Tuna Fisheries and their Resources in the Indian Ocean
V. Discussion on the Prospects of Tuna Fisheries
in the Indian Ocean
449
The stocks of large-sized tunas are considered to be more or less fully exploited and
substantial increases in catch may not be realized by increased effort (SHOMURA et ai.,
1967; FAO 1968, 1969; KIKAWA et ai., 1969). The observed relationships between catch
and effort indicate that the maximum sustainable yield for individual species may not be
farfrom the following estimates (FAO, 1969 a): yellowfin 30 - 35, albacore 20 - 25, bigeye
22 - 28, and southern bluefin 35 - 40 thousand tons. Analyses of catch-effort relationships and the size composition of commercial catches, together with the calculation of
yield per recruitment, suggest that the recent depletion of resources of the 4 large-sized
tuna species has been caused by increased fishing rather than by a decrease in recruitment.
It is the author's opinion that the present activities of the fisheries seem to be around the
level at which the maximum sustainable yield could be realized. Therefore, fixing the
activites of fisheries at around the level of recent years appears to be the first step
necessary for rational management.
Resources of the small-sized species are still under-utilized. However, we do not have
a useful means of examining more effective ways to exploit these resources because of the
lack of biological and environmental information. Among the small-sized species, little
tuna and frigate mackerel are coastal species and may be of possible concern to local
fisheries but not to fisheries which operate on a large scale in the open ocean.
V. Discussion on the Prospects of Tuna Fisheries
in the Indian Ocean
449
The stocks of large-sized tunas are considered to be more or less fully exploited and
substantial increases in catch may not be realized by increased effort (SHOMURA et ai.,
1967; FAO 1968, 1969; KIKAWA et ai., 1969). The observed relationships between catch
and effort indicate that the maximum sustainable yield for individual species may not be
farfrom the following estimates (FAO, 1969 a): yellowfin 30 - 35, albacore 20 - 25, bigeye
22 - 28, and southern bluefin 35 - 40 thousand tons. Analyses of catch-effort relationships and the size composition of commercial catches, together with the calculation of
yield per recruitment, suggest that the recent depletion of resources of the 4 large-sized
tuna species has been caused by increased fishing rather than by a decrease in recruitment.
It is the author's opinion that the present activities of the fisheries seem to be around the
level at which the maximum sustainable yield could be realized. Therefore, fixing the
activites of fisheries at around the level of recent years appears to be the first step
necessary for rational management.
Resources of the small-sized species are still under-utilized. However, we do not have
a useful means of examining more effective ways to exploit these resources because of the
lack of biological and environmental information. Among the small-sized species, little
tuna and frigate mackerel are coastal species and may be of possible concern to local
fisheries but not to fisheries which operate on a large scale in the open ocean.