448
A. SUDA:
and southern bluefin is understood as a reflection of the difference in primary productivity
among their domains. In practice, the relation between primary productivity and
abundance of tunas must be rather an obscure one because of time lags between the peak
seasons of productivity on different trophic levels; at the same time, a spatial dispersion
S.B.F
AI.
Fig. 6. Latitudinal changes in hooking-rates of the 4 large-sized tuna species in areas of south-north
direction on the whole year basis, a: between 55° and 60° E, b: between 90° and 95° E. Southern
bluefin: S.B.F., albacore: AI., bigeye: B.E., yellowfin: Y.F.
of the production is expected on higher levels. Therefore, the relation between primary
productivity and the abundance of tunas is probably an indirect one.
The manner of distribution of bigeye is much more complicated. Immature bigeye
occur along the southern boundary of the sub-tropical gyre. The mature group occurs
along the bottom of tropical surface water. Thus, the occurrence of this species is
characterized by its concentration along the boundary of a current system or a water
type. As already suggested, the domain of this species is neither the enriched Indian
equatorial water nor the fertilized subantarctic water. The manner of the distribution
of the species might be interpreted as an example of "Edge effect".
A. SUDA:
and southern bluefin is understood as a reflection of the difference in primary productivity
among their domains. In practice, the relation between primary productivity and
abundance of tunas must be rather an obscure one because of time lags between the peak
seasons of productivity on different trophic levels; at the same time, a spatial dispersion
S.B.F
AI.
Fig. 6. Latitudinal changes in hooking-rates of the 4 large-sized tuna species in areas of south-north
direction on the whole year basis, a: between 55° and 60° E, b: between 90° and 95° E. Southern
bluefin: S.B.F., albacore: AI., bigeye: B.E., yellowfin: Y.F.
of the production is expected on higher levels. Therefore, the relation between primary
productivity and the abundance of tunas is probably an indirect one.
The manner of distribution of bigeye is much more complicated. Immature bigeye
occur along the southern boundary of the sub-tropical gyre. The mature group occurs
along the bottom of tropical surface water. Thus, the occurrence of this species is
characterized by its concentration along the boundary of a current system or a water
type. As already suggested, the domain of this species is neither the enriched Indian
equatorial water nor the fertilized subantarctic water. The manner of the distribution
of the species might be interpreted as an example of "Edge effect".