FISHERY BULLETIN: VOL. 76, NO. 2 



I also obtained data on the uniformity of size in 

 individuals of the same school. The squid were 

 caught during a 1-wk period in August in locations 

 ranging from San Diego to Santa Catalina Island, 

 Calif. A night-light was placed off the stern of the 

 ship in the center of an L-shaped 3-m long mesh 

 net. Squid were attracted to the light and would 

 rush into the net. The net was then raised and the 

 squid could be removed with dip nets. The 

 "schools" were all of the squid which swam into the 

 net at the same time. Squid caught during this 

 period ranged from 5.8 to 17.3 cm dorsal mantle 

 length. But for a given school, they were much 

 more uniform in length. The average size range for 

 29 schools of 2 to 32 individuals was 2.5 cm. 



Maintenance of 

 School Structure and Orientation 



Experiments in the laboratory have indicated 

 that vision is sufficient sensory input to mediate 

 schooling behavior. Squid on different sides of a 

 clear, rigid Plexiglas barrier will readily school 

 with each other and they appear to maintain the 

 same type of parallel orientation that is present in 

 normal schooling behavior. Preliminary experi- 

 ments using such Plexiglas barriers were run to 

 try to elucidate the mechanisms by which spacing 

 is maintained. 



Two-Squid Experiments 



Experiments were run to determine whether 

 squid would school in the same manner with or 

 without a clear Plexiglas barrier in place. Mea- 

 surements were obtained for squid swimming to- 

 gether and for the same squid swimming on oppo- 

 site sides of a Plexiglas barrier which divided the 

 tank into two compartments. The order of the 

 treatment was randomized for each pair of squid. 

 Squid ranged in size from 7 to 13 cm mantle 

 length. For a given experiment, the two squid 

 were of similar length. Pictures were taken of the 

 squid in each treatment every 10 s for 3 min after 

 they first came together and again every 10 s for 3 

 min after the squid had been left undisturbed for 

 15 min. If the squid did not come together to within 

 at least 0.5 m within 1 min, the experiment was 

 terminated. 



Table 2 shows the results of five such experi- 

 ments. The first 3-min periods have been compared 

 with each other, as have the later runs. This was to 

 see if the pattern of schooling changed after the 



Table 2. — Median nearest distances and median separation 

 angles for two-squid (Loligo opalescens) experiments. 



'Significance of difference in medians from IVlann- Whitney U-test. 

 ^NS = no significant difference. P 0.05 



squid became more adapted to the experimental 

 regime. This table presents results for the median 

 nearest distance between the two squid for each 

 run and for the median separation angle for these 

 same runs. Separation angle for each frame is 

 simply a measurement of the angle between the 

 two squid and is a measure of orientation (0° sep- 

 aration angle indicating parallel alignment facing 

 the same direction). 



The barrier has an effect upon the separation 

 distance between the two squid. In all cases, there 

 was a significant difference between the distance 

 between squid with and without the barrier. When 

 the Plexiglas barrier was present, the squid 

 tended to space themselves farther apart. There is 

 not a clear relationship between angular separa- 

 tion and the presence of the barrier. Of the six runs 

 showing significant differences, three had greater 

 median separation angles with the barrier in place 

 and three had greater median separation angles 

 when the barrier was not present. 



Three-Squid Experiments 



The experimental tank was divided crosswise 

 into three equal compartments (1 x 2 m each) by 

 clear Plexiglas partitions. A squid was chosen 

 from the holding tank and was placed in the cen- 

 tral compartment. Then a squid for each of the 

 outer compartments was selected. These squid 

 were assigned at random to each of the outer com- 

 partments. The squid were allowed to adapt to the 

 experimental situation for 15 min and then were 

 filmed for 5 min (one picture every 10 s). The two 



438 



