, I will often kneel or even lie down on the bank to slip the hook free while the fish is still in the landing net in the water. A gentle lift gives me access for the antiseptic spray and I can then release it unharmed.

Don’t become carp myopic either. My carp cradle doubles for pike, tench or indeed any larger fish. I spray it afterwards with very hot water, wipe it over with disinfectant and then place it where the sun can dry it for a period of at least five days.
Module 1: Fish Biology
Module 2: Invasive Species
Module 3: Fish Diseases
Module 4: Topography - Lakes
The Sea
Module 5: Angling and Mental Health