Iodine deficiency cause hyperplasia (goiter) of the thyroid tissue. The cause is not always known. Some may be due to iodine deficiency (very difficult to produce) while others may be due to the affects of goitrogenic substances in the feed or to the presence of goitrogenic pollutants in the water.
Fatty
Acid Deficiency (Linolenic and linoleic acid deficiency)
Fish are capable of synthesizing most fatty acids but not the linolenic or lenoleic acid series. Deficiencies of these fatty acids lead to depigmentation, fin erosion, cardiomyopathy, fatty infiltration of the liver, and myxomatous degeneration of fat.
Ascorbic acid is an essential vitamin of fish. Deficiencies of this vitamin lead to poor wound healing, ulceration of skin on fins, hemorrhage, and skeletal deformity. This vitamin is very temperature sensitive and oxidizes readily in stored feed.
Vitamin E deficiency is associated with necrosis and degeneration of skeletal and cardiac muscle, steatitis, and lipoidal liver disease.
Pantothenic acid is a coenzyme need in the metabolism of fats and carbohydrates. Deficiencies lead to anorexia due to hyperplasia of the gill lamellar epithelium and fusion of secondary lamella (nutritional gill disease). Anemia is usually associated with the disease.
Methionine deficiency (primarily in salmonids) leads to reduced growth rate with the development of bilateral cataracts. It is felt that deficiencies of vitamin A and riboflavin also play a role in this lesion.