BREDER: FISH SCHOOLS AS OPERATIONAL STRUCTURES 



Figure 19.-A head on view of a loose school of Ckaetodipterus. 

 faber. From Herald (1961). Photo by Fritz Goro. 



poses, yielded the following comparative values: 

 Harengula humeralis (Cuvier) 12.5, Jenkinsia sp. 

 24.2, and Atherinomorus stipes (Miiller and Tros- 

 chel) 25.0. Additional data on Jenkinsia stolifera 

 (Jordan and Gilbert) taken from Breder and Bird 

 (1975), based on Grand Cayman fishes, gave 31.7. 

 All are below the 30% level of variation except the 

 last. A school of let ahirus nebulosus (LeSueur) still 

 being herded about by their parents, however, had 

 42.9. It is known that when several family groups 

 are present, the young fish often become mixed. 

 This may well be the cause of this greater varia- 

 tion, a similar feature being found in extra large 

 schools of adult clupeids, as discussed by Breder 

 (1967). 



The fate of injured and parasitized schooling 

 fish has not been given much attention and it has 

 generally been assumed that such unfortunates do 

 not long survive. This view has been nurtured by 

 the fact that a captured school of fish most often 

 contains no individuals that show either wounds or 

 evidence of gross parasitism. That there are 

 striking exceptions to this has been shown by 



Guthrie and Kroger (1974). They reported that 

 individuals of both Brevoortia tyrannus (Latrobe) 

 and B. patronus Goode, with vitality reduced 

 because of depletion caused by injury or para- 

 sitism, are to be found in estuaries schooling with 

 smaller, younger, but healthy individuals normally 

 present in these relatively protected areas. Out- 

 side waters yielded no such composed schools. 



The relative sizes of the healthy young fishes 

 and the handicapped older ones and the ratios 

 between the largest and smallest individuals are 

 given below as percentages. 



Young 



Estuarine 

 B. tyrannus 

 B. patronus 



Oceanic 

 Both spp. 



52.7 

 73.6 



Old 



10.4 

 44.2 



63.4 



Only one group has an index of low variation in 

 lengths, 10.4. The others all have indices of high 

 variation reaching to the extreme of 73.6. If the 

 schools of both young and old are each taken as a 

 whole then all groups would show very high varia- 

 tion, i.e., 63.6 for B. tyrannus and 80.0 for B. 

 patronus. 



There is only one way these figures can be 

 interpreted. The schools of both species are a 

 mixed lot of lesser schools, as would be expected of 

 fishes that persist in forming enormous schools 

 that mix broods from different spawning areas 

 and that are hatched at various times in waters of 

 different temperatures. This genus would seem to 

 be the most prodigious gatherer of huge ag- 

 gregates of a single species on the American 

 Atlantic coast. 



In the usual, more uniform schools, where the 

 variation is less than about 30%, the geometric 

 structure is observably more uniform. Theoret- 

 ically, at least, the smaller the variation in the size 

 of the fishes the nearer the lattice could approach 

 geometrical perfection. Schools of fishes where 

 there is larger variation in size tend to break in 

 direct proportion to the magnitude of the varia- 

 tion. In enormous schools with great size varia- 

 tions breaking up is not always possible but does 

 lead to considerable churning as individuals of 

 similar sizes gravitate together. 



Effects of Mirrors 



The confronting of animals with mirrors has 

 been practiced for many years, for both trivial and 

 serious purposes. The vast majority of such pre- 

 sentations has been made to one subject at a time. 



489 



