BREDER: FISH SCHOOLS AS OPERATIONAL STRUCTURES 



Furthermore, the drag reduction of a fish slime 

 diluted with water that produced a 25% reduction 

 just after its removal from the fish, was inert 3 h 

 later, according to Hoyt (1975). He also gave a 

 hydrodynamic explanation on why it is possible for 

 very small fishes to gain an advantage from their 

 mucus although the operational mechanics are 

 diff'erent than those available to larger fishes. This 

 concerns differences in the boundary-layer transi- 

 tion from laminar to turbulent flow in relation to 

 the Reynolds numbers. Fish mucus does not dilute 

 easily with water by mere contact, but does so 

 easily with agitation. Rosen (1959) used the term 

 "reluctance" to designate this condition. 

 Polymers, especially those manufactured to have 

 high drag-reducing characteristics as measured on 

 a rheometer, have drag reduction features that are 

 comparable to or exceed those of fishes' surface 

 mucus in the small quantities required to obtain 

 maximum effects. 



The material used was a water soluble resin, a 

 high polymer of ethylene oxide, from the Union 

 Carbide Corporation, and generally known by its 

 trade name Polyox^'^. The significant characteris- 

 tics, as given by Hoyt (1971) follow 



Molecular 

 weight 



6,000,000 (ca.) 



Polyox F.R.A. (Lot 1163) 



,, J Concentration (%) 

 Max drag L__ 



reduction (%) 



67.8 



Max D.R. 

 15 



V& max D.R. 

 1 



This particular grade of Polyox was used because 

 of its unusually high molecular weight as the 

 purpose here was merely to establish whether such 

 products would induce a change in the swimming 

 efficiency of the fishes. Hoyt (1975) considered a 

 minimum molecular weight of 50,000 of the drag- 

 reducing element to be necessary for friction 

 reduction to be expected. 



Polyox is reported to have very low, if any, 

 toxicity, (Smyth, et al. 1970, Wade 1970). For the 

 purposes of this study, toxicity tests were also run 

 on a variety of fishes. Nothing whatever occurred 

 that would suggest any physiological disturbance 

 on any of the test fishes. Both Poecilia reticulata 

 Peters (fresh water) and Hippocampus erectus 

 Perry (salt water) produced young when subjected 

 to concentrations far higher than any required 

 here. The only item showing obvious adjustments 



to the change in lubricity of the water was that 

 mature examples of Hippocampiis erectus were 

 unable to use their prehensile tails effectively on 

 the smaller supports provided in their aquaria. 

 That is, they simply slipped off plastic rods, of 

 circular cross section, if the rod diameters were 

 below a certain magnitude relative to the grasp of 

 their tails. With larger rods they had no trouble 

 and were readily able to "grasp" the supports and 

 hold on in normal fashion. Those that could not 

 find a suitably sized "perch" coiled their tails so 

 that about three-quarters of a circle was formed at 

 right angles to the body axis and then "sat" with 

 the partial circle laid on the bottom of their 

 aquarium. Apart from being somewhat restless, 

 they apparently were just as well off as the others. 

 The Poecilia moved about in what appeared to be 

 their normal random manner, but whether they 

 moved a little faster or not could have only been 

 determined with great difficulty and would not 

 have contributed to the problems under study. 

 None of the fishes tested after the preceding 

 preliminaries showed any distress from the addi- 

 tion of Polyox. 



The Gulf menhaden, Brevoortia patronus 

 Goode, was used for tests on drag reduction. This 

 species is an obligate schooler and, as with many 

 such schoolers, the ability to spread its caudal fin is 

 _ severely limited. There is a strong possibility that 

 none of them exercised this slight ability at all. 

 Also, these fish accommodate well to aquarium life 

 if provided adequate swimming room and a few 

 companion fishes, a feature not common in many 

 members of this family. The fishes selected for 

 testing were first established in a circular concrete 

 tank 4+ m diameter, with a water depth of 1 m. 



Specially made aquaria were used for these 

 experiments. They measured 25 x 25 x 90 cm and 

 were filled with synthetic seawater^^ to a depth of 

 20 cm providing a total water volume of 45,000 cm^. 

 These were established in a perfectly light-tight 

 room, actually a Navy Sea Van without windows, 

 remote from vibrations and sounds. Lights were 

 controlled by a time switch for day and night 

 effects and a thermostat controlled the tempera- 

 ture. The test aquarium was placed on the floor 

 and the others on rocks at a convenient height. 

 Precautions were taken to protect the fishes from 

 being startled by motions, vibrations, or other 



*'Klndly supplied gratis. Reference to trade names does not 

 imply endorsement by the National Marine Fisheries Service, 

 NOAA. 



'^'Kindly supplied gratis under the name "Instant Ocean" by 

 Aquarium Systems, Inc. 



495 



