Histological changes in the pituitary gland of the pacific salmon (genus Oncorhynchus) accompanying sexual maturation and spawning

One of the most remarkable phenomena in the whole realm of vertebrate biology is the regular occurrence of death of all adult Pacific salmons shortly after their first spawning. Other closely related anadromous fishes, the sea-run rainbow trout (steelhead) and the Atlantic salmon are capable of reproducing repeatedly, although most of them fail to return for a second spawning (Robertson and Wexler, '62). Physiological adjustments required in transition from salt to fresh water can be excluded as the principal lethal factor in this fish, since the kokanee (0. nerha kennerlyi), the landlocked offshoot of the blue back salmon (0. nerka nerka), which spends all its life in fresh water, migrates upstream from its resident lake to spawn and dies shortly thereafter . Likewise, physical influences such as stress from long migrations, discussed in a former paper (Robertson, '57) appear to bear no essential relationship to the initiation of the series of progressive degenerative changes which follow the spawning act. The first definite suggestion that variations in the functional activity of the pituitary gland might play an important role in this process came from an observation made by Roger E. Burrows of the U. S. Fish and Wildlife Service during the course of experiments on artificial acceleration of gonadal maturation in blue back salmon by means of pituitary gland injections. Certain of the injected fish, especially those receiving the larger doses 6 to 8 pituitaries exhibited all of the characteristic changes seen in the spent salmon under natural spawning conditions, e.g., deterioration of the skin with focal necroses and fungus infection, loss of muscular power and balance, and death before full gonadal development had been reached (personal communication). A second finding which pointed to distinctive activity of the pituitary gland at the time of sexual maturity in the adult salmon was the observed difference between the appearance of the precociously mature male king salmon p a r which occasionally occurs before a year of age, and the adult male normally spawning at the end of 4 years. The latter fish are characterized by a hooked jaw, hump back, brilliant coloration and absorption of the scales. The sexually ripe p a n show none of these changes and they survive following expulsion of milt, proportionally much greater in quantity than that expelled by the adult male (Robertson, '57). Presumably the increased pituitary hormone production in the small fish is limited principally to gonadotropins. Thirdly, the occurrence of cellular changes in the pituitary gland of the spawning rainbow and steelhead trout (Robertson and Wexler, '62) indicative of greatly increased functional activity accompanied by evidences of degeneration, suggested that the stress of reproduction had produced an overly intense stimulation of the pituitary. Furthermore, pituitary degeneration was most pronounced in steelhead trout which were held in captivity following spawning. These fish