222 Fish Behavior and Angling 



Identification of Common and Uncommon Odors. Fisheries researchers 

 at the University of Wisconsin^" demonstrated that Winded bluntnose 

 minnows, after a training of 2.5 months, were able to distinguish be- 

 tween odors of milfoil, Mtjriophijllum exalbescens, and coontail, Cerato- 

 phylhim demersum, when rewarded with food and punished with electric 

 shock. These minnows were able to detect a water rinse of a sprig of an 

 aquatic plant diluted 1 : 10,000. Hasler and Wisby ^^ found that bluntnose 

 minnows could detect phenols at concentrations of 0.01 ppm and could 

 discriminate between phenol and p-chlorophenol at concentrations of 

 5 X 10-4 ppm. 



These studies suggest that olfaction in fishes ( at least in some species ) 

 may be as important in the routine of day to day existence as is olfaction 

 in dogs. Fish may not be so obvious in their use of the sense of smell as 

 are dogs, but there is little doubt that they may recognize a "home" en- 

 vironment, identify individual fishes of their kind as well as those of other 

 kinds, and perform other functions through the use of olfactory organs. 

 Further experiments indicate that some fishes are "blood hounds" in their 

 use of the sense of smell. This is particularly true of fishes that make long 

 migrations. 



Odor as an Aid in Migration. Biologists have long searched for an 

 explanation for the return migration of salmon to a specific location in a 

 home stream. Tliese fish are spawned in fresh water streams, usually 

 many miles from the oceans. After spending several months in the stream 

 where they were spawned, they gradually work seaward, finally leaving 

 fresh water, for a period of several years in the ocean. When they reach 

 sexual maturity, they return to the stream system where they were 

 spawned, migrate upstream always selecting the right branch or tributary 

 until they return to the location of their origin where they in turn spawn. 

 Hasler and Wisby ^* have postulated that salmon must find the way 

 back to their place of origin through their retention of olfactory impres- 

 sions of the stream. In order for such a hypothesis to be true, investi- 

 gators would have to be able to give an affirmative answer to the follow- 

 ing questions: 



( 1 ) Do streams have characteristic odors to which fish can react, and, if so, 

 what is the nature of the odor? 



(2) Can salmon detect and discriminate between such odors if they do exist? 



(3) Can salmon retain odor impressions from youth to maturity? 



They began by testing unconditioned salmon fry with odoriferous 

 substances ^^*- to find ones that the salmon fry gave evidence of liking 

 or disliking. Salmon fry were placed in a central waterfilled compartment 

 of an apparatus and a test substance was introduced into one of four 

 tributary arms through which water flowed to supply the center compart- 



