Comparisons with previous cruises

In the Gulf of Oman the five acoustical surveys carried out by R/V "Dr. Fridtjof Nansen" during 1975-76 gave abundance estimates ranging between 8 and 20 million tonnes (mean 13 million tonnes). The survey in August 1979 gave 8 million tonnes and then in January 1981 11 million tonnes, provided that abundance off the coast of Iran, which was not covered, was similar to that on the Oman side (Fig. 14).

In the Gulf of Aden the previous cruises gave estimates ranging between 6 and 40 million tonnes (mean 20 million tonnes), while the cruises in 1979 and 1981 lead to an estimate of 4 million and 16 million tonnes respectively (Fig. 15). Although the total biomass usually has been larger in the Gulf of Aden than in the Gulf of Oman, the biomass per unit surface area has usually been highest in the Gulf of Oman.

Fig. 14. Estimated abundance of mesopelagic fish in the Gulf of Oman 1975 - 1981.

Fig. 15. Estimated abundance of mesopelagic fish in the Gulf of Aden 1975 - 1981.

It is more difficult to compare the trawl catches because during the cruises conducted in 1975-76 trawling was mainly conducted to identify the sound scatters, and not to obtain high catch rates. Trial fishing was, however, one of the objectives of the surveys in 1979 and 1981.

In spite of this difference in approach we had lower mean catch rates from the mesopelagic fish layer in the Gulf of Aden in these years than from most of the previous cruises (Table 12). In the Gulf of Oman the catch rates were low in 1979 but higher in 1981.

Table 12. Mean catch rates in kg/hour of trawling from the mesopelagic fish layers on cruises with R/V "Dr. Fridtjof Nansen".

Cruise	Catch	Gulf of Oman Myctophidae	No of hauls	Catch	Gulf of Aden Myctophidae	No of hauls
	kg/h	%		kg/h	%
1-2				120	94	7
3	280	89	4	4900	96	2
4	2470	99	10	230	90	6
5	1060	88	9	315	95	1
6	50	89	7	45	50	5
1979	200	93	52	40	40	28
1981	1010	94	74	54	60	23

In the Gulf of Aden there was also a difference in catch composition. With one exception (cruise 6) the family Myctophidae represented between 94 and 96% of the mean catches given in Table 2. In 1979 the corresponding proportion was about 40% and in 1981 60%. The percentage from 1979 is the lowest recorded. It seems therefore that the abundance of this family has decreased even more than the decrease in total echo abundance suggests. In the Gulf of Oman no such change was observed.

During the "Regional fishery survey and development Project" the echo abundance of mesopelagic fish in the Gulf of Oman was estimated by R/V "Lemuru", and the following results were obtained:

November	1977	4.6 million tonnes
May	1978	2.9 million tonnes
September	1978	1.7 million tonnes

These results are very much lower than those obtained by R/V "Dr. Fridtjof Nansen" both in 1975-76 and in 1979 and 1981.

In parts of the area R/V "Lemuru" observed denser concentrations than those found with the R/V "Dr. Fridtjof Nansen", but also large areas with no recordings of mesopelagic fish was reported from the "Lemuru" survey.

This difference is probably caused by the much lower performance, and the higher threshold used on the acoustical equipment on R/V "Lemuru" compared with that on R/V "Dr. Fridtjof Nansen" (FAO, 1981).