444
A.5uDA:
that the average depth of the thermocline over the equatorial waters is around 100 m,
accompanying ridges and troughs in a complex way. Such an indication helps to explain
the effective catch of bigeye in the equatorial waters of the Ocean north of 10 0 S. As is
suggested in the foregoing section, bigeye seems to be a deep-layer swimmer. WATANABE
(1961), depending on the data obtained mainly from the Pacific Ocean, observed the
appearance of scratches made on the smoked glass of a depth recorder set on a hookline
of longline gear when fish are caught, and examined the depth of formation of such
scratches. He suggested that bigeye was hooked, on average, at 80 - 100 m while yellowfin
was hooked in a significantly shallower layer. (Table 4).
Table 4. Depth of occurrence of the scratch on the smoked glass of a depth recorder set on a hookline of longline gear, which is supposedly caused when a fish is hooked. (After WATANABE, 1961)
Date
Locality
5pecies
Depth range of the
scratch in m
May 12, 1960
6° 28' 5, 120° 21' W
Bigeye
80-82-87
do.
do.
do.
58-98-112
May 13, 1960
5° 51'5, 120° 14'W
Yellowfin
0-32
May 25, 1960
4° 50'N, 119°05'W
Yellowfin?
0-30
May 26, 1960
5° 01'N, 118° 59'W
Yellowfin?
0-30
May 28, 1960
4°33'N,118°4TW
Yellowfin
0-44
May 29, 1960
4° lO'N, 118° 51'W
do.
0-26
do.
do.
do.
0-40
June
2, 1960
5°44'N,122°49'W
Bigeye
70-90-96
June
3, 1960
5°41'N,122°51'W
Yellowfin?
0-33
do.
do.
Yellowfin
0-32
June
5, 1960
5°49'N, 125° 51'W
Yellowfin?
0-45
June 18, 1960
4° 14'N, 136° ll'W
Yellowfin
0-20
June 16, 1960
4° 33'N, 137°08'W
do.
0-16
Oct.
7, 1960
6° 44'S, 58°23'E
Yellowfin?
12-30-66
Oct. 13, 1960
7°31'5, 48°41'E
Yellowfin?
0-40
Oct.
8, 1960
7" 50'S, 48°49'E
Yellowfin?
0-28
Oct. 19, 1960
7° 23' 5, 47°46'E
Yellowfin
0-30-63
Oct. 22, 1960
3°28'5, 48°23'E
do.
0-14
Oct. 23, 1960
3°26'5, 49° 08' E
do.
0-22
Nov. 3, 1960
3°13'5, 45° 22'E
Yellowfin?
0-44
Nov. 12, 1960
Z008'S, 48°04'E
Yellowfin
0-50
Nov. 14, 1960
5°4TS, 47"05'E
do.
0-36
Nov. 19, 1960
6° OT 5, 47° 49'E
Yellowfin?
0-58
Nov. 29, 1960
6° 20' 5, 47° 13'E
Yellowfin?
0-62
Therefore, it is supposed that bigeye inhabits greater depths than yellowfin. Thus,
though both species demonstrate similar patterns of geographical distribution, their
habitats appear to be separated vertically.
3. Upwelling and Concentration of Tunas
Occurrence of upwelling is reported from certain areas of the eastern (WYRTKI, 1961,
1962; ROCHFORD, 1962) and western parts (WOOSTER, SCHAEFER and ROBINSON, 1967;
CUSHING, 1969) of the Ocean. However, only few studies have been made of the
A.5uDA:
that the average depth of the thermocline over the equatorial waters is around 100 m,
accompanying ridges and troughs in a complex way. Such an indication helps to explain
the effective catch of bigeye in the equatorial waters of the Ocean north of 10 0 S. As is
suggested in the foregoing section, bigeye seems to be a deep-layer swimmer. WATANABE
(1961), depending on the data obtained mainly from the Pacific Ocean, observed the
appearance of scratches made on the smoked glass of a depth recorder set on a hookline
of longline gear when fish are caught, and examined the depth of formation of such
scratches. He suggested that bigeye was hooked, on average, at 80 - 100 m while yellowfin
was hooked in a significantly shallower layer. (Table 4).
Table 4. Depth of occurrence of the scratch on the smoked glass of a depth recorder set on a hookline of longline gear, which is supposedly caused when a fish is hooked. (After WATANABE, 1961)
Date
Locality
5pecies
Depth range of the
scratch in m
May 12, 1960
6° 28' 5, 120° 21' W
Bigeye
80-82-87
do.
do.
do.
58-98-112
May 13, 1960
5° 51'5, 120° 14'W
Yellowfin
0-32
May 25, 1960
4° 50'N, 119°05'W
Yellowfin?
0-30
May 26, 1960
5° 01'N, 118° 59'W
Yellowfin?
0-30
May 28, 1960
4°33'N,118°4TW
Yellowfin
0-44
May 29, 1960
4° lO'N, 118° 51'W
do.
0-26
do.
do.
do.
0-40
June
2, 1960
5°44'N,122°49'W
Bigeye
70-90-96
June
3, 1960
5°41'N,122°51'W
Yellowfin?
0-33
do.
do.
Yellowfin
0-32
June
5, 1960
5°49'N, 125° 51'W
Yellowfin?
0-45
June 18, 1960
4° 14'N, 136° ll'W
Yellowfin
0-20
June 16, 1960
4° 33'N, 137°08'W
do.
0-16
Oct.
7, 1960
6° 44'S, 58°23'E
Yellowfin?
12-30-66
Oct. 13, 1960
7°31'5, 48°41'E
Yellowfin?
0-40
Oct.
8, 1960
7" 50'S, 48°49'E
Yellowfin?
0-28
Oct. 19, 1960
7° 23' 5, 47°46'E
Yellowfin
0-30-63
Oct. 22, 1960
3°28'5, 48°23'E
do.
0-14
Oct. 23, 1960
3°26'5, 49° 08' E
do.
0-22
Nov. 3, 1960
3°13'5, 45° 22'E
Yellowfin?
0-44
Nov. 12, 1960
Z008'S, 48°04'E
Yellowfin
0-50
Nov. 14, 1960
5°4TS, 47"05'E
do.
0-36
Nov. 19, 1960
6° OT 5, 47° 49'E
Yellowfin?
0-58
Nov. 29, 1960
6° 20' 5, 47° 13'E
Yellowfin?
0-62
Therefore, it is supposed that bigeye inhabits greater depths than yellowfin. Thus,
though both species demonstrate similar patterns of geographical distribution, their
habitats appear to be separated vertically.
3. Upwelling and Concentration of Tunas
Occurrence of upwelling is reported from certain areas of the eastern (WYRTKI, 1961,
1962; ROCHFORD, 1962) and western parts (WOOSTER, SCHAEFER and ROBINSON, 1967;
CUSHING, 1969) of the Ocean. However, only few studies have been made of the