9. MESOPELAGIC FISH

9.1 Species composition and distribution
9.2 Behaviour
9.3 Biological observations
9.4 Abundance

9.1 Species composition and distribution

The species composition in the catches was only fully studied during a cruise in April-June 1978, and partly during January-March 1978. A list of species caught on these cruises is given in Table 9.1. Provisional identification carried out at sea during the other cruises suggests, however, that the dominant species were the same.

Table 9.1. Species identified during cruises 3 and 4 of “Dr. Fridtjof Nansen”.

1 and 2 indicate first and second species in abundance respectively, + indicates presence in sample.

B. pterotum was caught at one station (25°14’S 34°33,5’E), where it was the only species caught. The catch was about 7 kg. Previously, B. pterotum is known in the Arabian Sea south to about 3°N off east Africa (GJØSÆTER, 1978). The present record seems to be the first from the southern Indian Ocean.

The specimens could not be distinguished from B. pterotum from the Arabian Sea in their distribution of photophores, but they had a lower number of gill rakers on the first gill arch with a mean of 23.10 versus 26.56. Although the difference in this single character does not justify description of a new species, it warrants further studies on the taxonomy of B. pterotum stocks along the east coast of Africa.

Hygophum hygomi was caught at one station, about 20°S. This species apparently has a bipolar distribution (BEKKER, 1965). NAFPAKTITIS and NAFPAKTITIS (1969), caught this species between about 20° and 35°S in the Indian Ocean.

Myctophum spinosum and M. aurolaternatum were caught at one station each, both between 13° and 15°S. NAFPAKTITIS and NAFPAKTITIS (1969) caught both species southwards to 10°S in the Indian Ocean.

M. asperum and M. aurolaternatum were caught at one station, about 20°S. NAFPAKTITIS and NAFPAKTITIS (1969) caught these species between 10°N and 10°S and between 5°N and 10°S respectively, in the Indian Ocean.

Symbolophorus evermanni was the dominant species at one station at about 13 S and was present at one about 16°S. Previously NAFPAKTITIS and NAFPAKTITIS (1969) recorded this species south to about 15°S in offshore waters.

Diaphus garmani was caught between 16°S and 26°S. It was the most abundant species in a night haul at 50 m depth at about 16°S. It ranged second in a bottom trawl haul at 50 m depth. The records fall within the known range for this species as described by NAFPAKTITIS (1978).

D. nielseni was caught from about 15°S to 21°S. At about 15°S it was the dominant species. This species was only caught with pelagic trawls. D. nielseni is also previously known from this area (NAFPAKTITIS, 1978).

D. watasei was caught between about 22° and 26°S, where it ranged first or second in three bottom trawl hauls at depths between 265 and 460 m. It was never caught with pelagic trawls. NAFPAKTITIS (1978) recorded this species between about 5°S and 28°S.

D. perspicillatus was caught at five stations between 14°S and 21°S, at one station (14°S) it was the dominant species, and at two it ranged second. It was caught with both pelagic trawls and bottom trawls. NAFPAKTITIS (1978) also recorded D. perspicillatus from the same area.

D. suborbitale and D. thiollierei were caught at one and two stations respectively between 14°S and 17°S. Neither of them were abundant. D. suborbitale has previously been recorded between 7°N and 8°S in the Indian Ocean (NAFPAKTITIS 1978, GJØSÆTER 1978), therefore the present catch locality (17°S) seems to be a southern extention of the range of this species. D. thiollierei has also been previously recorded from this area.

Maurolicus muelleri was caught at four stations between 21°S and 27°S. At two of these stations it was the dominant species and at one it ranged second. It was caught both with pelagic and bottom trawls. M. muelleri has a world-wide distribution; it has been caught off South-Africa (GREY 1964), but apparently not off Mozambique before.

Polymetme corythaeola was caught in a bottom trawl at one station (26°S). This species is previously known from the Indian Ocean off Natal (about 29°-30°S) and in the northern Indian Ocean south to about 5ºS off Zanzibar. The present record suggests that it may be distributed continously along the east African coast.

9.2 Behaviour

Mesopelagic fish were observed over most of the area studied. In offshore waters a deep scattering layer (DSL) was usually observed in deep water. Sometimes this layer was found below 500 m, which was the lower limit for the echo integration carried out to estimate fish abundance. At sunset the DSL, or part of it, migrated towards the surface and during night-time it was situated in the upper 100 m (Fig. 9.1). Usually the DSLs consisted of dispersed fish and schools were seldom observed. Generally, the fish density was highest close to the continental slope.

Fig. 9.1. Mixture of plankton and mesopelagic fish during night-time.

Along most of the coast, a scattering layer was found above the bottom at depths between 300 and 350 m, while sometimes mesopelagic species were also observed close to the bottom in more shallow waters (Fig. 9.2). Diaphus watasei was found in this bottom layer both day and night. During daytime Benthosema fibulatum and Maurolicus muelleri were also caught in this layer.

Fig. 9.2. Recording of mesopelagic fish along the continental slope.

9.3 Biological observations

Observations on the biology of the mesopelagic fish species were made on the cruise during January-March 1978 and on the one during April-June 1978. Fig. 9.3 gives the length distribution of the species.

Benthosema fibulatum, which was the most abundant species, ranged in length between about 30 and 90 mm. There was no difference in size distribution between samples from March and May 1978. Primary growth zones in the otoliths which are supposed to be formed daily were studied in a few fish, and the results seem to confirm the conclusion that B. fibulatum reaches its maximum size in about one year (GJØSÆTER, 1978).

Fig. 9.3. Length distribution of mesopelagic fish.

BENTHOSEMA FIBULATUM - MARCH 1978

BENTHOSEMA PTEROTUM

BENTHOSEMA FIBULATUM - MAY 1978

DIAPHUS PERSPICILLATUS

DIAPHUS WATASEI - MARCH 1978

DIAPHUS NIELSENI

DIAPHUS WATASEI - MAY 1978

MAUROLICUS MUELLERI

Gonads were studied in a sample from March 1978, and most of them were mature or ripe. Stomach contents were studied in the same samples, copepods and euphausids appearing to be the most important food items.

The catch of B. pterotum consisted of large adult fish only. A few gonads were studied, and they were all maturing or ripe. There therefore seems to be little doubt that the species spawn in the area. The biology of these two species is further discussed by GJØSÆTER (1978).

Ranging after Benthosema fibulatum, Diaphus perspicillatus was the most abundant species in the area. The length of this species ranged between about 20 and 60 mm. NAFPAKTITIS (1978) found mature eggs in females ranging between 48 and 54 mm in the Indian Ocean. CLARK (1973) suggests that the species reach maturity after one year in Hawaiian waters.

D. nielseni was also fairly common in the catches. This species ranged in length between 30 and 50 mm with a mode between 35 and 40 mm. NAFPAKTITIS (1978) found that three females measuring 32-36 mm were gravid.

Diaphus watasei was often abundant in bottom trawl catches. This very large species ranged in length between 80 and 170 mm. Fish caught during March 1978 had a slightly lower modal length than those caught during May 1978. The gonads were studied in one sample from March 1978. Only females were caught and they were, with few exception, ripe. The smallest ripe females measured between 124 and 157 mm in the same area. Stomachs contained euphausids, prawns, small squids and cope-pods. Myctophids were also observed in one stomach, but the species could not be identified due to advanced digestion. D. watasei is usually caught in gears fishing on or close to the bottom. Juveniles are, however, supposed to live pelagicly.

Maurolicus muelleri was fairly common in parts of the area studied. Only adult specimens ranging in size between 40 and 60 mm were caught.

9.4 Abundance

The abundance of mesopelagic fish has been calculated from acoustical data and length composition in the catches. As length data are only available from two cruises, and no significant difference could be observed between these, the same lengths, 40 mm for the area north of 18°S and 50 mm for the area south of 18°S - were used for all surveys. The results are shown in Table 9.2.

Table 9.2. Abundance estimates (in million tonnes) of mesopelagic fish in zones of 0-30 n.miles and 30-200 n.miles off the 200 m depth contour off Mozambique.

Cruise	South of 18°S		North of 18°S		Total
	0-30 n.miles	30-200 n.miles	0-30 n.miles	30-200 n.miles
1	1.2	1.0	0.02	0.3	2.5
2	0.4	2.9	0.8	1.7	5.8
3	0.9	5.9	0.2	1.5	8.5
4	1.1	1.2	0.5	2.7	5.5
Mean	0.9	4.5	0.4	1.6	5.6

The estimate obtained from the first cruise is probably an underestimate as the source level of the acoustic equipment at that time was lower than on the latter cruises. Although this is partly compensated for in the calculations, the detection threshold would have been different in such a way that small concentrations of weak targets such as mesopelagic fish would not have been included in the integrated echo abundance. The other estimates, 5.5-8.5 million tonnes for the whole area, are fairly similar.

Based on the material available, it is not possible to demonstrate any consistant differences in abundance between seasons or between areas. In general, however, it seems that the area from the 200 m depth contour to 30 n.miles seaward of this line has a higher density of mesopelagic fish than the more offshore areas.

Averaging over the three last cruises (which are supposed to give the most reliable estimates), the following densities were observed:

	0-30 n.miles	30-200 n.miles
N of 18°S	9.1 g/m2	6.4 g/m2
S of 18°S	11.0 g/m2	4.5 g/m2

These mean densities are much lower than those generally observed in the Northern Arabian Sea (GJØSÆTER, 1978), but they are still high enough to be of commercial interest. An approximate estimate of maximum potential yield of an unexploited stock can be derived from the equation:

Y_max = 0.5 · M · B_O