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, provided that abundance of the coast of Iran, which was not covered, was similar to that on the Oman side.

In the Gulf of Aden the previous cruises gave estimates ranging between 6 and 40 million tonnes (mean 20 million tonnes), while the present cruises leads to an estimate of 4 million tonnes. Although the total biomass usually has been larger in the Gulf of Aden than the Gulf of Oman, the biomass per unit surface area has usually been highest in the Gulf of Oman (Fig. 8).

It is more difficult to compare the trawl catches because on the cruises conducted in 1975-76 trawling was mainly conducted to identify the sounder scatters, and there was no intention of getting catches. On the cruise 1979 we tried to get as large catcherates as possible, at least at most of the trawl stations.

In spite of this difference in approach, we had lower mean catch rates from the mesopelagic fish layer this year than from most of the previous cruises (Table 3).

Table 3. Catch rates in kg/hour of trawling from the mesopelagic fish layers on cruises with R/V "Dr. Fridtjof Nansen".

CRUISE	GULF OF OMAN			GULF OF ADEN
	Catch kg	Myctophidae %	No of hauls	Catch kg	Myctophidae %	No of hauls
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
Present cruise	200	93	47	40	40

In the Gulf of Aden there was also a difference in catch composition. With one exception (cruise 6) the family Myctophidae was between 94 and 96% of the mean catch rates given in Table 2. In 1979 it was about 40%. This is the lowest percentage recorded. Therefore, it seems that the abundance if 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 "Lemurn", and the following results were obtained.

November	1977	4.600.000 tonnes
May	1978	2.750.000 tonnes
September	1979	"still lower"

We have no surveys from the same years, but these results are very much lower than those obtained by R/V "Dr. Fridtjof Nansen" both in 1975-76 and in 1979.

For small pelagic fish the estimates made by R/V "Lemurn" and "Dr. Fridtjof Nansen" falls well within the same range.

If R/V "Lemurn" has a much lower source level than R/V "Dr. Fridtjof Nansen", mesopelagic fish at their day depth may be lost due to the threshold effect. If this is the explanation of the difference between the estimates obtained by the two vessels, we should expect that R/V "Lemurn" got much higher values for mesopelagic fish during night time that during day time. The reports available to us do; not state whether this was the case.

A further discussion of the differences must be postponed until the technical details of the acoustical equipment on R/V "Lemurn" are available.