Sardines, anchovies^ mackerel, mackerel-scadSj myctophids, flying- 

 fish, squid, Ryilkyii squid, shrimp, spotted shrimp, larval crabs, 

 schizopods, and amphipodSo Quite a few fish are found to have completely 

 empty stomachs. 



The state of the balance between the total population of skipjack in 

 Japanese waters and the available supply of natural food as well as the 

 factors which led to the establishment of the skipjack fishery are believed 

 to be implied in these facts. 



The following deductions can also be made from a consideration of 

 the skipjack's food habits. Schools which feed on such things as mackerel, 

 maokerel=scad, spotted shrim.p, larval crabs, and amphipods, all of which 

 are of a coastal character, are composed of "sedentary fish" or fish which 

 have stayed temporarily around islands, reefs, and shoals„ The schools 

 which feed on pelagic forms such as flyingfish, squid, and schizopods are 

 those which are accustomed to svTimming in the open sea„ Schools of fish 

 which have empty stomachs can be assumed to be pelagic schools which have 

 not been able to find food before being taken„ 



Section 4 Characteristics of the Skipjack 



Paragraph 1 The Skipjack's Eyes 



According to Mr, Shigeo Yamamoto's report of his researches, "The 

 study of the refraction characteristics and accommodation mechanism of 

 the eye of the skipjack reveals the following facts. Judging from the 

 fact that the crystalline body in the eye of the fish is almost spherical, 

 it has been said in the past that the fish are near-sighted, but this study 

 has revealed that they are on the contrary far-sighted, very much so in 

 facto This has been established in the case of the skipjack, tuna, sea- 

 bass, and striped marlin„ If we measure the crystalline body's refraction 

 of parallel rays of light in the air, we find that it has a very short 

 focal distance of only 0<,4- inm, however, if we measure it for the skipjack in the 

 water with apiece of glass fastened behind it f'], we find that it has a 

 long focal distance of 15 mm, differing markedly from the visual line of 

 about 6 vtimo This places the focal point about 7 mm behind the retina of 

 the eje, and means that the eye is far=sighted„ This is thought to be 

 the condition when the eye is at rest, and it is assumed that it is so 

 constructed as to be capable of even stronger far-sight. The fish can 

 se& clearly at least 30 or 35 feet ahead, but of course it is impossible 

 to see as far in the water as in the air, no matter how far-sighted it 

 nay be, because the water contains more fine particles than the air and 

 the sun's rays are weaker in the water^ The eye muscles of the skipjack, 

 like those of terrestrial mammals, include the superior direct, inferior 

 direct, inferior oblique, external direct, and internal direct muscles, 

 but great differences are apparent in their size and points of insertion. 

 These muscles net only move the eyeball forward and backward and up and 

 down, but they appear also to contribute to the adjustment of the vision ,,. 

 A statement of the c '.Irsicns bcssd on the results of the study would 

 indicate that the sicip jack does not have very good vision obliquely to 

 the rear. When relaxed or at rest,, all of the eye muscles are resting, 



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