BREDER: FISH SCHOOLS AS OPERATIONAL STRUCTURES 



DISCUSSION 



The two basic purposes of this paper are the 

 estabUshment of the primary space lattice formed 

 by schooling fishes and the role that their surface 

 mucus plays. Both features are supported by 

 theory and empirical data and both expedite the 

 swimming efforts of the fishes. This alone gives 

 sufficient reasons for the formation and the 

 maintenance of schools. 



The question of how much of the schooling 

 phenomenon is a simple following of the paths of 

 least resistance, with automatic avoidance of other 

 fishes, how much is social imitation, and how much 

 is mediated by communication between in- 

 dividuals is not answered here. The phrase "social 

 imitation" is discussed at length by Radakov 

 (1972) as is the status of the term "communica- 

 tion" discussed by Tavolga (1974). The latter 

 indicated that the mechanisms involved can begin 

 as the optomotor orientations of Shaw (1960, 1961). 

 He added that possibly the responses of the fishes 

 ". . . even as adults may be primarily taxic." The 

 rheotactic response to vortices and to fish mucus, 

 reported here, may be equal to or of greater 

 influence than the optical response, since they are 

 fully operable in the dark, but not nearly as precise 

 as the visual response. This could account for the 

 fact that schooling fishes do not fully lose contact 

 with each other in darkness even in species not 

 given to sound production (Breder 1967). 



It is recognized, of course, that there is more to 

 the activity of any fish than efforts to avoid 

 possible physical exhaustion. An evaluation of the 

 importance of other activities or even an enumer- 

 ation of those that are more evident will not be 

 attempted here. However, another approach to the 

 overall problem is noted as follows. The "following 

 reaction" of Crook (1961), based on bird flocks, has 

 been discussed in connection with fish schools by 

 Shaw (1960, 1962), Hemmings (1966), and van 01st 

 and Hunter (1970). The expression is evidently 

 very nearly, if not completely, identical with the 

 "social imitation" of Radakov (1972). 



These data suggest a hypothesis that could go as 

 follows. A group of fossil fishes, not living in 

 schools, but within swimming distances of each 

 other, may form the background. One fish crossing 

 in back of another and encountering its vortex 

 trail would find that self-propulsion required less 

 effort. It is not unreasonable to suppose that after 

 a few such encounters, a tendency to follow would 

 develop. This might be without any instinct to 



follow or imitate but not without prior experience 

 with the vagaries of water currents, which each 

 fish encounters on its first feeble swimming at- 

 tempts as a hatchling; nor is there any reason to 

 dismiss the alternative, that the order is opposite. 

 In Recent fishes the latter is most probably the 

 case. However in the early fishes, which are con- 

 sidered above, the first move to follow could have 

 been solely on a hydrodynamic basis. From here 

 on, with the establishment of a primitive school, 

 its continued existence and development or ex- 

 tinction would be regulated by selective processes, 

 depending basically on whether schooling hin- 

 dered or enhanced the species' ability to survive. It 

 is visualized that this process could have taken 

 place many times in various groups, especially 

 among fishes with relatively scanty mucus 

 production. Also, this process would probably be 

 easily reversible so that fish schools could appear 

 and disappear according to environmental or 

 physiological changes that made schooling or a 

 solitary life favor a species' survival. 



Detailed conparisons between schools of various 

 taxa, or between schools formed by a single species 

 at various times, or under varied conditions have 

 not been made. It would seem however, that all 

 schools are not necessarily isomorphic but are 

 probably at least homomorphic, in the sense of 

 Ashby (1956). 



In a fully theoretical paper, Hamilton (1971) 

 supported the view of Williams (1964, 1967), that 

 most types of animal aggregations owe their 

 existence basically to each animal (vertebrate or 

 invertebrate) trying to hide behind another. With 

 this we have no argument (Breder 1967) and our 

 presentation here, on the locomotor utility of fish 

 schools, exists comfortably with or without it. The 

 question of which came first, hiding or benefiting 

 from an enhancement of swimming efforts, in- 

 volves no interference. They could have developed 

 together or independently, each little advance- 

 ment of one helping the development of the other. 



ACKNOWLEDGMENTS 



Assistance came from many sources during 

 these studies and through the production of the 

 manuscript. The Director of the Mote Marine 

 Laboratory, Perry Gilbert, provided excellent 

 support in facilities and professional assistance 

 during the course of this investigation. H. David 

 Baldridge was most helpful in connection with 

 mathematical and physico-chemical matters. Pa- 



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