Acoustical abundance estimation

Gulf of Oman
Detailed surveys in 1981
Gulf of Aden

Gulf of Oman

During 1979 the southern part of the Gulf of Oman was covered during the period 4 July - 3 August. The northern part was not surveyed (Fig. 1).

The echo abundance was converted to biomass using the equations

where B is biomass, C' convertion factor, L average fish length, M average integrator reading and A is the corresponding area (see page 3). A mean fish length of L=35 mm was used for the transformation (see Fig. 16).

In the part of the Gulf covered, a biomass of about 6 million tonnes was found. If the densities recorded are representative for the whole Gulf, the total biomass of mesopelagic fish was about 8 million tonnes.

The density of fish per m² surface area was calculated for one degree squares. It ranged from 20 to 165 g/m² with a mean of 87 g/m².

In 1981 the abundance of mesopelagic fish in the Gulf of Oman was studied during three surveys (Fig. 3):

I	24-28 January
II	28-31 January
III	9-14 February

The period 1-9 February was used for a detailed survey with fishing experiments in the area south of 24°50'N and west of 57°40'E.

In 1981 the mean length of the fish was 39 mm (see Fig. 17) and the variation between areas and between the different layers were too small to warrant the use of different lengths, in the conversion factor.

During night time parts of the fish seemed to be found in the upper 10 m and were therefore lost. During day time parts of the organisms responsible for the echo obtained from the lower layer were not fish. No compensation is made for these biases in the following estimates. It is also supposed that those parts of the Gulf covered (about 2/3) are representative for the whole area. Based on these assumptions the following abundance of mesopelagic fish was estimated:

Survey	I	8	mill.	tonnes
	II	11	"	"
	III	13	"	"

The variances are rather high (Table 5) and the differences between the estimates may not be significant. The mean of the three estimates,

11 million tonnes

Table 5. Echo abundance (mm integ. deflection) from the three main surveys in the Gulf of Oman.

Survey	No. five miles	DAY					NIGHT
		DI		D II		No. five miles	NI		N II
I		mean	SD	mean	SD		mean	SD	mean	SD
W58°E	25	50	52	183	240	32	117	47	88	173
E58°E	14	57	186	30	63	29	79	59	1.4	2.2
II
W58°E	27	23	26	360	407	38	175	79	82	127
E58°E						19	94	45	31	57
III
W58°E	41	63	74	286	181	28	175	79	73	88
E58°E	34	55	89	152	250	36	122	64	10	18

Detailed surveys in 1981

An area in the Gulf of Oman south of 24°50'N and west of 57°40'E was covered six times during the period 1-9 February. The coverage differed, so the six surveys are not directly comparable. Several trends are, however, evident (Table 6). The upper day layer usually had the lowest echo abundance and high variance. The lower day layer always showed the highest echo abundance and usually high variance. The two night layers had similar abundance. The upper layer had low variance, the lower higher. In general these trends can also be seen in the data from the main surveys.

Table 6. Echo abundance from the detail surveys 1-9 February 1981.

Date	No. five miles	DAY				No. five miles	NIGHT
		D I		D II			N I		N II
		mean	SD	mean	SD		mean	SD	mean	SD
1-2	13	138	104	924	691	7	147	57	103	254
2-3	12	146	133	533	210	8	187	59	156	116
3-4	6	86	61	523	348	9	268	116	252	169
4-5	9	70	36	1039	678	10	302	53	303	115
5-6	13	32	43	348	315	19	232	74	226	137
6-9	9	120	118	569	327	17	221	80	161	141
Mean	62	100	100	654	518	70	230	86	204	158

Totally the day recordings were higher than the night recordings. No significant increase or decrease in abundance could be traced during this period.

Converted to biomass of mesopelagic fish in the area (2120 n. miles²) the following results are obtained:

Detail survey	1	3.7	million	tonnes
	2	2.7	"	"
	3	2.3	"	"
	4	4.2	"	"
	5	2.1	"	"
	6	2.4	"	"
Mean			2.9 million tonnes
Standard deviation			0.8 million tonnes

Gulf of Aden

The Gulf of Aden was surveyed during the period 8-29 August 1979 (Fig. 2). The integrated echo abundance was converted to biomass as described above (page 18).

In the western part of the Gulf of Aden, B. pterotum with mean length about 4 cm was an important component of the scattering layers, and this length is used for the calculation. In the eastern part of the Gulf there was much variation in species and size composition, and fishes with no swim bladder (eg. Chauliodus) had an important position in the fauna. It is therefore difficult to estimate an equivalent mean size. Four centimeters was used, however, although it probably gives an underestimate of the true biomass.

The total biomass of mesopelagic fish was estimated to 2.6 x 10 tonnes in the Gulf of Aden west of 47°E and 1.6 x 10 tonnes between 47° and 51°E. According to the acoustical surveys the total biomass was therefore about 4 million tonnes.

The density of fish per m² surface area was calculated for one degree squares. It varied from 50 g/m² and 7 g/m² within the Gulf of Aden. The mean density was 17 g/m².

Five one degree squares in the inner part of the Gulf of Aden were covered three times during the survey with the following results:

Period		Fish density
8 - 19	August	23	g/m2
20 - 25	"	41	"
26 - 29	"	42	"

As a total of 14 one degree squares were covered more than once and the estimated biomass increased in 13 of these, the increase seems to be significant. The performance of the equipment was followed carefully throughout the survey, and no change was recorded. During the last part of the cruise a different setting of the equipment was used (see Annex 2). On very scattered recordings in deep water this can account for a 30% increase in estimated abundance due to a lower threshold effect. For dense recordings, however, there is little difference between the settings. This change can therefore only partly explain the increase in estimated abundance.

A few squares in the central and eastern part of the Gulf were also covered more than once. These too, indicated a rise in biomass, although less pronounced. Therefore, migration cannot account for the increase. Individual growth can explain at least part of the increase. Recruitment of juveniles, which according to plankton samples were abundant during the first part of the cruise, may also be part of the explanation.

In 1981 the abundance was estimated during one survey conducted 17 to 26 February (Fig. 4).

The integrated echo abundance (Table 7) was converted to biomass using the same conversion factors as in the Gulf of Oman. The mean length of the targets was supposed to be 33 mm.

Table 7. Mean echo abundance (mm integ. deflection) from the scattering layers in the Gulf of Aden 1981.

		West of 47°E			East of 47°E
		No. of five miles	mean	SD	No. of five miles	mean	SD
Day	D I	30	1.3	3.1	94	11.3	15.6
	D II	30	91	32	94	136	132
Night	N I (excl. plankton)	48	50	44	90	110	65
	N II	48	44	20	90	45	34

The following estimates were obtained:

Area west of 47°E	9 million tonnes
Area east of 47°E	8 million tonnes
Total Gulf of Aden	16 million tonnes

These estimates include all sound scatterers excluding the plankton in the N I layer. No compensation is made for the fact that a considerable part of the fishes caught in the deep layers have no swim bladder, and therefore lower target strength