Longline fishing for tuna off the coast of east Africa 1958–1960

Abstract

Previous work of the East African Marine Fisheries Research Organisation (EAMFRO) Zanzibar, on the pelagic fishes of the East African coastal region, lead to the conclusion that considerable subsurface stocks of tuna might exist. Exploratory longline fishing was carried out by the Japanese in the West Indian Ocean, the closeset point being 200 miles off the East African coast, and high catch rates of yellowfin tuna were obtained. A programme of research longlining was then initiated and carried out by EMFRO between October 1958 and March 1960. In theory the area of operations covered the whole of East Africa but in practice was restricted to the area from Lat. 2°50’s. to 8°40’s. up to 30 miles offshore. Unfortunately the survey was bed evilled by lack of finance and staff, and an erratic fishing programme due ta repeated mechanical failure in RV MANIHINE. Oceanographic conditions in the area are briefly outlined and are based on work previously carried out by EAMFRO. Charts are given showing the general distribution of surface currents in the area and the stratification of the water masses in the coastal region. Reference is also made to the properties of the north flowing East African Coastal Current (EACC) and the main thermocline between that mass and the underlying Arabian Sea Water (ASW). Bathythermograph casts were made several times a day but otherwise there was a lack of oceanographic work during the survey. A detailed record of weather encountered whilst longlining was maintained and is full tabulated and discussed. The type of longline required by EAMFRO needed to be cheap, easy to make up and effective for exploratory fishing; thus the design was based on American (POFI) research gear, altered to suit East African conditions and to fish 250 hooks. Each basket of EAMFRO gear consisted of a 120 fm. mainline, 511 fm. hook droppers and a 60 fm. buoy line and buoy. The cost and construction of the gear, assembly and setting from the vessel is discussed in detail, Unlike the American gear, which has short buoy lines and is set slack, it was decided that EAMFRO gear should have long buoy lines and be set rather tighter than normal. Comments are made on the reason for this decision. The speed of the line hauler on MANIHINE was totally unsuitable, being about one eighth of that of modern style Japanese longline haulers. Experience showed that the longline was best set along the line of the EACC which runs parallel to the coast (that is roughly in a northerly direction). The most satisfactory directions for setting and hauling the longline in relation to current, wind sea and the vessel are given. Eight men were used in setting and hauling procedure which are detailed. The greatest defect of the gear was as found by POFI workers the relatively short life of the cotton fishing line caused by (i) overstraining of the line; (ii) wear and tear during hauling; (iii) deterioration due to lack of proper care and maintenance. In addition, there was the non-selectivity of the gear and the unsatisfactory longline hauler. The use of EAMFRO gear is discussed fully; many faultswere put right during the survey and little change is needed in the material and make-up of longline for future operations. Full details are given of setting and hauling times for the 54 longline stations, together with a summary for each cruise. Hauling speeds did not seem to be ‘affected by a‘heavy catch, provided the fish did not tangle the line. Operating times are. compared’ with’ those for POFI gear and vessels in the Pacific. The effect of weather and deteriorating gear on hauling times is briefly discussed. Lack of finance restricted the amount of spare ‘gear’ which ‘should have been carried, and for the same reason virtually no fishing of experimental gear was undertaken. Methods used to determine maximum fishing depth of longline gear in the Pacific Ocean are reviewed. Chemical sounding tubes, the commonest and most ‘successful method, were used by EAMFRO. Data on depths of gear obtained by sounding ‘tubes and comparisons with theoretical and overall range of depth are tabulated. Asin other ‘areas it is shown that in the water EAMFRO gear lay roughly in a cantenary but that hauling procedures and not the eavironment caused the line to be skewed from the catenary. Shoaling of the gear (shearing) caused by normal environmental factors is not great in East African ‘waters, although it is not known how much shearing must take place at the main EACC/ASW ‘thermocline due to internal waves and differential current speeds and directions. Daily drifis of longline gear were measured and are tabulated, the overall average rate of drift being 1° 97 ~ knots. ' The invention and construction of the BELL TRIGGERING DEPTH GAUGE is ‘discussed briefly, together with its potential value to marine science, especially, longlining. Only two prototypes were available during the survey and were never on hooks catching fish. ‘One of the greatest problems at EAMFRO was the lack of local supplies of a silvery pelagic bait fish such as a sardine or mackerel. Instead it was necessary to use the scavengear Lethrinus variegatus, caught locally in quantity by MANIHINE. The yellow-green L. variegatus is a good holding bait, does not need to be double hooked and remains firm after long immersionin water. Other more suitable baits were used when available and it is believed that Lethrinus bait did limit tuna catches, although high catches of billfish were made with this species. Artificial baits of polythene were made in the laboratory, but the numbers were.so few that no definite results were achieved. A list is given of the scientific and common names, of the 19 species of fish and shark taken during the survey. Catch rates (expressed as catch per 100 hooks) for each of the important groups of fish (yellowfin tuna, striped marlin and shark), together with number of hook Set and noon positions, are tabulated by cruises and accompained by an overall summary. The ‘complete catch records and summaries are given in the Appendices. Also tabulated is the breakdown of the catch by numbers and by weight for each cruise. Location of longline station are shown on charts for each cruise undertaken. Some remarks are made on the present state of taxonomy of tunas and the one species concept in relation to yellowfin, bigeye, albacore and skipjack. Only yellowfin tuna, albacore and skipjack were taken off East Africa and there was no sign of T. tonggol reported from Aden and Somalia or the bigeye tuna, T. obesus, taken by the Japanese 200 miles off the East African ‘coast. Recent work has shown that morphometric differences between yellowfin tuna, Thunnus albacares, from different geographical locations can also occur between fishes in the same area over a period of time. Nevertheless, standardised morphometric measurements were madé on all longline caught yellowfin (tabulated in the Appendices) to allow comparisons to be made with yellowfin from South Africa and other parts of the Indian Ocean. - Fin and gill rakey counts are also given. Analysis of length/weight data on yellowfin showed no significant difference between the regression equations for the sexes. The overall catch rate of yellowfin tuna for the survey was 0-89/100 hooks (average weight 49lb.). Although catch rates were never high, there is a marked seasonal abundance of yellowfin tuna, almost alllongline catche being made during the SE monsoon (1-42/100 hooks), when the EACC is coldest, occupies the greatest depth range and is isothermal down to 60/70 fm. Due to the few cruises with hig yellowfin catches, little can be deduced regarding spatial distribution, although the general impression was of more tuna in southern areas and where there were extensions of the usually narrow continental shelf, or obstructions to ocean currents such as islands, banks, etc. As in the centYal Pacific there was a considerable daily variability in catch rates at the same position. Reasons are advanced why it is thought unlikely that catches of yellowfin are caused by localfood enrichment. Lack of yellowfin tuna in the NE.monsoon is striking and a possible connection with the current system is suggested. The level of correlation of spatial and temporal distribution and abundance of yellowfin with enviromental conditions is very low due to the limitation of the survey. It is hoped that the Bell Triggering Depth Gauge will solve the problem of estimation of the depth at which hooks are taken by fish. In Equatorial areas yellowfin are the least stratified of tuna and chemical sounding tubes showed the maximum range of yellowfin was from the surface to the top of the ASW. It is thought, however, that the yellowfin were mostly taken where gear fished longest—that is from the EACC/ASW thermocline to maximum depth of the gear. Although total numbers were small, two very similar length frequency modes were found in the SE monsoons of 1958 and 1959. These modes, when trans ferred to agrowth curve of yellowfin from the Equatorial Pacific, showed that the East African fish were in-age groups I and II and were typical of coastal or insular deep swimming stocks. The overall sex ratio for the survey was males to females 1 : 1-06. Gonad maturity states are defined and discussed. Only 4% of the yellowfin were immature and all were less than 68 cm.; spent fish were found as small as 71cm. These findings are much the same as throughout the geographical range of the species. Spatial distribution is connected with the fact that yellowfin spawn in East African waters and the records indicate a long spawning season at least from June to December, Random ‘night light stations were made but no tuna larvae were caught, although large numbers of juvenile yellowfin of about 1 Ib. were seen at the surface between December and April. The yellowfin had stomach contents from 4 to 827 cc. with the average about 1oo‘cc. Preliminary analysis shows percentage volume breakdown as Fish 42%, Cephalopds 37%. Crustaceans 10°5°/o and unidentifiable animal matter 10°5%. Comparisons are made with results from other parts of the Indo-Pacific and show the similarity of diet. Trematode, Nematode and Cestode parasite have been identified from the stomachs of yellowfin. . Detailed information is given on the single specimen of albacore (Thunnus alalunga caught during the survey. It appears that the albacore was taken at a greater depth than the yellowfin; this is expected ia equatorial zones as the optimum water temperature for alba core is much lower than that for yellowfin. Only two badly damaged specimens of skipjack (Katsuwonus pelamis) were taken and notes are given on these fish. The gonads of billifish are deseribed anc the maturity states used are defined.