The Behaviour of Pacific Salmon Fry During Their Downstream Migration to Freshwater and Saltwater Nursery Areas

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Abstract
The downstream migration of sockeye, coho, pink and chum salmon fry is initially nocturnal and appears to be regulated quite precisely by changes in light intensity. Downstream movement is seen to arise from a displacement by the current when firm visual contact with fixed objects in the stream is lost. Once the migration is under way the distribution of the fry varies. The lateral distribution of pink and sockeye, but not chum and coho, was closely and positively related to current speed, above a threshold of 1.3 ft/sec (0.4 m/sec). Pink fry were found to be distributed throughout the total depth of water but greatest catches were made at intermediate depths. The negative response of fry to light appears to change after exposure to it, and pink and chum fry were found to extend their movements into and throughout the daylight hours where the migration route was lengthy. Feeding and schooling activity is probably associated with this change in response to light. Both pink and chum fry were observed to school only near the end of their seaward movement. Pink fry were found to feed to some extent in the natal areas but to a greater extent as the sea was approached.

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