ROLE OF ETHOLOGY 251 



repeatedly expressed doubts as to whether a decline in the catches 

 of certain trawl fishes was indeed caused by overfishing, to which 

 it was (and is) generally ascribed, or to some behavioral factor. It 

 was at least a possibility, he thought, that a fish which had been 

 a "near miss" in the net when it was small would learn from this 

 experience and make an effort to avoid a repetition of the likeli- 

 hood of capture. There v/as the further possibility that the then 

 recently introduced widespread use of echo-sounders in trawlers 

 was a contributing factor — again by the process of association — 

 but in this instance of the regular onset of the sound with the 

 arrival of the trawl. Against this was to be set the fact that there 

 was no recorded example of any fish being able to detect sounds of 

 these high frequencies. 



Conditioning experiments of the kind described in the chapter 

 on "Conditioned Responses" in Brown's Physiology of Fishes 

 (Bull, 1957) therefore were carried out at Cullercoats into the 

 ability of a number of fishes to perceive the sounds from two 

 commercial echo-sounding transmitters, the Kelvin & Hughes 

 MS 24 and MS 22, the first transmitting at a frequency of 14.25 

 kc sec, the second at 30 kc/sec, the pulses in both going out at 

 66.7 /minute. Neither of these transmitters submerged in the 

 aquarium tanks and set transmitting at high intensity evoked any 

 reaction or alarm or interest in any of the many kinds of fish then 

 living in the aquarium, including a very high proportion of our 

 common British food fishes. 



These experiments showed that the fishes studied, Blennius 

 pholis L., Pleiironedes platessa L., Gobius minutus Pallas, Gadus 

 virens L., and Gadus callarias L., readily formed conditioned 

 responses to 14.25 kc/sec and some of them also to 30 kc/sec. 

 They therefore not only perceive these sounds but are able to 

 use them in forming associations with other vital actions. This 

 means that the suggestion that commercial trawl fishes might, by 

 conditioning, become more wary of capture and, hence, lead to 

 reduce catches cannot wholly be ruled out of consideration. 



When one passes from the study of the behavior of adult 

 animals relatively easy to keep and maintain to that of their 

 larvae, ethology reaches its most vigorous challenge. First class 



