FISHERY BULLETIN: VOL. 74, NO. 3 



20 40 60 80 



% DRAG REDUCTION 



Figure 20.-Graph of the effectiveness of fish mucus on drag 

 reduction. Based on the data of Rosen and Cornford (1970, 1971). 

 The numbers and letters at each point are explained in Table 3, 

 giving the name and number of each fish in the left hand column. 

 See text for full explanation. 



ponents of fish mucus, that the hydrodynamic 

 efficiency of the fishes they studied varied directly 

 with the extent of the basicity of their surface 

 mucus. The fishes measured were Atlantic bonito, 

 Sarda sarda (Bloch), sea bass, Serranus scriba 

 (Linnaeus), and stargazer, Uranoscopus scaber 

 (Linnaeus), given here in the order of descending 

 basicity. This is consistent with the present 

 studies based on the lubricity of certain polymers. 

 The mucus of a fish in a school does more than 

 reduce the drag on its producer since it washes 

 over those that follow. This means that the 

 "leaders" have only their own mucus to ease their 

 passages while the "laggards" receive all the 

 benefits bestowed by those ahead of them. The net 

 effect is to produce a lubricity gradient from zero 

 to the maximum which is dependent on the size of 

 the school. To maintain a steady pace, fishes in the 

 forepart of the school must use more muscular 

 power than the others while the last members 

 require the least effort to hold their positions. As 

 fatigue sets in, the "front runners" would have a 

 choice of accelerating their efforts or holding a 

 steady pace and thus permit those following to 

 pass ahead of them until they find a place requir- 

 ing an effort compatible with the magnitude of 

 their tiring, which could carry them to the trailing 



end positions of the school, if necessary. Zuev and 

 Belyaev (1970) indicated that in a school of Tra- 

 churus, the individuals in the front part beat their 

 tails faster than those in the rear. This condition 

 would naturally follow the lattice-vortex-mucus 

 thesis as developed here. 



Thus, this condition of graded positions in 

 respect to ease of swimming and the matter of 

 muscular fatigue may be a large factor in the 

 maintenance of the integrity of a school and 

 explain the internal churning so often seen in fish 

 schools. The very general changes in positions of 

 individuals within the structure of a school could 

 thus be impelled to a large extent by the individual 

 urge to attain a position demanding the least 

 swimming effort. Also this urge would insure the 

 usual prompt reassembly of a school after being 

 violently dispersed and suggests that the closed 

 figure "mills" of schooling fishes, that would 

 otheru'ise seem to be trivial and pointless, form a 

 relatively quiescent rest period in a favored place. 

 Fish mills have been noted by many students, 

 beginning with Parr (1927). They can be developed 

 from many other sources than the one noted above. 

 Often they are derived directly from extrinsic 

 events, as discussed by Breder (1965). The 

 development of an evidently intrinsic mill is 

 shown there by three photographs that may truly 

 represent the formation of a true "resting mill" as 

 suggested above. 



There is too little known about the complexities 

 of fish mucus to permit much further progress into 

 the details of its relation to school formation and 

 maintenance or its importance to other matters. 

 For instance, how constant are its characteristics 

 and are there rhythmic variations in them related 

 to season, reproductive periods, or type of food 

 ingested? Are there changes in the mucus with age 

 or condition of the fish? Is the mucus of marine 

 fishes more stable than that of freshwater fishes? 

 Since ocean water is chemically more uniform 

 than fresh water it might be expected that these 

 features were reflected in the mucus. 



Experiments with Drag-Reducing Polymers 



Fish mucus, in the amounts necessary for these 

 experiments, is difficult, if not impossible, to 

 obtain and handle without some decomposition 

 and reduction of the long-chain molecules. Addi- 

 tives of some bacteriostatic chemical or refrigera- 

 tion merely introduces other difficulties that could 

 make interpretations uncertain. 



494 



