Hydrography
Acoustic survey
Demersal trawl hauls
Fishing experiments with traps and longline
The temperature and salinity in the surface layer was about 3 C warmer and about 0.2 ‰ higher than in August. The thermocline in August was situated in the depth 100 - 150 in the two southern sections, while in December it was in the depth region 50 - 125 m (Figs. 3-4). In the two northern sections (Figs. 5,6) the thermocline was at about the same depth during both surveys (100-125 m)
The upper layer of relatively high oxygen content (4.5 ml/l) in the three southern sections was about 50 m thinner this time than during the former survey. In the northern section this layer had similar thickness as in August. The oxygen content observed in the water column 500 -5 m has no limiting effect on the fish distribution.
As in August the abundance of fish observed acoustically was rather low (Fig. 7). Aimed pelagic trawl hauls for identification of scattering layers resulted in fish larvae, Myctophids, squids and on one occasion (St. no. 370) at about 60 m depth, 20 kg of Leiognathidae, Clupeidae and Carangidae were caught.
Several small fish schools were spotted by eye along the coast, especially in the area north of Lamu towards Somalia.
The area south of Malindi, between St.ns. 367 and 370, was not well covered due to rather high fishing activity of small fishing boats. Neither was the bottom suitable for demersal trawling.
Based on a target strength; 1 mm integrator deflection corresponds 13.6 tons per square nautical mile, the biomass of fish in the acoustically investigated area was estimated to about the same as in August, 25 000 - 30 000 tons. This is an underestimate of the fish biomass because fish close to the bottom or the surface is out of range of the acoustic equipment. Besides the area close to the coast was too shallow for “Dr. Fridtjof Nansen” to enter, and the noise of the vessel will scare away fish and thereby a lower fish density than the actual one will be recorded by the equipment.
33% of the hauls gave catches below 50 kg per hour, while only 7% gave catches above 1000 kg per hour. The best catch was made 5 nautical miles north of Malindi (St.no. 371). The catch rate was about 1700 kg per hour and the main species caught were Pellona ditchela, Secutor insidiator, Leiognathus equulus and Gazza minuta. The trawl stations close to the coast north of Malindi gave rather good catches of big fish as Lutjanus coccineus, Epinephelus malabaricus, Pomadasys multimaculatum, but also Sardinella gibbosa was abundant in the area.
In the Malindi area 14 of the hauls from the August survey were repeated. The average catch rates for these stations in August and December were 230 kg and 360 kg per hour respectively. The increase in December was mainly caused by the big catch of clupeids at St.ns. 371 and 385 (1700 kg and 1500 kg per hour). Excluding these two stations give an average catch rate of 160 kg per hour for December. Except these two rather big catches of Pellona ditchella (st.371) and Sardinella gibbosa (st. 385) the species composition in the catches was similar to that observed in August.
In the North Kenya Bank area 6 of the trawl hauls from August were repeated. The average catch rates were 50 kg and 40 kg per hour for December and August respectively.
The average catch rate in the Malindi area was reduced by 30% in December while the two surveys gave similar catch rates for the North Kenya Bank area.
According to the trawl and acoustic data the magnitude of fish biomass was similar or slightly reduced from August to December in the area investigated by “Dr. Fridtjof Nansen”.
Two longline (70 hooks) and 6 fish trap (5 traps) stations were made. The bait was squid (Loliginidae). The two longline stations (St.ns. 389 and 398) were not very successful. The line was in the sea for one hour, and during that time most of the bait was gone. At St.398 two sharks (Carcharhinus sealei) of 4.2 kg was caught and at St.389 nothing was caught. The fish trap which is collapsible is shown in Fig.8.
The traps were in the sea for about 10 hours and the catch varied between 4 kg and 40 kg. Big fish as Epinephelus sp. (6.2 - 10.4 kg) and Lutjanus sp. (2.0 - 7.0 kg) were caught at trap St.399 at North Kenya Bank. The bottom at this station was rather rough and not suitable for trawling.
It may be concluded that the traps and probably the longlines will fish rather well if placed in proper areas.
Figure 1. Cruise track and stations. Fishery resources survey, Kenya, 7-15 December 1982.
Figure 8. The collapsible fish trap made of an upper alluminium frame with floats and a bottom iron frame. The size: 130 x 45 x 45 cm.
