What are fish abnormalities?

Abnormalities in fish include any changes that are obvious by looking at the outside (external) or inside (internal) of the fish.  External abnormalities include growths, lesions, wounds, unusual scale patterns, body colour changes and physical deformities such as curved spine or blindness. Examples of internal abnormalities include such things as changes in the liver, the intestines, the reproductive organs, or the presence of parasites.

Some abnormalities are due to natural causes and some can be caused by human activity.

Examples of abnormalities include:

• Growths or tumors (a swelling found on the body that may or may not be cancer).

• Lesions (red sores found on the body).

• Obvious colour changes in the skin.

• Wounds

• Unusual scale pattern.

• Frayed, torn or damaged fins.

• Damage to the eye (pop-eye, blindness).

Body deformities
• Curved spine, shortened body, missing or additional fins.

• Other deformities

• Parasites

Abnormalities may occur for many reasons:

• Injury caused by other fish, birds, mammals or humans (for example, motor propeller or gill net damage).

• Natural injuries from spawning for example, digging spawning beds.

• Infections from bacteria, fungus, viruses or parasites.

• The abnormalities is inherited. A certain percentage of any fish population will have genetic anomalies that cause deformities.

• Natural or human-induced increases in water temperature.  For example, increased temperatures may increase fungal or bacterial growth, which may cause more infections in fish.

• Chemicals in the water, found naturally or from people’s activities, may cause abnormalities to fish such as damage to gills or enlarged liver.

Because many of these reasons for abnormalities are not related to human activities, most fish populations have a natural number of anomalies. For example, 4% of Arctic grayling captured in an “unspoiled” lake in the Northwest Territories (Lac de Sauvage) in 1996 had growths on their bodies (Golder 1997a). In addition, 4% of lake trout captured in the same lake had a deformity including missing fins, shortened opercle (flap that covers the gills) and eye blindness (Golder 1997a).