water temperaturo is around 21°C '^^>'^'o During the winter season in 

 January^ r'ebruary, and March the different depths (at least down to SOm) 

 have nearly the same temperature and the differ.encc between .r.e tempera- 

 ture at the level where the fish are taken and that at the surface is 

 slight, but through the spring and summer in i»ay,, June^ and July the 

 water temperatures in the upper layer rise rapidly from month to month 

 and their difference from the temperatures of the lower levels increases o 

 As stratification becomes distinct, the water temperatures at the depth 

 where the fish are caught and at the surface m.ust become markedly dif- 

 ferent o The difference in temperatures is still fairly great in IJovember, 

 but in December it decreases rapidly and the two levels become almost 

 homothermalo The object of the author in this p&per has been to show the 

 degree of error in the estimation of favorable temperatures for fishing 

 based on such surface observations, the favorable fishing temperatures cor- 

 responding to the depths actually fished,, and a method of estimating at 

 will the approximate temperature of the middle layer when only surface 

 observations have been taken o 



Depths at -ahich tuna longlines catch fish., A scanning of Table 1, 

 based on data ''' from the Chiba Prefecture Fisheries Experiment Station, 

 reveals chat the estimated positions of the hooks lie within a range of 

 30m. at the shallowest to 170m at the deepest^ with most of them within the 

 range of 50-lOOm, In northern waters some hooks fish at the comparatively 

 shallow depths of 50-7Cm (as a matter of fact., in the coastal waters of 

 Hokkaido and the southern Kuriles many tuna leap on the surface and drift 

 nets and trolling lines are much used \ while the farther south one goes 

 the deeper are the levels at which fish are takeno Off Kochi and liiyazaki 

 prefectures they are at the comparatively deep levels of 80-170m ' o) 



It appears also that the depth at which fish are taken varies from 

 month to month within the same sea areac J?or example, the season of the 



2) lakayama., Itaro and Seiji Ando£ An Observation on the Tuna Fishing 

 Situation in IBSOo -^"isheries Experiment Station Eeport Noo5(38) 1934 <> 



3) Shizuoka Prefecture Fisheries Experiment Stations On the Albacore" 

 (study of 1S?1 on the data of the three years 1929, 1930, and 1231 o ) also 

 states that fish are caught most frequently at 21°C, thus differing 

 greatly from the figure of 18°C published by the 3ate Dr,;. Kishinouye 4), 



4) Kishinouye, K„s Contributions to the comparative study of the so-called 

 scombroid fisheso Jouro Gollo Agriculo Imp, UniVo 'Tokyo^ 8(3)^ 293-475o 



5) Chiba Prefecture Fisheries Experiment Station Report (photostat) "Long- 

 lines Used by the kost Successful Fishing Vessels in Each Prefecture in 

 1932," 



6) Kawana, Takeshi s The Tuna Fishing Situation am.d Oceanographical Condi- 

 "cions in Hokkaido'^ J.okkaido Fisheries Experiment Station, Fisheries 

 Survey Reports o Volo 31, 1934 o 



7) IViarukawa, Hisatoshis "Developing the Sea" pc 165 says that this is 

 because the thermocline is deeper than it is in the souths. 



