RESULTS

Hydrography

The general tendency was that the surface layer was about 1-2°C warmer and the thermocline was not as distinct as in June-July. The salinity in the upper layer was also higher (0.1 - 0.2 ‰). The oxygen content was similar for the sections I-VI (Figs. 3-8) as observed in June-July, but for sections VII and VIII (Figs. 9, 10) the oxygen content was up to 2 ml/I higher this time. An oxygen minimum (2.5 ml/1) was observed at 200 m in section IV (Fig. 6) in June-July; now this minimum was 3.5 ml/I. The oxygen content for the water column 5-500 m was above 2.4 ml/1. The oxygen content observed in this water column has probably no limiting effect on the fish distribution.

Acoustic survey

The abundance of fish as observed from acoustic data was rather low (Fig. 11).

The fish biomass observed acoustically was estimated to about 100 000 tons. This was done by integrating the different areas within the isolines (Fig. 11) and multiply these areas by the intermediate integrator deflection value, 0.5, 5, 15 and the observed value of the areas above 20 mm was 25 mm. This was raised to a biomass estimate by applying the value:

1 mm integrator deflection = 13.6 tons/square nautical mile

The fish biomass estimates for the two periods are probably too low due to the following facts:

- A rather big proportion of the coastal area is too shallow for “Dr. Fridtjof Nansen” to enter
- Fish either close to bottom or to the surface are not observed by the acoustic equipment
- Noise from the steaming vessel will scare some of the fish away and thereby dilute the fish concentrations available for the acoustic equipment

Some surface schools were observed by eye in the Mafia region, the Zanzibar Channel, along the east coast of Zanzibar, and quite many in the shallow area northeast of Tanga.

Demersal trawl hauls in shallow waters

The trawl hauls reflected a similar species composition as in June-July. The average catch rates was less than oh the first survey. As demonstrated in Table 1 the reduction in the Zanzibar Channel was about 30%, in the Mafia Region about 40% and in the area south of 8 °S the reduction was about 70%.

Table 1 The average catch rates for demersal trawl hauls (depth 100m) in different regions during June-July and November 1982.

N = number of trawl hauls
C = catch per trawl hour (kg)

	Zanzibar channel		Mafia region		South of 8° S
	N	C	N	C	N	C
June/July	26	296	9	650	8	918
November	33	194	9	372	7	288

However, during the first survey a few very high catch rates (1500 kg/hour) were observed. If these catches are excluded the average catch rates for the three regions were 182 kg, 433 kg and 512 kg respectively. Two stations in the Zanzibar Channel (st.81, 82) one station in the Mafia region (st.37) and one station south of 8°C (St.33) were excluded. Thus the catch rates observed the Zanzibar Channel were similar for July and November. In the Mafia region and the area south of 8°S the catch rates were reduced by 15% and 45% respectively in November.

About 3% of the demersal trawl hauls gave catches above 1000 kg per hour, while about 22% of the hauls gave catches below 50 kg per hour. The best catch (1730 kg per hour) was made west of Latham Island (st.no. 292 Fig. 1) at about 140 m depth. The main species were Pristipomoides filamentosus and P. sindensis. The individual weights were 1-4.5 kg.

The dominating families in the catches were Leiognathidae, Mullidae, Carangida, Clupeidae and Lutjanidae. In the Zanzibar Channel the dominating species were small fish as Leiognathus leuciscus, L. bindus, Gazza minuta, Secutor insidiator, Gerres spp and different Mullidae. In the central part of the Channel catches of 200-300 kg per hour of damselfish (Teixeirichthys jordani) were made. This is also a small fish with a length range in the catches of 5 - 10 cm.

In the Mafia region the dominating species were similar as in the Zanzibar Channel except for the damselfish.

In the southern area the dominating species were bigger fish as Scombroides commersonianus and Epinephelus sp. In this area the largest catch was caught at st.269 (19 m). The main species in this catch of about 900 kg per hour were Sardinella albella and Leiognathus fasciatus.

Demersal trawl hauls in deeper waters

Due to earlier observations that little fish were in deeper water, just 18 trawl hauls were made in waters deeper than 150 m. The average catch rates were about 100 kg per hour (2-560 kg per hour). The dominating species at st.300 (Fig. 1) which gave 560 kg per hour were Dasyatis sp. and Decapterus Kurroides. The last species dominated another catch, st.321. Other important species in this depth stratum Saurida undosquamis, S. tumbil, Choropthalmus agassizi shrimp and prawns. If big sharks and skates are excluded from the catches the catch rates were reduced by about 30%.

Comparing trawl hauls with the Mbegani training vessel “Mafunzo”

Four demersal trawl stations in the western part of the Zanzibar Channel (Stations 312 - 315) were worked by F/V “Mafunzo” and R/V “Dr. Fridtjof Nansen”. The depth of the trawl hauls ranged from 19 to 45 m. In this depth range with the actual species composition (small fish, Leiognathidae and Mullidae, etc.), it seems that F/V “Mafunzo” caught on average about twice as much as R/V “Dr. Fridtjof Nansen”. This is probably caused by the following facts: The “Mafunzo” trawl has no bobbins and operates further away from the vessel than the “Dr. Fridtjof Nansen” trawl. Having no bobbins, the “Mafunzo” trawl operates closer to the bottom. Besides, in shallow waters “Dr. Fridtjof Nansen” will scare away more fish than the much smaller vessel “Mafunzo”. Comparison of trawl hauls in deeper water was planned but could not be carried out because of technical problems onboard “Mafunzo”.

Fishing experiments with traps and longline

Three longline and 13 trap stations were made The bait was mainly squid, but small fish was used in some of the traps. No obvious difference in catch according to bait was observed. The fish trap which is collapsible is shown in Fig.12.

At St.No.276 large fish, such as Epinephelus malabaricus (76 cm/6.3 kg) and Lutjanus coccineus (62 cm/3.5 kg, 74 cm/5.4 kg) were caught in the traps. Usually smaller fish were caught.

It may be concluded that the traps and the longline will fish rather well if placed in proper areas.

Figure 1. Survey courses and stations. Second fisheries resources survey, Tanzania. 10 Nov - 3 Dec 1982.

Figure 2. Survey courses and stations (Mafia - Kenya border). Second fisheries resources survey, Tanzania. 17 Nov-3 Dec 1982.

Figure 3. Temperature, salinity, density and oxygen at hydrographic section I: Moa Bay - Pemba (North). R/V “Dr. Fridtjof Nansen”, 30 Nov - 1 Dec 1982.

Figure 4. Temperature, salinity, density and oxygen at hydrographic section II: Pangani - Zanzibar Island - East. R/V “Dr. Fridtjof Nansen”, 29-30 Nov 1982.

Figure 5. Temperature, salinity, density and oxygen at hydrographic section III: Wami - Zanzibar. R/V “Dr. Fridtjof Nansen”, 25 Nov 1982.

Figure 6. Temperature, salinity, density and oxygen at hydrographic section IV: Dar-es-Salaam - Northeast. R/V “Dr. Fridtjof Nansen”, 21 Nov 1982.

Figure 7. Temperature, salinity, density and oxygen at hydrographic section V: Kwale Island - East. R/V “Dr. Fridtjof Nansen”, 18-19 Nov 1982.

Figure 8. Temperature, salinity, density and oxygen at hydrographic section VI: Mafia Island (South) - Northeast. R/V “Dr. Fridtjof Nansen”, 16-17 Nov 1982.

Figure 9. Temperature, salinity, density and oxygen at hydrographic section VII: Ruvuma River - North. R/V “Dr. Fridtjof Nansen”, 13-14 Nov 1982.

Figure 10. Temperature, salinity, density and oxygen at hydrographic section VIII: Ruvuma River - Northeast. R/V “Dr. Fridtjof Nansen”, 14 Nov 1982.

Figure 11. Integrated echo intensities (mm per nautical mile) classified as fish. R/V “Dr. Fridtjof Nansen”, 10 Nov - 3 Dec 1982.

Figure 12. The collapsible fish trap made of an upper aluminium frame with floats and a bottom iron frame. The size: 130 × 45 × 45 cm.