The Development of Ceylon's fisheries (a symposium) Item Type article Download date 02/10/2021 15:21:52 Link to Item http://hdl.handle.net/1834/32499 279 The central equatorial region which showed the heaviest decline right from the b5ginning 0:111 be expected to yield a catch of 1 to 2 tons per unit effort. However with reduction in the fishing intensity in this region due to the spreading of the effort westwards, the catch rate in the central equatorial region though low, appears to be approaching stabilisation. With declining catch rates, the size of the tuna was also becoming smaller ; however, this phenomenon too does not seem to be getting worse. This condition setting in for this region can be expected to influence favourably the fishery in the Bay of Bengal as well. In the eastern part of the Arabian Sea fishing was good during the period 1957 to 1958 Gradually the good fishing areas have shifted westwards and the area immediately west of Ceylon has become relatively poor now. The Arabian Sea and the west equatorial region west of the Maldives and Chagos Islands, east of the African coast and north of Madagascar is a very rich ground at present (Fig. 4). With the decline in the average value of catch of yellowfin tuna per unit effort, tmn boJ.ts have to spend more days fishing in order to fill their holds and at the same time boa.ts leaving J apJ.nese bases have to sail the entire width of the Indian Ocean to get to the rich yellowfin grounds off Africa. This problem iE: being overcome by operating the tmu boats from African bases and by refuelling out at sea. Presently the number o+ boats operating from African bases is increasing rapidly, and their catches are transhipped either to ,Japan or directly to countries like the United States to which about 4,000 tons of the tm1a are thus shipped from the Indian Ocean. Section 2-Big eye tuna Biology.- The life span of this species also is about seven years. This kind of tuna grows to a length of about 2:30 em. (150 Kgrms.) and attains maturity when it reaches a length of 91-100 em. (14-20 Kgrms). The size group entering the fishery is 80 to 180 em. with the mode at 140 em. The 120 to 180 em. group is dominant in the equatorial region. Tho size becomes smaller ill the south where bigeys over 1:30 em. are scarce. The mature female has a fecundity of 2.8 to 3.6 million eggs. The spawr1ing occurs both in the eastern and western waters and the bigeye in these two regions are considered as two sub-populations. The members of the eastern sub-population resemble the Pacific bigeye in their ch&racters. In the Indian Ocean as a whole mR.les predominate over the females, both among the smaller and the larger sized fish. Age group 0 I II III IV v VI Size 35 55 75 90 115 130 145 ± 5·9 ± 4·1 ± 2·6 ± 1·8 ± 2·2 ± 3·5 Distribution.-This species also has a wide distribution like the yellowfin and is relatively more concentrated in the northern waters than in the southern half of the Indian Ocean. However, this specief does not seem to be dominant in any part of this ocean. Best bigeye tuna fishing areas in tbe Indian Ocean are the mouth of the Arabian Sea west of the Maldive~:, and south of the Sunda. Islands. No bigeye is caught ir the antcrc+ic waters or waters affected by that flow. Though roughly similar k the yellowfin in distribution pattern, the bigeye perhaps lives at a relatively greater depth and less densely too. This species also has characters rather distinctly different from tho::;e of yellowfin and alb2 core. H has been shown in the c&se of the Pacific bigeye that ik distribution is not necessarily related to the plankton diE:ttibution. 1n the India::J. Ocean the fiflhing grounds are spawning gr0unds and the distribution of the feeding groups is still unknown. Catch per Unit Effort.-The hooked-rate in the Indian Ocean for higeye tuna ranges between 0.6 and 2.5 per cent., the average being a little below 1 per cent. There have been no marked changes in the stock through these years even though there have been fluctuations. The fluctuation of the hooked rate in different regions is shown below: Area 1952 1.963 1.954 1956 19[.6 1957 1958 1969 1960 60°-70°E 0·81 0·85 l·Ol 1.07 1 ·04 0·67 0·81 70°-80°E 0·75 0·67 0·75 0.84 0·75 0·83 0·75 80°-90°E 0·83 0·81 0·76 0·93 0.83 0·79 0·63 0·63 7°S-12°S," 90°-100°E 0·97 1·18 1·45 1·65 1.51 1·37 1·08 1·17 l00°-ll0°E 1·09 1·82 1·64 1·54 1.37 1·25 1•22 1·52: " ll0°-120°E 0·81 0·93 1·52 1·19 0·76 0.55 0·60 0·76 0·75 " 6--R 951 (9/64) 280 Though they are caught almost in the same area as the yellowfin tuna, the annual fluctuation 'Df the hooked rate is markedly dissimilar to that of yellowfin tuna. The significant decrease of the hooked :rate with continued exploitation of the fishing ground, as seen in the case of yellowfin tuna, is not noticed in this species. Section 3-Blue:fin tuna Biology.-This species is the largest of the tunas reaching a length of 2·5 m. and a weight of 300 to 350 kilograms. The dominant size group is 140-170 em. with the mode at 155 em. April to August, fish of 150 em. length or more are rarely caught. The fish caught during the active :fishing season are the spawning group. Until recently the bluefin tuna off the west, south and east coasts of Australia were considered to be completely independent populations and were called the " Indian bluefin " (West coast), " Southern bluefin " (South coast), and the "Australian bluefin" (East coast). Recently from morphmetric and tagging studies it has been shown that the fish from these three :regions cannot be distinguished morphologically though different spawning groups occur on either side of the continent, and also that intermingling occurs between those from the west and the south. Distribution and Fishing Grounds.-In the Indian Ocean this species was thought to be found along the west coast of Australia, 95° to 125°E longitude and 8°S to 20°S latitude, since 1952. In 1958, a "new ground" was discovered south of the "old ground", between 20°S and 30°S (Fig. 5). Very reeently it has been found that the bluefin tuna exists between 30°S and 40°S and longitudinally between 50°E and 90°E. Whether it is sufficient for commercial exploitation yet remains to be studied. The seasonal changes in the location of the " old ground " is as follows :- September to Octobe:r : Between 8°S-l5°S, ll0°E November to Feb:rua.:ry : 8°8-20°8, l00°E March 8°8-20°8, ll0°E After April the fish is supposed to move southwards. The fishing reaches a peak during October, in the old ground. In the new ground the fishing commences in September and reaches a peak in Februaryland then declines. A north-south migration between the two grounds is evident. 20~----~----4-----+-~--~ ... 60 70 90 100 110 120 130 Fig. 5 : Indian Ocean Tuna Fishing Grounds : Density distribution of blue fin tuna (catch per 100 hooks). Gatch per Unit E:ffort.--Roughly half the effort applied in the Indian Ocean is concentrated in the areas described above. There are two reasons for this. One is that the hooked rate of bluefin tuna is very high in the Indian Ocean and the other is that this tuna reaches an enormous size. These 281 ilwo combine to g1ve very high values for the catch per unit effort in terms of weight. An average of 3 to 4 per cent. hooked-rate for the old ground and 7 to 8 p::lr cent. for the new ground is being realised. The :fishing effort directed at catching bluefin increased steadily from about 48 operations in 1952 to 1,930 operations in 1959. Beginning from 1960 the effort shot up to 4,275 operations, with the discovery of the new ground. Presently the effort is in the region of 5,200 operations per year. Over half the total production of tunas from the Indian Ocean comes from the bluefin :fishery. Biology.-This species is the smallest of the tunas and is uncommon in the warmer waters of the Indian Ocean. It has a life span of roughly ten years. Those caught north of 11he equator are smaller (mode 95 em.) than those from the southern part. In the region 0° to 4°S the modal size is 102 em. and this increases southwards to the 20°-25°8 latitude. Further south the modal size decreases. This probably could be attributed to the existence of a convergence around 24°S. The albacore above this convergenee belon~ to the spawning group and those below it belong to the feeding group. This condition resembles the pattern in the North Pacific where the sub-tropical convergence is the boundary separating the spawning group from the feeding group north of it.