Discrimination of Stream Odors by Fishes and Its Relation to Parent Stream Behavior

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Abstract
To explain the mechanism by which migrating salmon return to their parent stream, it was suggested that some characteristic odor of the stream guides the returning migrants. This hypothesis required investigation of the existence in streams of characteristic odors (organic, inorganic, or a combination of these) to which fish can react, and of the salmon''s capacity to detect and discriminate between such odors, if they do exist. To answer the first question, a conditioned response training program was started with the bluntnose minnow. The fishes were able to discriminate successfully between chemical differences in waters from two Wisconsin creeks after two months'' training. Extinction tests indicated that these minnows would respond to the stream odors after a "forgetting period," which was longer in fishes trained when young than in those trained in senility. Thermo cautery of the olfactory epithelium eliminated response to the training odors, proving that olfaction was the sole means of discrimination in these tests. Chemical analysis of the stream waters indicated a total absence of CO2; proving that this compound was not that which was detected. Fractionation of the stream waters proved that the fish did not react to the inorganic ash, or to the distillate or residue of water fractionated at 100[degree]C. They reacted to the distillate, but not the residue, of water fractionated by vacuum distillation at 25[degree]C; a strong indication that the odorous stimulant is a volatile, aromatic substance. Preliminary tests with salmon showed that they could detect the stream odors, and that they were able to discriminate between them. It was postulated that the nature of the guiding odor must be such that it have meaning only for those salmon conditioned to it during their freshwater sojourn. Any substance which was merely a general attractant could not guide salmon to their "home" tributary.