While it is a pleasure to land a healthy, fully-scaled, fin-perfect fish it is unfortunate that you do sometimes hook one that is a sorry sight.
It is quite natural for fish to carry a wide range of parasites and diseases but these organisms rarely threaten their lives unless the fish are stressed. If the water in which they swim is polluted, for example, their internal resistance to disease is reduced.
Common fish parasites
A parasite is an animal which gets its food by living on or in another animal – known as its host. One of the most common skin parasites is the fish louse Argulus. This louse uses its piercing mouthparts to drill through the fish’s scales and skin and suck up the blood underneath.
Another familiar parasite is the fish leech. This worm-like animal uses a sucker at the front end of its body to feed on the fish’s blood.
In small numbers both of these parasites are harmless, but when large infestations occur they cause numerous puncture wounds which can allow other diseases to enter.
Among the largest internal fish parasites are tapeworms. These live in the fish’s intestines. Roach and bream often become infested with Ligula – a white, ribbon-like tapeworm which can grow to an incredible length. Infected fish are often slow swim- ming and their distended bodies present and easy target to fish-eating birds – the parasite’s final host.
Viruses and bacteria
Most of the serious disorders in fish are caused by viruses and bacteria. They can spread directly from one fish to another, making them difficult to control.
Common bacterial diseases include tail rot – which causes the fins of infected fish to become ragged. Among viral diseases is Spring Viraemia of Carp (SVC) which has been responsible for the death of many carp in recent years. Symptoms usually include swollen eyes, swelling of the abdomen and a loss of balance.
Where diseases and parasites are concerned, prevention is much better than cure. The most successful means of prevention is by protecting the fish’s environment and managing fish stocks correctly. Problems arise when fisheries are overstocked and when fish are illegally introduced to waters. Fish should never be transferred from one water to another without permission, and a health check, from officers of the National Rivers Authority.
Handle with care
Anglers can help to keep fish stocks in good condition by handling fish as carefully as possible. A good disgorger or a set of artery forceps are essential. If you use them correctly you can ensure that unnecessary damage to fish is prevented.
Fish slime (mucus) forms an important protective layer around the fish’s body. Handling a fish with dry hands or a cloth, or keeping fish in a net for too long, removes the slime and leaves the fish at the mercy of diseases and parasites.
Nature cures its own
Where fish are kept in confined quarters, such as aquaria or on fish farms, it may be possible to cure most of the common ailments by good husbandry or by drugs and chemicals. In fisheries it is rarely possible to cure fish diseases or parasites. Nature takes its own course, and it is unusual for fisheries to be subjected to prolonged problems.