A laboratory study of the response to current of juvenile Atlantic salmon (salmo salar)

Abstract

Laboratory experiments were conducted in 1978 on rheotropic behavior of juvenile Atlantic salmon from Placentia Bay, Newfoundland rivers. Experiments were designed to investigate the role of water current in directing migrations and to determine the relative importance of environmental factors which may control migration. Smolts, precocious male parr and autumn immature parr were captured for this purpose during their normal periods of migration. Rheotropic behavior was quantified by exposing subjects to current in an experimental flume. Trials were conducted during spring-summer for smolts and autumn for parr. -- For smolts, temperature, light intensity and salinity were selected as experimental variables in a fully crossed experiment. Temperature and time were the only variables in parr experiments. Directional preference, latency of the response and orientation of the downstream response were recorded for all trials. -- Downstream was the most prominent directional response for all juveniles. Response latency and orientation of the downstream response were generally in agreement with directional preference. Experimental findings were consistent with field observations from this and other studies. -- For smolts, experimental variables interacted in regulating rheotropic behavior. The upstream response occurred significantly more frequently in freshwater than at higher salinities. In freshwater latency of the downstream response was shorter where it occurred most frequently. Negative orientation of the downstream response (active movement with the current) occurred significantly more frequently in freshwater than at higher salinities. Positive orientation (passive movement) was more common at the higher salinities. -- For immature parr downstream was the only directional response displayed. This response was also most common for precocious male parr, but some upstream response also occurred. Northeast River precocious male parr showed significantly more upstream response at 8ﾟC than at 12ﾟC. Also for these precocious males, latency of the downstream response was significantly shorter later in the season and at lower temperatures. Orientation of the downstream response for parr was negative. -- Water current may be a directional cue in juvenile Atlantic salmon migrations. Environmental factors probably regulate rheotropic behavior and so control the timing and intensity of migrations.