SENSORY FACTORS IN THE SIDE-TO-SIDE SPACING AND 

 POSITIONAL ORIENTATION OF THE TUNA, Euthynnus affinis, 



DURING SCHOOLING 



i 



Phyllis H. Cahn^ 



ABSTRACT 



This study was designed to demonstrate the role of the lateral line sensory system in fish schooling. Groups 

 of 2, 3, 4, and 6 adult Euthynnus affinis were filmed during schooling orientation in the presence or 

 absence of transparent partitions between them. When the hydrodynamic field between orienting fish 

 was blocked by the partition, they significantly increased their side-to-side spacing, and changed their 

 diagonal to abeam position ratios so that abeam orientation assumed increased importance. Apparently, 

 hydrodynamic contact is essential for the typical spacing and positional orientation in schooling, and the 

 lateral line, as the water turbulence detector, plays a prime regulatory role. 



The primary role of vision for the initial ap- 

 proach and prolonged maintenance of parallel 

 orientation of fish in a school has been well known 

 for many years (Parr, 1927) . The accessory role 

 of other sensory systems, especially the acousti- 

 co-lateralis, for maintenance of fish-to-fish dis- 

 tance during schooling, has been implicated by 

 numerous investigators (Breder, 1959, 1965; 

 Cahn, Shaw, and Atz, 1968; and Moulton, 1960) , 

 but direct evidence has been difficult to obtain. 

 Breder (1965) showed that the side-to-side spac- 

 ing of danios in a school {Brachydanio albo- 

 lineatus) is "usually just a little over twice the 

 distance from the side of each fish to the outer 

 edge of the trail of vortices" each generates 

 while swimming. He considered that the fish\ 

 space themselves so as to "respect these vortices 

 or suffer a considerable reduction in locomotor 

 efficiency". Thus, the sensory system primarily 

 involved in hydrodynamic detection should play 

 a key role in regulation of this spacing. Our 

 work (Cahn, Siler, and Fujiya, in press), and 

 that of Dijkgraaf (1963), strongly implicates 

 the lateral line mechanoreceptors in this func-' 

 tion. 



In a preliminary report on tuna schooling 

 (Cahn, 1967), it was found that a transparent 



* Graduate Department of Marine Science, Long 

 Island University, P.O. Greenvale, L.I., NY 11548. 



partition that blocked hydrodynamic contact be- 

 tween orienting fish also resulted in somewhat 

 diminished fish-to-fish attraction, and in changed 

 spacing and positional relationships. Prior to 

 this study, many other investigators of fish 

 schooling tried to separate the sensory compo- 

 nents involved, and used transparent plastic and 

 glass boxes, tubes and plates, as well as mirrors 

 (reviewed by Shaw, 1970). Variable results 

 were obtained, with little quantification, except 

 for Shaw's study on Caranx hippos (1969), 

 which showed a reduced duration of schooling 

 when the fish were separated by transparent 

 partitions. 



The present study quantifies the changes in 

 spacing and positional orientation of the tuna, 

 Euthijnnus affinis (common name, kawakawa), 

 observed in the earlier report, when transparent 

 partitions blocked non-visual schooling cues. The 

 results strengthen the role of hydrodynamic de- 

 tectors in control of fish positional relationships 

 and side-to-side distance between fish in a school. 



MATERIALS AND METHODS 



This work was carried out at the Honolulu 

 Biological Laboratory of the National Marine 

 Fisheries Service, where the facilities are avail- 

 able for experimental studies on scombroid 

 fishes. Special transport containers (Nakamura, 



Manuscript accepted October, 1971. 



FISHERY BULLETIN: VOL. 70, NO. 1, 1972. 



197 



