Owing to the lack of observations both of the accurate depth
d and of temperature
θ of the layer in which tunnies off the Pacific coasts of Japan are angled with long-lines, the catch has customarily been discussed, reference being made only to the surface temperature
θ0.
From July to Dec. 1930, off the North-eastern Japan, and from Dec. 1930 to March 1931, off south to Bôsyû, several fishery research boats carried out the said observations for the tunny, including
Thunnus thunnus L.,
Thunnus alalunga (Gmelin),
Parathunnus sibi (T. & S.) and Germo macropterus (T. & S.). Their reports enabled the writer to examine the difference between the distributions of total catch
N as well as of the frequency of catch
f, referred to the temperature
θ and those referred to
θ0.
Off the North-eastern Japan, both
N and
f show the mode at 19-20°C of
θ0, while at 18-19°C of
θ (Fig. 1). Evidently the difference is due to the remarkable stratification of the water in temperature during summer time. No definite explanation can be given to the appearance of a secondary mode at 13-15°C of
θ. Off south, to Bôsyû, both the mode of
f at 19-20°C and of n (=
N/
f) at 17-19°C are not changed in
θ and
θ0, showing that the stratification in temperature is but slight during winter (Fig. 2).
Estimation, if not very accurate, of
θ without direct observation can yet be made by calculating preliminarily the value of
θ0-
θ from other serial observations of temperature for each region and for each month, provided d is known. From the records of
θ0, submitted by a fishing company “Tôkai-Enyô Kabusiki Kwaisya, ” for
Thunnus alalunga (Gxmelin) off south to Bosyû in 1930,
θ was calculated in such a way as just stated, with the result that the mode of
N lies att about 18°C,
d having been taken as 50-100m or 100m (Fig. 3 a). Remarkably enough, the variation of
N in
θ is much smaller than that in
θ0. A mode at the same temperature is seen in the frequency distribution of
θ, at which the catch was maximum in every month (Fig. 3 b) Some other records were also treated similarly and again the mode of
N at 18°C was obtained.
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