Tuna Fisheries and their Resources in the Indian Ocean
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b) Albacore
The sub-tropical gyre and its neighbouring waters are the territory of albacore.
The northern half of the gyre, that is the South Equatorial Current area, forms its
spawning ground. The southern part of the gyre, which is the area of the eastward
current, is a habitat of young albacore.
c) Yellowfin
The concentration of yellowfin is observed over a wide area north of 10° S including
the North Equatorial Current area (NE monsoon drift during the northern hemisphere
winter period), the Equatorial Countercurrent area (during the northern hemisphere
winter period), the SW monsoon drift area and the northernmost part of the South
Equatorial Current area. Consequently, it is suggested that the occurrence of the species
coincides with the Indian Equatorial Water (SVERDRUP, JOHNSON and FLEMING, 1942).
The seasonal expansion of the distribution along both the African and Australian coasts
is perhaps to be interpreted as a seasonal expansion of the influence of the Indian
Equatorial Waters towards higher latitudes.
d) Bigeye
Distribution of this species is somewhat complicated because the species occurs in
2 separate waters, one in the area north of the northern boundary of the South Equatorial
Current and the other in the area of the subtropical gyre. The immature group of this
species occurs in the southern half of the sub-tropical gyre together with immature
albacore, and the mature fish in the equatorial water along with yellowfin. However,
as will be discussed in the next section, over the area north of the northern boundary
of the South Equatorial Current, the species is supposed to inhabit a deeper layer than
yellowfin and a vertical separation between the 2 species is indicated.
Thus, these 4 large-sized tuna species share their own domains independently. It is
notable that the way of sharing is, more or less, correlated with the surface current
system and or water type. Also, it is pointed out that the stage of sexual development is
another factor of differentiation of distribution.
2. Permanent Thermocline and Efficiency of Fishing for Bigeye by Longline Gear
SUDA, KUME and SHIOHAMA (1969) discussed, mainly on the basis of information
from longline fishing in the Pacific Ocean, that the formation of the longline ground of
bigeye is likely to be related to the depth of the permanent thermocline over the area
where the tropical surface layer is well developed. In such a case, where the thermocline
occurs at about 100 m below the surface, a high hooking-rate of bigeye is experienced.
The following is an interpretation of the good coincidence between effective catch by
longline and thermocline depth of 100 m:
a) longline-caught bigeye do not scatter in the tropical surface layer but occur in the
thermocline itself, or just below it;
b) when the thermocline is situated around a depth of 100 m and coincides with the
depth of hooks, the longline gear works well in catching bigeye.
In the Indian Ocean, the information available on the depth of the thermocline is not
sufficient for the present study. The results of past research, for instance bathythermograph data compiled by the u.s. National Oceanographic Data Center (1966), indicate
443
b) Albacore
The sub-tropical gyre and its neighbouring waters are the territory of albacore.
The northern half of the gyre, that is the South Equatorial Current area, forms its
spawning ground. The southern part of the gyre, which is the area of the eastward
current, is a habitat of young albacore.
c) Yellowfin
The concentration of yellowfin is observed over a wide area north of 10° S including
the North Equatorial Current area (NE monsoon drift during the northern hemisphere
winter period), the Equatorial Countercurrent area (during the northern hemisphere
winter period), the SW monsoon drift area and the northernmost part of the South
Equatorial Current area. Consequently, it is suggested that the occurrence of the species
coincides with the Indian Equatorial Water (SVERDRUP, JOHNSON and FLEMING, 1942).
The seasonal expansion of the distribution along both the African and Australian coasts
is perhaps to be interpreted as a seasonal expansion of the influence of the Indian
Equatorial Waters towards higher latitudes.
d) Bigeye
Distribution of this species is somewhat complicated because the species occurs in
2 separate waters, one in the area north of the northern boundary of the South Equatorial
Current and the other in the area of the subtropical gyre. The immature group of this
species occurs in the southern half of the sub-tropical gyre together with immature
albacore, and the mature fish in the equatorial water along with yellowfin. However,
as will be discussed in the next section, over the area north of the northern boundary
of the South Equatorial Current, the species is supposed to inhabit a deeper layer than
yellowfin and a vertical separation between the 2 species is indicated.
Thus, these 4 large-sized tuna species share their own domains independently. It is
notable that the way of sharing is, more or less, correlated with the surface current
system and or water type. Also, it is pointed out that the stage of sexual development is
another factor of differentiation of distribution.
2. Permanent Thermocline and Efficiency of Fishing for Bigeye by Longline Gear
SUDA, KUME and SHIOHAMA (1969) discussed, mainly on the basis of information
from longline fishing in the Pacific Ocean, that the formation of the longline ground of
bigeye is likely to be related to the depth of the permanent thermocline over the area
where the tropical surface layer is well developed. In such a case, where the thermocline
occurs at about 100 m below the surface, a high hooking-rate of bigeye is experienced.
The following is an interpretation of the good coincidence between effective catch by
longline and thermocline depth of 100 m:
a) longline-caught bigeye do not scatter in the tropical surface layer but occur in the
thermocline itself, or just below it;
b) when the thermocline is situated around a depth of 100 m and coincides with the
depth of hooks, the longline gear works well in catching bigeye.
In the Indian Ocean, the information available on the depth of the thermocline is not
sufficient for the present study. The results of past research, for instance bathythermograph data compiled by the u.s. National Oceanographic Data Center (1966), indicate