Behaviour and species composition

Gulf of Oman
Gulf of Aden
Reaction to light

Gulf of Oman

Mesopelagic fish were found off the continental shelf where the water was deeper than about 200 m. During day time two sound scattering layers were usually observed in addition to a weak plankton layer in the surface. The upper layer (D I) was usually found at about 150 m depth and had a vertical extention of 20-40 m (Table 1, Fig. 5). It consisted of small schools and dense concentrations. Below this layer and usually cantered about 250 m depth a second scattering layer (D II) appeared. This usually had a vertical extention of 70-100 m. Although it sometimes gave very high echo recordings, it had a more "smoky-appearance on the echograms than D I. About one hour before sunset the upper layer started to migrate slowly upwards. About half an hour before sunset it usually reached about 100 m. At this time both this layer and parts of the lower layer moved rapidly towards the surface where they mixed with the plankton layer. Parts of the deep day layer stayed in deeper waters also during night time (Fig. 6). Sometimes fish seem to migrate from the D I to the D II layer during daytime (Fig. 7).

Table 1. Vertical distribution of mesopelagic fish layers in 1979. Ranges are given in parenthesis.

Area	Time	Day				Night
		DI		D II		N I	N II
		Depth	Vertical extention	Depth	Vertical extention	Vertical extention	Depth	Vertical extention
Gulf of Oman	July-79	181	43	265	86	72	219	65
		(150-215)	(30-60)	(225-295)	(50-105)	(60-90)	(185-245)	(35-100)
Gulf of Aden	Aug.-79	-	-	363	161	66	354	156
				(275-400)	(100-220)	(50-90)	(260-400)	(100-250)

Fig. 5. Shallow day layer of mesopelagic fish (D I) in the Gulf of Oman.

Fig. 6. Vertical migration of scattering layers in the Gulf of Oman. The distance between the vertical lines is one nautical mile.

Fig. 7. Echogram showing fish migrating from a D I to a D II layer. The ship was laying still during the recording.

The shallow day layer (D I) consisted almost exclusively of Benthosema pterotum (Table 2). The salps observed in the trawl catches from this layer in 1981 may be contamination from the surface layer where they were very numerous. In the deep day layer (D II) B. pterotum was also the dominant form, although salps also made up a large proportion of the catches. In tows from the lower part of the D II layer shrimps were sometimes numerous, and in contrast to the salps, these were probably partly responsible for the echoes received from these layers.

During night time B. pterotum was second to salps in abundance in 1981, but among those animals which are supposed to give sound scattering, B. pterotum was dominant (Table 2). A towed transducer sounding upwards showed that parts of the night layer (N I) extended upwards beyond 10 m depth which is the upper limit for the echo integration. This experiment was carried out at new moon, and probably most of the fish will stay deeper during the other parts of the moon cycle.

Table 2. Composition (percentage of weight) of trawl catches in the sound scattering layers in the Gulf of Oman. Figures in the brackets give the percentage of the catch exclusive of salps, + present but less than 0.1% of weight.

	1979				1981
	Day		Night		Day		Night
	D I	D II	N I	N II	D I	D II	N I	N II
Myctophidae	99.2	73.2	94.6	44.5	96.0 (100)	44.9 (60.1)	46.8 (90.4)	55.2 (86.0)
Champsodon	0.1	1.1	0.7	1.4		2.9	0.6	1.4
Harpadontidae		6.0	+	33.1		0.8	+	0.2
Cubiseps		0.4	0.6			1.1	1.7	0.1
Lestidium	+	0.7	0.6	+	+	1.1	0.3	1.7
Trichiurus	0.3	3.7	+	0.4	+	0.2	0.1	+
Vinciguerria						0.1	+	+
Epinnula					+	0.7	0.3
Synagrops						0.2	+	0.1
Other fishes	0.1	0.2	0.4	1.9	+	0.5	+	1.0
Shrimps/Krill		8.5	+	8.5	+	21.0 (28.1)	0.1	2.5
Squid	0.3	1.8	0.9	1.3		1.0	1.9	0.6
Salps/Jellyfish		4.0	2.0	7.6	3.9	25.2	48.3	36.5
No. of hauls	16	11	17	3	30	14	23	8

In catches from the deep night layer (N II) B. pterotum made up 55% of the weight, or 86% if the salps were excluded in 1981 and about 45% in 1979. Harpadon contributed 33% in 1979, but only 0.2% in 1981.

Tows taken between the depths of the scattering layers or at the surface during day time in 1981 gave large numbers of salps, but no fishes. Tows with Juday net showed that small krill and other plankton organisms, too small to be caught in the trawl, could be numerous. It seems, however, safe to conclude that fish, and mainly B. pterotum were responsible for most of the sound scattering. This view is supported by the pictures taken in these layers during the cruise in 1981, showing mainly fish and salps (Fig. 8-9).

Fig. 8. Photo showing myctophids in a D I layer.

Fig. 9. Photo showing myctophids in a D II layer.

Gulf of Aden

During day time a surface plankton layer and one deep scattering layer were found in most of the area. The DSL corresponding to the D II in the Gulf of Oman was usually found at depth of about 350-400 m in 1979 and 300-350 m in 1981. It had a vertical extension varying between 100 and 200 m (Table 1). In the western part of the Gulf, scattered schools corresponding the the D I layer of the Gulf of Oman were observed in 1981, but not in 1979.

At dusk, parts of the mesopelagic layers migrated towards the surface and mixed with the plankton.

The surface layer (N I) extended from the surface down to about 100 m, with the highest concentrations between about 30 and 70 m. Parts of the fish remained in deep water (N II) (Table 1).

The shallow day layer (D I) observed in 1981 consisted almost exclusively of Benthosema pterotum (Table 3). In the deepest layer (D II) Myctophidae were dominant followed by Chauliodus sp. In the shallow night layer (N I) Myctophidae were still the dominant fish. In 1979 Cubiceps sp and other fishes were also important while swimming crabs and salps made up a large part of the total catch in 1981.

Table 3. Composition (percentage of weight) of trawl catches in the sound scattering layers in the Gulf of Aden. +, present, but less than 0.1% of weight.

	1979				1981
	Day		Night		Day		Night
	D I	D II	N I	N II	D I	D II	N I	N II
Myctophidae		34.6	56.1	33.2	91.5	64.5	45.2	20.4
Gonostomatidae		3.0		1.6		1.2	0.4	10.2
Sternoptychidae		6.3		2.8	0.5	2.1	+	10.2
Cubiceps		0.8	10.8				2.3
Lestidium		0.6	4.1	0.3		1.2	2.7
Chauliodus		21.2		5.5		9.1	+
Trichiuridae							0.4	6.8
Other fish		6.0	10.2	4.6		2.1	0.7	2.3
Shrimps/krill		4.5	4.2	11.3	2.8	5.8	8.7	2.0
Squids		1.5	2.2	0.4	2.1	1.8	4.7	1.4
Salps/Jellyfish		19.7	12.4	40.2	3.1	9.1	16.7	40.8
Swimming crabs						3.0	17.9	+
No. of hauls		15	11	3	4	8	10	1

Table 4. Preliminary list of Myctophidae caught in the Gulf of Aden, February 1981.

	August 1979	February 1981
Electrona rissoi	x	x
Hygophum proximum	x	x
Benthosema pterotum	x	x
B. fibulatum	x	x
Myctophum spinosum	x	x
M. nitidulum	x	x
Symbolophorus evermanni		x
Diaphus garmani	x	x
D. regani	x	x
D. coeruleus	x	x
D. sp. nov.	x	x
D. diademophilus	x	x
D. lucidus	x	x
D. megalops	x	x
D. parri	?	x
D. lobatus	x	x
D. problematicus	x
Lampadena sp.		x
Lampadena luminosa	x
Lampanyctus spp.		x
L. macropterus	x
L. indet.	x
L. tenuiformis	x
L. nobilis	x
Lepidophanes sp.		x
L. longipes	x
Ceratoscopelus warmingi	x	x
Bolinichtys blacki	x
Diogenichtys panurgus	x

The deep night layer (N II) was sampled only a few times. Both years jellyfish made up a large part of the catches. Among the fish, Myctophidae were dominant. In 1979 Gonostomatidae and Sternoptychidae made up 10% each.

A preliminary list of the Myctophidae identified so far is given in Table 4. A complete list must await a more thorough analysis of some samples.

Quantitatively Benthosema pterotum was the dominant species in the western part of the Gulf (W of 47°E) while various Diaphus species dominated in the eastern part. In 1979 Lampanyctus spp. were also rather important.

Reaction to light

The reaction of mesopelagic fishes to light was studied twice in the Gulf Of Oman. A 1000 W high pressure sodium lamp was used as a light source.

The first experiment was carried out at new moon. The light was mounted at the side of the vessel and the echo sounder was used to monitor the reaction of the fish. Underwater photography was used to identify the organisms aggregating. The light made the scattering layer sink and concentrate at a depth of about 50 m (Fig. 10). Photographs show that myctophids were the dominant organisms in this layer (Fig. 11).

In the second experiment the light was mounted on a small boat. This experiment was carried out about 10 days after new moon. This time the fish concentrated in a cone around the light (Fig. 12), but the concentrations were not as dense as those observed during the first experiment.

Fig. 10. Echogram from a light station, showing the dispersion of fish when the light is turned off. The ship was laying still during the recording.

Fig. 11. Photo taken in 50 m depth at the light station shown in Fig. 10.

Fig. 12. Echogram from a light station showing slightly increased concentration at some distance from the light source.

During both experiments most of the myctophids present were adult B. pterotum with length between 35 and 43 mm. Most of them had mature gonads.