are expressed as the sum of the positive areas 

 under the response wave form. 



Beginning on 25 February 1971, one fish was 

 examined per day with 1 % and 0.01 % morpho- 

 line stimuli. Fourteen fish were used. Each test 

 was started at approximately 1000 hr. Every 

 subject was tested with the morpholine concen- 

 trations and the responses compared with re- 

 sponses to 0.06 N NaCl. Stimuli were randomly 

 ordered and presented for 10 sec followed by 

 75 sec of tap water rinse. The stimulus series 

 was then repeated seven times. 



Results 



Fish that had been exposed to 10 ~' ppm of 

 morpholine as smolting juveniles evidenced sig- 

 nificantly higher bulbar EEG activity over con- 

 trols when tested with 1% and 0.01% concen- 

 trations of morpholine (Table 1). Responses to 

 1% morpholine gave a Mann- Whitney value of 

 U = 5 (Siegel, 1956) with probability of 0.006 

 that the control group and the experimental 

 group were drawn from the same treatment pop- 

 ulation. Responses to the 0.01 % level were less 

 markedly, but still significantly, different (U = 

 11, P = 0.049). 



Discussion 



Exposure to low concentrations of morpholine 

 produced a sensitization which lasted at least 10 

 months. But, we did not attempt to determine 

 whether this observed sensitization was exclu- 

 sively to morpholine or to other stimulatory 



products. Casual observation of our data did 

 not reveal the experimental subjects to be more 

 responsive to NaCl than the controls; but this 

 comparison was difficult to make in our experi- 

 mental design because of the changing relation- 

 ship between response amplitudes and back- 

 ground activity levels. (Hence, the necessity of 

 continual comparison of morpholine response 

 with NaCl response, our reference.) Even if 

 overall olfactory responsiveness is increased as 

 a result of pretreatment, sensitization to mor- 

 pholine is still proportionally greater (experi- 

 mental vs. control) 10 months later. We hy- 

 pothesize that exposure to morpholine imprinted 

 the fish during one of the critical periods in the 

 life of the coho salmon, a period when the fish 

 is undergoing physiological changes in prepara- 

 tion for entering a marine environment. 



Independent evidence indicates the existence 

 of a critical period. The Wisconsin Department 

 of Natural Resources allows approximately 1 

 month of imprinting during the period of smolt- 

 ification before releasing the fish into the river 

 system. Imprinting for less time or at different 

 stages of the life cycle seems to result in more 

 straying (Peck, 1970). 



Morpholine was chosen as the imprinting sub- 

 stance since the responses of fingerling coho 

 salmon to it have been investigated (Wisby, 

 1952). Consequently, the concentration could 

 be chosen with knowledge of the performance 

 parameters of the coho salmon. It was necessary 

 to be above threshold but not so high as to cause 

 enthusiastic avoidance or sublethal damage. Be- 

 cause of the vagaries of the flow measurements 



Table 1. — Morpholine-elicited EEG responses of morpholine-imprinted coho salmon compared with those of controls. 

 E designated subjects were imprinted with morpholine at a concentration of 10 "^ ppm; C designated were controls. 

 Median EEG responses of each fish to 1% morpholine and 0.01% morpholine stimuli are ranked (ties carry aver- 

 aged ranks) and Mann- Whitney U values and probability values are shown for each treatment level. 



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