139736eo.pdf (20MB) - Japan Oceanographic Data Center

By April/May the demersal stocks had migrated to greater depths. The main concentrationwas now between 100 and 120 m, where Sparidae and cuttlefish were dominant. It is assumed thatthe crabs had moved deeper although a few were found. At 30 m the CPUE was reduced to 40 kg hrlcompared with 381 kg hr-l in October. The dominant families at 30 m were Sparidae, Sepia andCarangidae, with a much reduced speciation compared to October. At 114 m depth the CPUE was thehighest recorded, 660 kg hrl, where the main taxa were Sepia, Sparidae and Serranidae (groupers).During upwelling at the same locality at Ras Fartak, catches deeper than 20 m depth weredominated by Callionymidae (97%), while in shallower water Nemipteridae dominated (90%). Sincethe latter normally occurred between 70 and 100 m at this locality, a strong inshore migration wasindicated.Locality ChangesLocal changes have been indicated for the west, central and east sectors in Figure 17.Variations along the coast in the non-upwelling period were too complex to discuss here, but somegroups were only found at Ras Fartak (e.g. Drepanidae, moonfish), while others reached theirmaximum abundances here and were scarce elsewhere (e.g. Trichiuridae, Indian threadfins). In othergroups the speciation was different along the coast (e.g. Carangidae).Standing StocksEstimates of the quantities of demersal fish on the shelf were made using the swept areamethod of SHINDO (1972) with a catchability coefficient of 0.5, or an assumed escapage of fish boththrough the trawl meshes and avoidance of the net of 50%. During upwelling demersal trawl data mayhave been misleading due to intrusion of deeper water species on to the coastal shelf. Further, therestricted survey of R/V Dr. F. Nansen in August may have missed pockets of inshore demersal fish.The average standing stock estimates for depths to 120 m are given in Table 2, while the seasonalvariations (with depth) in standing stocks at Ras Fartak are given in Table 3, where movement byseason can be seen. Higher stocks occurred on the eastern trawl grounds, which were the mainlocation of cuttlefish stocks. The shelf area of this coast is around 26 x 1@ km2 to a depth of 200 m.R/V Dr. F. Nansen surveys in 1975/76 gave a demersal stock estimate of 164 - 238 x 1@ tons usingintegrated echosounding. The results of the R/V Ibin Magid surveys have indicated a minimum stockof 116 x 103 tons, partly using data from R/V Dr. F. Nansen to calculate stocks between 120 and 200m.A decrease in stocks, probably due to trawling for cuttlefish, has occurred since 1975.Formerly cuttlefish were trawled over a wider area, with grounds existing to the west of Aden, but arenow trawled between Mukalla and Ras Fartak. The stock estimates given here for the east sector inTable 2 coincide with those in FAO (1981) for neighbouring Omani waters.Other Demersal ResourcesIn shallow coastal areas with muddy bottoms, the shrimp Penaeus semisulcatus was found.East of Ras Fartak catch rates reached 17 kg hr-1 during spawning in October. The deep sea lobster,Puerulus sewelli, is trawled at 200 to 300 m depth off Mukalla and Ras Fartak.Table 2.Average stock estimates of demersal fish for the coastal shelf to a depth of'120 m (tons km-2).Area/SeasonNon UpwellingUpwellingWest3.61.9Central East3.2 6.01.4 2.5267