FISHERY BULLETIN: VOL. 74, NO. 3 



e.g., Svendsen and Armitage (1973). There have 

 been few cases of mirrors being introduced to 

 groups, such as fish schools. Pitcher (1973), using 

 mirrors for certain photographic purposes, noted 

 some of the reactions of his fish subjects. In both 

 cases above and almost all others, the studies have 

 been made on captive animals. 



Information on the reactions of individual fishes 

 vv^ithin a tightly organized school is not readily 

 obtained. Experiments on captive schools yield 

 results that are naturally suspect, primarily 

 because of the usually gross changes in the 

 behavior of schools confined to small quarters and 

 the length of time in days or weeks, even in a 

 relatively enormous container, that it takes to 

 reach apparent stability. Analysis of motion pic- 

 tures taken of feral schools cannot be expected to 

 supply much more than occasionally fortunate 

 sequences. One difficulty is the interference of 

 other members of the school or of other species 

 exterior to it. Mirrors introduce something to 

 which fishes generally respond and thus the pos- 

 sibility of reasonably interpreting their responses 

 exists. The experiments and their results follow. 



A submerged mirror, 39 x 57 cm, was hung near 

 an observation dock or other suitable location 

 which yielded data on fishes in schools in their 

 native habitats. The school's presence was in no 

 way forced, nor were they present because of any 

 attractiveness mirrors may have, since the sites 

 selected were normally visited daily by these 

 schools. 



Four species, three of the Clupeoidei and one of 

 the Mugiloidei, reacted to this mirror, each in a 

 different manner, as follows. 



Anchoa hepsetus (Linnaeus) showed the most 

 striking reactions. All the schools of this species 

 were large, at least containing 1,000 fishes and 

 usually far above that number. The schools ap- 

 peared at this place only during the daylight hours 

 and moved off to deeper water for the night. These 

 movements were independent of the tidal stages. 

 The horizontal component of the tidal flow clearly 

 regimented these fishes because at slack tide they 

 became somewhat disorganized. 



If the mirror was submerged while this species 

 was absent, the fishes schooled on arrival would 

 regard it simply as any other solid object, such as a 

 pile, that had to be avoided by changing their 

 course. In doing this, schooling fishes normally 

 leave a clear space between them and the object. In 

 this case it averaged close to 20 fish lengths. If the 

 fishes were present before the mirror could be 



lowered, it was allowed to slide directly into the 

 school, which produced little disturbance, other 

 than a few transient "shock waves" as the normal 

 space was formed around the mirror. It was 

 noticed early that the distance kept by the fishes 

 from the back of the mirror, painted black, was a 

 little greater than that kept from the face of the 

 mirror. 



After the elapse of about 1 h after the introduc- 

 tion of the mirror, the portion of the school oppo- 

 site the mirror's face made slight "bulges" toward 

 it, which were promptly resorbed. Nothing like 

 this appeared on the part of the school opposite the 

 black backing of the mirror. 



After another hour, the school had moved closer 

 to the face of the mirror, approximately 10 fish 

 lengths away. When this was once established, 

 individual fishes would sally forth from the pe- 

 rimeter of the school opposite the mirror and swim 

 to within four fish lengths of the mirror and 

 momentarily run parallel with their reflection. 

 This would be followed by a hasty retreat to the 

 school. The action, repeated frequently by various 

 individuals, would seem to be explicable as follows. 

 A peripheral member of the school could see the 

 school's reflection twice as far as the mirror 

 surface. To join that "other" school required that 

 the adventuresome individual had to negotiate 

 that apparent distance. The fish traveled about 

 nine fish lengths before it turned back. Here the 

 fish found that one fish in the reflection is coming 

 at him and running side-by-side with him, at an 

 apparent distance of two fish lengths. This kind of 

 behavior is not the "normal" in the situation of a 

 few or one fish attempting to join a much larger 

 group, at least in any of the species under study. 

 The usual manner in which one or a few fishes join 

 a large school is to quietly approach the larger 

 body and pick up its rate of speed and slowly 

 merge into the main body. There is never any 

 evident specific act on the part of the affected 

 fishes of the school. They seem to react to the 

 "intruders" as they do to the other members of the 

 school, constantly adjusting their positions by 

 small amounts. 



The above is not true of two schools of more 

 nearly equal size when in the process of merging. 

 The smaller will approach the larger at a rate of 

 speed apparently inversely proportional to the 

 volume of the smaller school. The larger school will 

 approach the smaller at a much slower speed also 

 inversely proportional to its volume. When the two 

 schools come within a distance equivalent to about 



490 



