BREDER: FISH SCHOOLS AS OPERATIONAL STRUCTURES 



factors include other schools, large predatory 

 fishes, fish-catching birds, and rich plankton 

 streaks. 



Theoretically at least, fish schools could take any 

 shape. Considered as three dimensional "blobs" 

 they have been described and photographed in a 

 wide variety of shapes, including even the nearly 

 spherical (Breder 1959). The latter mostly occurs in 

 open water some distance from the influence of the 

 water's surface and the bottom of the body of 

 water. These are rare and suggest almost exactly 

 balanced forces. Under such conditions the school 

 formation in the ordinary sense breaks down? The 

 form of organization within such near-spheres has 

 not been analyzed, nor has their manner of for- 

 mation or eventual dissolution. Other shapes not 



^There is in this case a question as to the propriety of including 

 this assemblage as a school in any sense. At least the fishes that 

 form this ball are in a solid school formation as they rush in to 

 form these structures. 



readily described in simple geometrical terms, as 

 that shown in Figure 15, seem to illustrate the 

 presence of either spiral arms or "smoke ring" 

 formations. 



Much more frequently encountered are schools 

 close to the water's surface or the bottom. These 

 often show a more or less oblate spheroidal form 

 from which a portion has apparently been planed 

 off, where near contact with surface or bottom 

 necessarily caused flattening. Otherwise, the op- 

 posite side follows the contour of the flattened side 

 so that the school takes the form of a flattened 

 sheet of rather uniform thickness. These often 

 take the form of a sheet one-fish deep, the school 

 practically reducing to a nearly two-dimensional 

 figure. These all may occur in open water, either 

 near the surface or bottom. They are, however, 

 more usually seen in very shallow water where 

 both surface and bottom influences impinge on the 

 school. These schools in which the horizontal 

 dimensions greatly exceed the small vertical one 



^ 



Figure 15.- An unusual and not readily explicable maneuver of Jenkensia stolifera, seen at Grand Cayman from scuba gear under very 



calm conditions. 



483 



