Retinal Responses of Pink Salmon associated with its Downstream Migration

Abstract

THE seaward migration of juvenile Pacific salmon is usually nocturnal and confined to a relatively brief portion of the night¹. It has been suggested that “as the light intensity falls rheotactic responses, which are to a large degree dependent on vision, fail, and these fish pass down stream in shoals. The fact that such mass movements occur during a rather precise period of the night is probably due to the dark adaptation of the eye and a period of night blindness”². In support of this view, a recent histophysiological examination of the retinæ from several species of Oncorhynchus has shown an incompletely dark-adapted condition of the retinæ at the time of most active downstream migration^3,4. As a further test of this hypothesis, the retinæ from fish killed at different light intensities during the evening and early morning have been compared with the retinæ from fish which were completely adapted to the same light intensities in the laboratory. The pink salmon fry (Oncorhynchus gorbuscha), probably the most specialized of the downstream migrant salmon¹, was selected for the study.