BEHAVIOUR AND SPECIES COMPOSITION

Gulf of Aden

During day-time one scattering layer was usually observed in the mesopelagic zone. This layer has a mean depth of about 350 m and a vertical extension varying between about 100 and 200 m. In addition to this layer there was another layer near the surface caused by plankton organisms and various fish larvae and juveniles and also sometimes an additional layer was observed between these two, but mesopelagic fishes was not observed in this layer. The upper mesopelagic day layer, characteristic for the Gulf of Oman, was never observed.

At dusk, parts of the mesopelagic layer migrated towards the surface and mixed with the plankton. This migration took place in several groups indicating that several species were involved.

On the surface the mesopelagic fish mixed with plankton in a layer extending from the surface to about 70 m. Part of the deep layer remained at its day-time depth.

Table 1. Composition of the scattering layers in the Gulf of Aden in percentage of weight.

	DAY (15 stations)	NIGHT UPPER (11 stations)	NIGHT LOWER (3 stations)
FISHES:
Myctophidae	34,6	56,1	33,2
Gonostomatidae	3,0	-	1,6
Sternoptychidae	6,3	-	2,8
Cubiseps	0,8	10,8	-
Lestidium	0,6	4,1	0,3
Chauliodus	21,2	-	5,5
Other fish	6,0	10,2	4,6
OTHER ORGANISMS:
Shrimps and krill	4,5	4,2	11,3
Squids	1,5	2,2	0,4
Jellyfish etc.	17,2	12,2	40,2
Salps	2,5	0,2	-

Table 1 shows the composition of the sound scattering layers based on trawl stations. About 75% of the total catch taken during day consisted of fish. During the night 80% of the total catch consisted of fish in the upper layer and it was about 50% in the deeper layer. About 35% of the fish caught during day belonged to the Myctophidae.

Chauliodus and Argyropelecus ranged next with about 20% and 6% respectively.

In the shallow night-layer Myctophidae was about 60% of the total catch, with Cubiceps (10%) and Lestidium (4%) ranging next. In the deeper layer Myctophidae was only 33%, Chauliodus and Argyropelecus about 6% and 3% respectively Cubiceps and Lestidium were very sparce.

The composition of the Myctophidae varied both geographically and in time. In the south-eastern part of the Gulf where mesopelagic fish were most abundant, Benthosema pterotum was the dominant species, and its abundance increased both in relative and absolute terms during the period of investigation. In other parts of the Gulf of Aden Lampanyctus spp. and Diaphus spp. were most; abundant.

Gulf of Oman

Mesopelagic fish were found off the continental shelf where the water was deeper than 200 m. During daytime two layers of mesopelagic fish were identified. The upper layer consisted of small scools. The average depth of this layer was 180 m, with an average vertical extension of about 40 m. A deeper layer was also frequently observed during daytime. The average depth of this layer was 270 m and the average vertical extension was 90 m. On the echo-paper this layer gave a more "smoky" appearance than the upper one.

The upper layer consists of nearly only mesopelagic fish. 98% of the catch was Benthosema pterotum, and 1% of the catch was Benthosema fibulatum. Benthosema pterotum was also the main component of the deeper layer constituting 74% of it by weight. Other important species were Harpoodon sp. 6%, Trichiurus lepturus, 4%, and shrimps and krill 9%. More details are given in table 2.

Table 2. Composition of the scattering layers in the Gulf of Oman in per cent weight.

	DAY		NIGHT
	Shallow (16 stations)	Deep (11 stations)	Shallow (17 stations)	Deep (3 station)
FISHES
Myctophidae	99,2	73,6	94,6	44,5
Champsodon cap.	0,1	1,1	0,7	1,4
Harpoodon sp.	-	6,0	P	33,1
Cubiceps	-	0,4	0,6	-
Lestidium nudum	P	0,7	0,6	P
Trichiurus lepturus	0,3	3,7	P	0,4
Other fish	0,1	0,2	0,4	1,9
OTHER ORGANISMS
Shrimpd and krill	-	8,5	P	8,5
Squids	0,3	1,8	0,9	1,3
Jellyfish	-	0,5	-	2,2
Salps	-	3,5	2,0	5,4

P: Present in very small quantities.

The layer found in the upper meters during night was dominated by Benthosema pterotum, constituting 95% of it by weight. Other fish organisms in this layer were Champsodon capensis, Cubiseps sp. and Lestidium nudum. Squids and salps were 1% and 2%, respectively.

Only three trawl stations were taken in the deep layer in Gulf of Oman at night. In this layer Benthosema pterotum was the most important species in one station, but in the other two stations Harpoodon sp. was the most abundant fish species. Shrimps and krill were also important constituents on these two stations.