Tuna Fisheries and their Resources in the Indian Ocean
447
indicate in any quarter of the year such poor productivity of tunas as was observed in the
Arabian Sea. MIMURA and NAKAMURA (1959) reported a fairly high hooking-rate of
bigeye in the area during the northern hemisphere summer when upwelling occurs, as is
pointed out by WYRTKI (1962) and ROCHFORD (1962).
It is the author's opinion that the linkage between upwelling and the occurrence of
tunas is not to be explained in a simple way but in a complicated one, which is connected
with, perhaps, fishing methods, the ecological condition of the fish, and other
environmental conditions apart from upwelling.
4. Tentative Considerations on the Factors Determining
Distribution and Abundance of Tunas
In the foregoing discussion, it was indicated that yellow fin is the most predominant
among the 4 large-sized tuna species. Less abundant are southern bluefin, bigeye and
albacore, respectively. The occurrence of the 4 tuna species clearly differs in its
geographical distribution. The subantarctic water is the territory of the southern bluefin.
The subtropical gyre is the habitat of albacore. The distribution of yellowfin coincides
with that of the Indian equatorial water. Bigeye live together with albacore in the subtropical gyre when they are immature, and with yellowfin in the Indian equatorial water
when they are mature.
It seems that the best explanation for the manner of distribution of tunas is, as
suggested by NAKAMURA (1954), that each species inhabits its own area of a certain
ocean current or water type. In Fig. 6, it is clear that boundaries between distributions of
species are almost the same as those which exist between ocean currents, water types, or
ocean current and water type. This appears to control the distribution of tunas more
efficiently than the primary productivity of the Ocean. For instance, in spite of the
indication of low primary productivity, albacore occurs exclusively in the subtropical gyre.
This is in contrast to the common anticipation that there must be a good coincidence of
high primary productivity with high concentrations of tunas. Even a negative correlation is
sometimes observed, as already demonstrated in the Arabian Sea where upwelling brings
high primary productivity.
However, the following consideration leads to some ideas about the relationship
between primary productivity and the abundance of tunas. It is observed that the Indian
equatorial water supports the yellowfin stock which appears to be the largest biomass
among the 4 large-sized tuna species in the Indian Ocean. On the other hand, the subtropical gyre maintains the smallest species stock, that of albacore. The biomass of southern
bluefin, which is the second in size among the 4 species, is maintained in the subantarctic
waters. Such a combination of size of species biomass and current system or water type as
a domain suggests that a relationship between primary productivity and abundance of
tunas should be interpreted less through the geographical or superficial correspondence of
the relationship than through the comparison between the size of biomass and the
accumulated productivity in the habitat by species. It is noted by MCGILL (1966),
KETCHUM (1969) and CUSHING (1969) that upwellings are observed in the Arabian Sea
and the Bay of Bengal and along the southern coast of the Great and Small Sunda Islands
and that the low latitudinal area north of 100 S is more enriched than the area of the subtropical gyre. In the subantarctic area, primary productivity is higher than that in the
subtropical gyre. The difference of stock size among populations of yellowfin, albacore
447
indicate in any quarter of the year such poor productivity of tunas as was observed in the
Arabian Sea. MIMURA and NAKAMURA (1959) reported a fairly high hooking-rate of
bigeye in the area during the northern hemisphere summer when upwelling occurs, as is
pointed out by WYRTKI (1962) and ROCHFORD (1962).
It is the author's opinion that the linkage between upwelling and the occurrence of
tunas is not to be explained in a simple way but in a complicated one, which is connected
with, perhaps, fishing methods, the ecological condition of the fish, and other
environmental conditions apart from upwelling.
4. Tentative Considerations on the Factors Determining
Distribution and Abundance of Tunas
In the foregoing discussion, it was indicated that yellow fin is the most predominant
among the 4 large-sized tuna species. Less abundant are southern bluefin, bigeye and
albacore, respectively. The occurrence of the 4 tuna species clearly differs in its
geographical distribution. The subantarctic water is the territory of the southern bluefin.
The subtropical gyre is the habitat of albacore. The distribution of yellowfin coincides
with that of the Indian equatorial water. Bigeye live together with albacore in the subtropical gyre when they are immature, and with yellowfin in the Indian equatorial water
when they are mature.
It seems that the best explanation for the manner of distribution of tunas is, as
suggested by NAKAMURA (1954), that each species inhabits its own area of a certain
ocean current or water type. In Fig. 6, it is clear that boundaries between distributions of
species are almost the same as those which exist between ocean currents, water types, or
ocean current and water type. This appears to control the distribution of tunas more
efficiently than the primary productivity of the Ocean. For instance, in spite of the
indication of low primary productivity, albacore occurs exclusively in the subtropical gyre.
This is in contrast to the common anticipation that there must be a good coincidence of
high primary productivity with high concentrations of tunas. Even a negative correlation is
sometimes observed, as already demonstrated in the Arabian Sea where upwelling brings
high primary productivity.
However, the following consideration leads to some ideas about the relationship
between primary productivity and the abundance of tunas. It is observed that the Indian
equatorial water supports the yellowfin stock which appears to be the largest biomass
among the 4 large-sized tuna species in the Indian Ocean. On the other hand, the subtropical gyre maintains the smallest species stock, that of albacore. The biomass of southern
bluefin, which is the second in size among the 4 species, is maintained in the subantarctic
waters. Such a combination of size of species biomass and current system or water type as
a domain suggests that a relationship between primary productivity and abundance of
tunas should be interpreted less through the geographical or superficial correspondence of
the relationship than through the comparison between the size of biomass and the
accumulated productivity in the habitat by species. It is noted by MCGILL (1966),
KETCHUM (1969) and CUSHING (1969) that upwellings are observed in the Arabian Sea
and the Bay of Bengal and along the southern coast of the Great and Small Sunda Islands
and that the low latitudinal area north of 100 S is more enriched than the area of the subtropical gyre. In the subantarctic area, primary productivity is higher than that in the
subtropical gyre. The difference of stock size among populations of yellowfin, albacore