On the so-called “Cobalt” Variant of Rainbow Trout

Abstract

The “Cobalt” or blue colored variant of rainbow trout are rarely found at trout farms located throughout Japan. Histological studies were made on the ventral brain regions of six such fish. It was found that the fish were abnormal because of the absence of a pituitary gland. They were further characterized by the lack of yolk deposition in oocytes and suppression of spermatogenesis, deposition of fat on intestinal organs, remarkable changes in the liver cells, retardness of growth and lack of black spots and parr marks on the body. Some of these changes have been found in hypophysectomized teleost fish of various species. The fact that this variant is able to live in fresh water for at least five years, that interrenal tissues and thyroid glands are histologically normal, and that growth and formation of circuli of scales occur normally, suggests that more detailed studies are required regarding the involvement of pituitary hormones in these biological phenomena.