442
A. SUDA:
season and consequently compensates for the others. The seasonal change in the amount
of catch per 1 000 hooks for individual species is remarkable, even in the case of
yellowfin which inhabit the tropical zone with its comparatively small seasonal change
in oceanographic conditions. This species peaks in the first quarter in the western part
of the tropical waters and in the third quarter in the eastern part of the Ocean.
Concerning the skipjack, though insufficient information is available, the more
frequent occurrences of larval fish from larvae net sampling and of juvenile fish from
examination of the stomach contents of billfishes, as compared to those of the 4 largesized tuna species, suggest a larger potential yield of this species than of large-sized tunas.
IV. Discussion of the Relationship between Distribution of Catches
of Tunas and Environmental Conditions
In the Indian Ocean, as mentioned in the previous chapter, remarkable seasonal
changes in longline catches of species are observed even in the lower latitudes. The
amplitude of seasonal fluctuations seems larger than in other oceans. The changes are
probably connected with changes in environmental conditions.
However, detailed examination of this relationship has not yet been carried out.
In the present paper, the distribution of catch is related to the surface current system,
the pattern of permanent thermocline and the upwelling.
1. Surface Current System, Water Type and Distribution of Tunas
NAKAMURA, YABUTA and MIMURA (1955) and MIMURA and NAKAMURA (1959)
discussed this relationship. They suggested that different species occur in the different
current systems.
a) In the South Equatorial Current area, albacore is abundant but yellowfin is not.
b) A remarkable concentration of yellowfin occurs during the northern hemisphere
summer, in the Equatorial Countercurrent area and in the zone with variable currents
between the South Equatorial Current and the SE monsoon drift. Albacore is absent
in these areas.
c) Over the NE and SW monsoon current areas, which are other habitats of yellowfin,
albacore seldom occurs.
On the basis of recent data (Fig. 1 a - h), the general relationship between species
distribution and ocean current systems, and/or water type, is summarized as follows:
The relationship between species distribution and ocean current system, and/or water
type, is more clearly observed with a species which occurs in the sub-tropical or subAntarctic area.
a) Southern Bluefin
The longline fishing occurs in the west-wind drift area, where individuals are caught
before maturing. When they mature, they move northward to the area off the NW coast
of Australia where they are influenced in some way by the west-wind drift. Very young
fish are found in the coastal waters off Sand SE Australia. It was revealed by ROCHFORD
(1958) and ROBINS (1958) that the concentration of young fish in the coastal area is
related to the sub-Antarctic water. Thus the feature of the occurrence of this species is
closely linked with the sub-Antarctic water (SHINGU, 1970).
A. SUDA:
season and consequently compensates for the others. The seasonal change in the amount
of catch per 1 000 hooks for individual species is remarkable, even in the case of
yellowfin which inhabit the tropical zone with its comparatively small seasonal change
in oceanographic conditions. This species peaks in the first quarter in the western part
of the tropical waters and in the third quarter in the eastern part of the Ocean.
Concerning the skipjack, though insufficient information is available, the more
frequent occurrences of larval fish from larvae net sampling and of juvenile fish from
examination of the stomach contents of billfishes, as compared to those of the 4 largesized tuna species, suggest a larger potential yield of this species than of large-sized tunas.
IV. Discussion of the Relationship between Distribution of Catches
of Tunas and Environmental Conditions
In the Indian Ocean, as mentioned in the previous chapter, remarkable seasonal
changes in longline catches of species are observed even in the lower latitudes. The
amplitude of seasonal fluctuations seems larger than in other oceans. The changes are
probably connected with changes in environmental conditions.
However, detailed examination of this relationship has not yet been carried out.
In the present paper, the distribution of catch is related to the surface current system,
the pattern of permanent thermocline and the upwelling.
1. Surface Current System, Water Type and Distribution of Tunas
NAKAMURA, YABUTA and MIMURA (1955) and MIMURA and NAKAMURA (1959)
discussed this relationship. They suggested that different species occur in the different
current systems.
a) In the South Equatorial Current area, albacore is abundant but yellowfin is not.
b) A remarkable concentration of yellowfin occurs during the northern hemisphere
summer, in the Equatorial Countercurrent area and in the zone with variable currents
between the South Equatorial Current and the SE monsoon drift. Albacore is absent
in these areas.
c) Over the NE and SW monsoon current areas, which are other habitats of yellowfin,
albacore seldom occurs.
On the basis of recent data (Fig. 1 a - h), the general relationship between species
distribution and ocean current systems, and/or water type, is summarized as follows:
The relationship between species distribution and ocean current system, and/or water
type, is more clearly observed with a species which occurs in the sub-tropical or subAntarctic area.
a) Southern Bluefin
The longline fishing occurs in the west-wind drift area, where individuals are caught
before maturing. When they mature, they move northward to the area off the NW coast
of Australia where they are influenced in some way by the west-wind drift. Very young
fish are found in the coastal waters off Sand SE Australia. It was revealed by ROCHFORD
(1958) and ROBINS (1958) that the concentration of young fish in the coastal area is
related to the sub-Antarctic water. Thus the feature of the occurrence of this species is
closely linked with the sub-Antarctic water (SHINGU, 1970).