BREDER: FISH SCHOOLS AS OPERATIONAL STRUCTURES 



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5 20 



10 



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02468 02468 



BEATS/SEC BEATS/SEC 



Figure 21. -Graph of experimental analysis of the relationship 

 of tail beats to speed in waters of various degrees of lubricity. 

 S = synthetic sea water. B = bay water. P = Polyox added. S2 in 

 graph A is shown in graph C by black spots. See text for 

 explanation. 



fishes were similarly tested. These were somewhat 

 larger than the first. Grossly polluted but sand- 

 filtered bay water was used. The results were in 

 good accord with the first set, the readings run- 

 ning a little higher and the slope of proportionality 

 being a little steeper. 



Measures of the varying amplitudes reached by 

 swings of the tails were not made as they vary 

 with the tempo of the cycles, as noted by Bain- 

 bridge (1958), and contribute no additional infor- 

 mation germane to this study. 



A direct result of these experiments is very 

 clear. The fishes had a choice of two possible 

 extreme responses to an increase in the water's 

 lubricity. They could maintain their former speed 

 by appropriately reducing the frequency of the tail 

 cycles, or they could so increase their tail beat rate 

 and thus their rate of translation. Obviously they 



could respond by some intermediate response by 

 partially using each of the above two responses. 

 Present data cannot be used to determine these 

 finer distinctions. However, the amounts of the 

 speed increase in both cases strongly suggests that 

 most, if not all, of the gain was by increase of 

 speed. 



It might be thought that the fishes were swim- 

 ming at their accustomed rate in the situation of 

 these experiments and so would not change their 

 rate of swimming even when the changed drag 

 effects reduced the effort required. The phrase 

 "accustomed rate" may or may not be the same as 

 their "optimum speed" as defined by Weihs 

 (1973a). As they did change their pace it seems 

 most probable that the fish were swimming close 

 to their maximum, possibly induced by the in- 

 creased illumination. 



The differences in speed of the fishes between 

 the nontreated water and that with Polyox added, 

 expressed in percentages, is impressive. Exper- 

 iments 1 to 5 and 6 to 9 (synthetic seawater) show a 

 mean increase of 66+%. Experiments 10 to 11 and 

 12 to 13 (bay water) show a mean increase of 63+%. 

 The crude percentage figures show no significant 

 differences between the two cases. The equivalent 

 figures, using the correction values for size of the 

 individual, follow: Synthetic seawater, exper- 

 iments 1 to 9, mean increase 58+%. Bay water, 

 experiments 10 to 13, mean increase 35+%. Further 

 analysis may show this to be a real difference. 



Cahn's (1972) studies tend to confirm the impor- 

 tance of both the hydrodynamic and mucus 

 elements in the formation and maintenance of 

 schools of Euthynnus affinis (Cantor). The fish 

 used by her were about 40 cm in total length and 

 the project was concerned with lateral line studies. 

 She found that placing a transparent plastic 

 partition between two fish that had been swim- 

 ming in parallel courses with the partition, with 

 one somewhat ahead of the other as the first point 

 and one of the side points of Weihs' (1973a) 

 diamond, resulted in the fishes changing to a side 

 to side position. Without questioning the value of 

 the lateral line organs, there is also the value of the 

 mucus and vortices and the "cues" from them 

 which may be handled by the lateral line system. 

 How much these sense organs are directly in- 

 volved with the maintenance of fish schools is not 

 yet clear. Williams (1967)1^ did not, "... believe 



'^Williams (1964), followed by Hamilton (1971), believes that 

 schooling is primarily a matter of cover seeking. 



497 



