ELECTRICAL SENSITIVITY OF FISH 



At present no information appears to be avail- 

 able on the electrical sensitivity of salmonids. 

 Even though experiments in the electrical guidance 

 of salmon smolts migrating downstream have been 

 large and intensive, this work seems not to have 

 included aiij- examination of possible eleotrore- 

 ceptor organs. Certain other fish, however, which 

 have electric organs as well as electrical receptors 

 have been investigated extensively. 



One group of electric fish includes the electric 

 eel. Hagiwara, Szabo, and Enger (1965a) de- 

 scribed the. physiological properties of electrore- 

 ceptoi-s in the electric eel, EJectrophoi^us. Two 

 types of electroreceptors reside in the lateral line. 

 Some of them respond only to electrical stimuli, 

 but others are mechanically and electrically sen- 

 sitive. The tlireshold of the "pure" electroreceptors 

 to imposed square pulses was 2 to 30 mv./cm. 

 and the discrimination threshold was 1.5 to 5 mv./ 

 cm. Since the electric eel's low-voltage pulses that it 

 uses for electrolocation are still relatively high 

 voltages — 20 to 50 v. — Hagiwara et al. (1965a) 

 suggested that the response of the mechanorecep- 

 tors to these electric signals may be only incidental 

 and not biologically significant. Because salmon 

 liardly experience signals of this magnitude, how- 

 ever, more useful interpretations can be gained 

 from information on a group of weakly electric 

 fish. 



Members of several genera of the weakly electric 

 gymnotid eels have electric organs that produce 

 electrical pulses of about 30 mv. at 40 to 600 per 

 second. Lissmann (1951) recorded such signals 

 from GymnarcMi^ and played back the fish's own 

 signals through a pair of electrodes in the water 

 at a distance from the fish. At a strength of about 

 30 jLiv. the fish responded to the signals by imme- 

 diately attacking the electrqdes. Lissmann and 

 Machin (1958) performed behavioral experiments 

 with Gymnarchus which indicated that the fish 

 discriminated changes in the electrical field of 

 0.02 juv./cm. 



In contrast, the sensitivity of individual recep- 

 tors is much less than that indicated by behavioral 

 experiments. Hagiwara and Morita (1963), who 

 recorded electrical activity from individual neu- 

 rons of the lateral-line nerve in two other gym- 

 notids, Staetogenes and Gymnotus, found a 

 threshold for an imposed electrical field of about 



10 mv./cm. along the long axis and a discrimina- 

 tion threshold of 0.1 to 1 mv./cm. Hagiwara 

 et al. (1965b) found that the electroreceptore of 

 Stemarclius responded phasically and tonically to 

 stimuli. The rate of response was highest in the 

 presence of a metal conductor and lowest in the 

 presence of a plastic plate over the receptor. When 

 an electrical field was applied between head and 

 tail, response rate changed for different polarities. 

 At the site of greatest electrical sensitivity they 

 found only one kind of sensoiy organ. Tlie organ 

 included several cells gi-ouped around a single 

 opening on the lateral line and imiervated by a 

 single sensory nerve fiber. 



Enger and Szabo (1965), who recorded electri- 

 cal activity in the medulla of several gymnotids, 

 concluded that tonic responses can yield informa- 

 tion to the fish about the presence and position of 

 objects within the electric field and that phasic 

 responses give information on movement, direc- 

 tion of movement, the size of an object, and the 

 position of the front edge of that object. Dijkgi'aaf 

 (1963) described the electrical sensitivity of the 

 gymnotid group as about 0.05 /xv./cm. or 0.05 

 luamps. (microamperes). Machin (1962) concluded 

 that such small signals could be distinguished from 

 background "noise" if about 40,000 receptors were 

 involved and that this was a reasonable number 

 of electroreceptors which might occur in the 

 lateral-line system. Thus, it appears reasonable to 

 conclude that at least some fish possess electro- 

 receptors of adequate sensitivity to detect and 

 determine the polarity of electrical voltages of the 

 magnitude produced by ocean currents. 



PREDICTION OF ELECTRICAL SENSITIVITY 

 FOR SALMON 



As indicated above, no investigations are known 

 concerning the electrical sensitivity of adult salm- 

 on or of the presence of receptor organs in the 

 lateral line of salmon which might be electrically 

 sensitive. In his recent review of lateral-line func- 

 tion, however, Dijkgraaf (1963) came to several 

 conclusions that might apply to salmon. His first 

 suggestion was that the basic electroreceptor 

 organ is a modified mechanoreeeptor which has 

 achieved maximal response to electrical stimuli. 

 Secondly, because electrical receptors occur in the 

 lateral line of several widely separated groups of 

 fish, he suggested that they may have evolved 



MODELS OF OCEANIC MIGRATIONS OF PACIFIC SALMON 



459 



