CAHN: SENSORY FACTORS 



be multiplied by five to reflect the number of 

 frames examined. 



Eighteen fish out of 22 adapted to the test 

 conditions; they were tested in groups of 2, 3, 

 4, and 6, and each fish was used in more than 

 one test. To determine the side-to-side distance 

 between fish that were in parallel orientation, 

 the position of each fish and his body axis were 

 drawn from the motion picture frames. The 

 perpendicular distance between the axes of pairs 

 of orienting fish was measured (Figure 2). 



u 



I  I  I  I  I 



10 20 30 40 



cm 



Figure 2. — Scale diagram showing tunas swimming 

 without a partition in rectangular channel. The dimen- 

 sions were taken from a fish of 43 cm fork length. Body 

 axes are shown as dashed lines. The side-to-side distance 

 of 25.9 cm (solid line) is the group mean for a school 

 size of 2 fish. 



When occasional non-parallel axes were en- 

 countered, the inter-fish spacing was measured 

 from the snout of the leading fish perpendicular 

 to the axis of the trailing fish, or from snout- 

 to-snout if the fish were abeam. The slight 

 body undulations produced while swimming and 

 when each fish turned resulted in slight angular 

 differences in the headings of two otherwise par- 

 allel fish. For our purposes it was not necessary 

 to measure these angles, although we spot- 

 checked the first frame of each sequence used 

 to be sure that the angular headings from one 

 another were approximately 26° Or less. This 



was the angular value which Hunter (1968) and 

 Shaw (1969) considered small enough to be 

 counted as parallel fish. 



To determine how often two fish in parallel 

 were in an abeam position (directly alongside 

 each other, as in Figure 3) , or in a diagonal re- 

 lationship (alongside, but slightly in front or 

 behind), we scored each frame of the measured 

 sequences as abeam or diagonal, and counted 

 the total number of frames the fish were in these 

 positions. Initially, we measured the diagonal 

 distances to get some idea of how far ahead or 

 behind each other the fish were keeping. At the 

 turns, however, these values became difficult to 

 keep track of. For our purposes, we found it 

 satisfactory to score as diagonal all frames 

 where fish in parallel were more than 5° dis- 

 placed from a strictly abeam position. We did 



.y*»': 



 ( 



Figure 3. — Single frame from test on a pair of fish 

 separated by a Plexiglas partition (top of column in 

 Table 2). Upper edge of partition is obscured by re- 

 flection from overhead lights. In this frame the fish 

 are about 93 cm apart, just about the mean distance for 

 this test. 



199 



