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

1. Various theories have been advanced to explain the mechanism by which migrating salmon return to their parent stream. One of these postulates the presence of some characteristic odor of the stream which guides the returning migrants. This theory presents two distinct problems: (1) Do streams have characteristic odors to which fish can react? If so, is the odor organic or inorganic in nature, or a combination of both? (2) Can salmon detect and discriminate between such odors, if they do exist? 2. In an attempt 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 of two Wisconsin creeks after two months' training. 3. Extinction tests indicated that these minnows would respond to the stream odors after a "for getting period," which was longer in fishes trained when young than in those trained in senility. 4. Heat cautery of the olfactory epithelium produced fish which were no longer able to respond to the training odors; proving that olfaction was the sole means of discrimination in these tests. 5. Chemical analysis of the stream waters indicated a total absence of CO2; proving that this compound was not that which was detected. 6. 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⚬C. They reacted to the distillate but not the residue, of water fractionated by vacuum distillation at 25⚬C.; a strong indication that the odorous stimulant is a volatile, aromatic substance. 7. Preliminary tests with salmon proved that they can detect the stream odors, and that they were able to discriminate between them. 8. It is 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.