246 POPULATIONS OF THE SEA 



acteristic of so many marine organisms and economically important 

 fishes of living together in shoals or schools. Neither does this 

 challenging subject receive treatment in Margaret Brown's (1957) 

 The Physiology of Fishes. This is not a phenomenon of interest 

 only to the biologist. No matter for what purposes echo-sounding 

 or Asdic devices are used, the presence of this type of congregation 

 can be either illuminating or complicating, or both. The ethologi- 

 cal mechanisms by which the schools are brought together, held 

 together, or dispersed, or by which they can be induced to be so 

 held or dispersed, could have an immense significance both 

 economically and militarily. 



Now, Steven (1959) has shown experimentally, under laboratory 

 conditions, that in two shallow water shoaling tropical western 

 Atlantic fishes, Hepsitia stipes (Miiller and Troschel) and Bathystoma 

 rimator, shoals disperse when light intensity falls below about 0.1 

 ft-c, but that they also disperse with similar behavior patterns 

 when stimulated olfactorily by weak solutions of various tissue 

 extracts. He makes a number of interesting speculations such as 

 that "it seems equally likely that the fish learn to associate the 

 regular diurnal changes in light intensity with \'ertical movements 

 of their food and, having become so conditioned, continue to 

 respond to the light stimulus even when not feeding. Another 

 possibility is that non-feeding as well as feeding fish may continue 

 to follow the concentration gradient of substances liberated by 

 plankton in their diurnal vertical migrations." These speculative 

 conclusions from his paper are quoted here, not with acceptance 

 or approval, but merely to show how packed with unanswered 

 ethological questions these two inoffensive sentences really are. 

 They are very typical of discussions on the ecology and behavior 

 of marine organisms. Further examples can be found in many of 

 the papers in several sections of this Congress. Zelikman (1959) 

 drew attention to a well known phenomenon and that is their 

 keeping together by groups which maintain their general identity 

 under a number of disrupting circumstances, and she attempts an 

 explanation in terms of "reflexes" and mechanical action of cur- 

 rents. Dr. Jenner (1959), on the other hand, discussed schooling 

 behavior in the marine snail, Nassariiis obsoletus, and dealt directly 

 with the environmental factors responsible. 



