MESOPELAGIC FISH

The Oman portion of the Gulf of Oman was surveyed from the UAE border to Ras al Hadd between November 9-19, 1983.

The mesopelagic fish are found off the continental shelf in waters deeper than 100 m. During previous surveys in the Gulf of Oman two sound-scattering layers were usually observed during both daytime and nighttime, with a typical depth distribution as presented in Table 6.

Table 6. Depth distribution of the mesopelagic fish layers in The Gulf of Oman.

Time	Layer	Depth	Vertical extension
Day	DI	150	40
	DII	250	80
Night	NII	50	70
	NIl	225	70

During the present survey DI, DII and NI, NII were only observed in a few areas close to the continental slope in the inner part of the Gulf and north of Muscat (see Fig. 9). The average integrator deflection of the DI layer was 136 mm/n.mile and only 7 mm/n.mile for NII layer in areas where these layers were observed. However, in the main part of the surveyed area only one layer was found and was identified as DII at daytime and NI at night. The distribution of the mesopelagic fish in these layers is presented in Fig 9.

To estimate the amount of mesopelagic fish in the upper mixed layer, the recordings of plankton during daytime in the same or a nearby area were subtracted. The average echo abundances are shown in Table 7 where they are compared to the results of the surveys in February and March 1983.

Table 7. Echo abundance (mm integrator deflection) in the Gulf of Oman from the present survey compared to the surveys in February and March 1983.

				DAY			NIGHT
Survey				No. five n. miles	DII		No. five n. miles	NI
					mean	SD		mean	SD
Feb.	W	58	E	25	252	145	40	94	75
	E	58	E	50	155	151	25	115	91
Mar.	V	58	E	26	119	88	51	100	40
	E	58	E	56	67	91	25	61	40
Nov.	V	58	E	27	272	222	51	89	57
	E	58	E	54	104	70	19	66	19

Figure 9. Distribution of mesopelagic fish. Echo intensity by levels of integrator deflection.

The echo abundance was converted to biomass using the equation

The area of the inner part of the Gulf (W of 58 E) 7000 sq.miles and the outer part (E of 58 E) to 15000 sq.miles. The results are compared to those from February and March 1983 and are presented in Table 8.

Table 8. Estimated biomass of mesopelagic fish in the Gulf of Oman in February, March and November 1983 (million tonnes).

	DAY	NIGHT	AVERAGE
Feb.	9,6	6,4	8,0
Mar.	5,6	3,8	4,7
Nov.	5,8	2,7	4,3

Gjøssæter and Tilseth (1983) proposed that the average value of 6,4 mill. tonnes probably gave the best estimate of the total biomass in the Gulf of Oman in February-March 1983. They based their calculations on the assumption that the cruise tracks provided a representative coverage of the whole Gulf including the Iranian waters. During the present survey the same cruise tracks were repeated and the biomass calculated according to Gjøssæter and Tilseth (1983). The variance of the estimates from the present survey was at about the same level as that from the surveys in February and March. The average standard length of the fish, however, was only 2,8 cm compared to 3,5 cm in February - March which gives 2,1 mill. tonnes lower average biomass for November.

From a fisheries point of view, a nearly absent DI layer during the present survey would indicate areas of lower catch than in February-March as fishing trials gave the best results in the presence of this layer (Scharfe, 1983).

During previous cruises carried out in 1981 by Aglen et al. (1981) and 1983 by Gjøsæter and Tilseth (1983) day recordings were higher than night recordings, as was the case during the present survey (see Table 8). This could be caused by some of the fish lying close enough to the surface to escape detection.

The fish became more concentrated during full moon, confirming previous observations. Under such conditions and with the addition of artificial lights good catches of mesopelagic fish could be made (Gjøsæter and Tilseth, 1983).

During the present survey Juday net (80 cm in diam, 375 um mesh size) hauls were taken to identify the spawning areas of mesopelagic fish. (Benthosoma pterotum). Eggs were, however, only found in one haul (see Fig. 10) and at extremely low density. The density of larvae in the area was also significantly lower than in February-March and consequently the month of November does not seem to be an important spawning period.

Figure 10. Distribution of the DI and NII layers.