6. DEMERSAL FISH

6.1. Species composition and distribution
6.2. Biological characteristics
6.3. Abundance

6.1. Species composition and distribution

Demersal fish of commercial interest are mainly confined to the upper 200 m. The splitting of the total catch into main groups and the species composition of the demersal catch from the surveys of “Aelita”, “Kattegat” and “Dr. Fridtjof Nansen” for summer and winter appear in Tables 6.1 and 6.2 respectively. Table 6.3 gives the same for the “Professor Mesyatsev” survey, and Table 6.4 shows the demersal fish composition for some months from the Soviet commercial trawlers. During the period June-November 1978 a survey was carried out by SIP (Servico de Investigacoes Pesqueiras, Maputo) to investigate the species composition of the group “Mixed fish” from these vessels. Some results appear in Table 6.5. As seen, small pelagic fish dominate, mostly scad (Decapterus spp.) below 15 cm. Of demersal fish goatfish and lizard fish are the most important. In general, the fish in the “Mixed fish” group are the smallest fish from the catches.

Table 6.1. Catch composition and composition of demersal catches in bottom trawls from “Aelita” “Kattegat” and “Dr. Fridtjof Nansen” during summer - October to March (% weight).

Area	Sofala			Bazaruto		Delagoa		Inhaca
Depth zones	10-50	50-100	>200	<200	>200	<200	>200	>200
Demersal fish	56.8	32.1	100	65.8	91.9	8.4	63.6	86.7
Pelagic fish	41.1	60.4		32.6	6.5	88.4	18.4	8.7
Sharks/rays	2.1	7.5		1.6	1.6	3.2	18.0	4.6
Breams - SPARIDAE/NEMIPTERIDAE	1.0	2.9		4.9		1.4
Croakers - SCIAENIDAE	7.0	0.4		3.2	1.8	5.4
Gurnards/Crocodile fish
TRIGLIDAE/PERISTEDIIDAE	0.2				0.8		0.8	4.7
Goatfish - MULLIDAE	1.2	4.7		0.8		0.4		0.5
Groupers - SERRANIDAE	0.6	1.7		4.4		0.3
Grunters - POMADASYIDAE	3.1	1.8		15.5
Hairtails - TRICHIURIDAE	0.8
Lizard fish - SYNODONTIDAE	2.4	1.2	25.4	2.0	14.5		14.8	5.5
Snappers - LUTIANIDAE	0.1	0.4		1.0
Others	40.4	19.0	74.6	34.1	74.8	0.9	48.0	75.9
Number of hauls	100	22	14	13	42	77	114	44

Table 6.2. Catch composition and composition of demersal catches in bottom trawls from “Aelita” and “Dr. Fridtjof Nansen” during winter - April to September (% weight).

Area	Sofala			Bazaruto		Delagoa		Inhaca
Depth zones	10-50	50-100	>200	<200	>200	<200	>200	>200
Demersal fish	53.7	54.0	76.5	33.4	38.8	37.4	76.9	70.2
Pelagic fish	33.1	36.3		61.8	53.2	59.6	13.3	19.9
Sharks/rays	13.2	9.7	23.5	4.8	8.0	3.0	9.8	9.9
Breams - SPARIDAE/NEMIPTERIDAE	2.8	1.6				4.2	0.2
Croakers - SCIAENIDAE	10.8		7.8	0.8	0.7
Gurnards/Crocodile fish
TRIGLIDAE/PERISTEDIIDAE							0.1	4.9
Goatfish - MULLIDAE	0.9							0.2
Groupers - SERRANIDAE	3.5	0.6
Grunters - POMADASYIDAE	6.3	31.9		8.1	6.9
Hairtails - TRICHIURIDAE	3.7
Lizard fish - SYNODONTIDAE	4.6	1.4		1.5	3.3	1.8	12.8	6.2
Snappers - LUTIANIDAE	0.1
Others	21.0	18.5	68.7	23.0	27.9	31.4	63.8	58.7
Number of hauls	76	11	1	21	63	65	60	24

Table 6.3. Catch composition and composition of the demersal catches in bottom trawls from “Professor Mesyatsev” Jan 76 - Aug 77 (% weight).

Area	Sofala				Delagoa
	0-50		50-100		25-200		200-300 m		300-530 m
Depth zones	Jan 76	Aug 77	Jan 76	Aug 77	Jan 76	Aug 77	Jan 76	Aug 77	Jan 76 Aug 77
Demersal fish	33.0	45.3	50.4	25.5	47.9	66.8	16.4	94.5	65.3
Pelagic fish	64.4	42.6	45.5	70.9	40.9	18.5	78.0	+	+
Sharks/rays	2.6	12.1	4.1	3.6	11.2	14.7	5.6	5.5	34.7
Breams - NEMIPTERIDAE	2.9	+	+	1.5
Goatfish - MULLIDAE	+	9.2	+	5.9	2.6	+
Groupers - SERRANIDA	1.0	+	6.3	+	+	1.9
Grunters - POMADASYIDAE	1.6	2.4	2.7	0.9
Lizard fish - SYNODONTIDAE	5.5	6.6	1.7	2.1	9.2	+	6.0	12.8	+
Scavengers - LETHRINIDAE	+	+	+	1.1	1.6
Snappers - LUTIANIDAE	2.6	+	15.9	1.8	+	8.9
Others	19.4	27.1	23.8	12.2	34.5	56.0	10.4	81.7	65.3
Number of hauls	24	51	8	24	4	8	6	4	17

+) indicates presence in the catch.

Area	Sofala 25-100 m		Delagoa 48-200 m
Depth zones	Dec 77	Jun 78	Nov 77
Demersal fish	21.6	33.9	13.1
Pelagic fish	60.9	39.3	59.5
Mixed fish	16.8	20.8	26.5
Sharks/rays	0.7	6.0	0.9
Cornet fish - FISTULARIIDAE		8.2
Croakers - SCIAENIDAE			0.7
Goatfish - MULLIDAE	9.3	21.7	5.0
Grunters - POMADASYIDAE	11.5	3.3	5.4
Lizard fish - SYNODONTIDAE	0.3	0.1	2.0
Snappers - LUTIANIDAE	0.5	0.6
Hours of trawling	560	948	620

Tables 6.1-6.4 indicate that the proportion of demersal species in the catches is less during summer at Sofala Bank and in Delagoa Bay than during winter. This tendency is especially pronounced in Delagoa Bay, where less than 10% of the total catch is demersal species in summer. In the Bazaruto area this seasonal variation seems to be reversed.

Table 6.5. Composition of the group “Mixed fish” sampled from landings of the Soviet trawlers in 1978 (SOUSA, 1979. Pers. comm.) (% in weight).

Area	Sofala		Delagoa
Depth zones	Jun 78	Nov 78	Aug 78
Demersal fish	52.5	24.8	12.0
Pelagic fish	44.7	72.8	87.3
Other items	2.8	2.4	0.7
Goat fish - MULLIDAE	49.6	6.2	5.5
Lizard fish - SYNODONTIDAE	2.2	18.4	4.4
Breams - PAGELLUS sp.	0.7	0.2	2.1

The northern area of Mozambique is not included in Tables 6.1-6.4. The shelf north of Angoche is narrow and unsuitable for trawling, the demersal fish composition is therefore not satisfactory known. At the St. Lazarus Bank, however, the demersal stock consists nearly exclusively of the two-spot red snapper (Lutjanus bohar). There seems to be an accumulated stock of snapper at this shallow bank. Table 6.6 shows the species composition in the pot catches in January 1978 at St. Lazarus Bank.

Table 6.6. The species composition in the pot catches at St. Lazarus Bank - January 1978. (W = weight - kg, N = numbers.)

Species	W	N	W%	N%
Twospot red snapper Lutjanus bohar	188.7	65	44.8	77.3
Speckled snapper L. rivulatus	6.1	1	1.5	1.2
Humpback red snapper L. gibbus	0.9	1	0.2	1.2
Moontail seabass Variola louti	5.5	2	1.3	2.4
Mottled brown seabass Promicrops lanceolatus	93.5	3	22.2	3.6
Epinephelus andersoni	11.8	1	2.8	1.2
Mud grouper E. brunneus	16.5	1	3.9	1.2
Squaretail seabass Plecopterus truncatus	17.6	1	4.2	1.2
Mooray eel Lycodontis tesselata	14.5	1	3.4	1.2
Mooray eel L. flavomarginatus	22.4	2	5.3	2.4
Orange spotted emperor Lethrinus kallopterus	2.5	1	0.6	1.2
Longface emperor L. miniatus	7.4	3	1.8	3.6
Shark Carcharinus tjutjot	34.2	2	8.0	2.3
	421.6	84	100	100

The upper 200 m

At Sofala Bank the most important demersal species of commercial value are croakers, grunters and lizard fish. The most abundant species of the first group are Johnius belangerii, J. dussumieri and Otholithes ruber. Of the second group the blotched grunt (Pomadasys maculatus) and the lined silver grunt (P. hasta) dominate. The catches of lizard fish consisted nearly exclusively of the brushtooth lizard fish (Saurida undosquamis). From Tables 6.3 and 6.4 it can be seen that goatfish, of which Upeneus vittatus dominate, are abundant during winter.

Table 6.7. The most important demersal species observed above 200 m during the cruises of “Dr. Fridtjof Nansen”.

x) The most abundant species.

In Delagoa Bay, breams dominate in winter, and croakers during summer. The number of trawl hauls in the upper 200 m off Inhaca were very few, due to the rough bottom. The species composition of this depth zone has therefore been deleted. Table 6.7 lists the most abundant commercially important species observed during the survey of “Dr. Fridtjof Nansen”.

Table 6.8. Dominant families observed below 200 m.

^x) Commercially most important, either by species or by abundance.

The main part of the ichthofauna below 200 m consists of the families listed in Table 6.8. The Myctophidae has been deleted, as this family will be treated separately under the chapter MESOPELAGIC FISH. It is difficult to establish any geographical distribution as only a few deep trawl hauls were carried out north of Bazaruto. Snake mackerel, however, were abundant between Nacala and Pemba and seem to be distributed along the whole coast. Spiny sharks were caught at depths between 30 and 450 m, but were especially abundant at 250-300 m off Inhaca. The most abundant species of commercial value seemed to be Neoscombrops annectens.

6.2. Biological characteristics

Figs. 6.1 and 6.2 give the mode and range of length as well as some observations on maturity stages for the most important demersal species.

Lutianidae

The most important species of this family was the two-spot red snapper (Lutjanus bohar) which dominated the stock of the shallow St. Lazarus Bank. The length of the fish varied between 33 and 79 cm with most being in the range 55 to 70 cm (Fig. 6.1). Spawning specimens were not observed.

Blood snappers (L. sanguineus) 55 to 70 cm were found between Angoche and Pebane and at the southern part of Sofala Bank at about 50 m in February 1978. In the same area, humpback red snappers (L. gibbus) 33 to 74 cm were caught in November 1977 at depths of about 50 m.

Mullidae

The yellow-striped goatfish (Upeneus vittatus) was the most abundant species from this family. It occurred frequently in trawl catches from 10-100 m at Sofala Bank. From the length distribution in Fig. 6.1 it can be seen that specimens from 6 to 24 cm were recorded. The main part of the stock, however, seemed to be between 10 and 15 cm. The smallest individuals were observed in April.

Nemipteridae

This family was represented in the catches by one species only, namely the Delagoa threadfin bream (Nemipterus delagoa) of lengths 13 to 28 cm, most being between 18 and 22 cm. There are some indications of a peak in the spawning activity in December-January.

Fig. 6.1. Distribution of modal length and range for some demersal species. (A)

Fig. 6.1. Distribution of modal length and range for some demersal species. (B)

Fig. 6.1. Distribution of modal length and range for some demersal species. (C)

Fig. 6.1. Distribution of modal length and range for some demersal species. (D)

Fig. 6.2. Distribution of modal length and range for some demersal species.

OTHOLITHES RUBER

JOHNIUS BELANGERII

NEOSCOMBROPS ANNECTENS

SAURIDA UNDOSQUAMIS

Pomadasyidae

The blotched grunt (Pomadasys maculatus) was observed at lengths between 6 and 31 cm. The smallest individuals were observed during summer and the length distribution gives some support for the assumption of a spawning period in December-January.

Lined silver grunts (Pomadasys hasta) had lengths between 10 and 58 cm, with the smallest individuals occurring in September-November. The samples in April-May with very wide length range suggest the presence of at least two year-classes.

Rock grunts (Pomadasys olivaceum) have a length of 18-22 cm. A peak in the spawning activity seems to occur between August and December, according to the maturity observations of “Aelita”, (BUDNICHENKO et al, 1977).

Sciaenidae

The length of the Belangers croaker (Johnius belangerii) was between 6 and 20 cm. The smallest specimens were found in November. No observations of any spawning were made.

The tiger-toothed croaker (Otholithes ruber) was observed at lengths between 6 and 42 cm. The majority was between 25 and 35 cm. Observations on maturity taken both on “Dr. Fridtjof Nansen” and the “Aelita” (BUDNICHENKO et al, 1977) indicate a peak spawning in December-January and maybe another in June-July. With some boldness one might construct the growth curves of Fig. 6.1, showing a growth to 25-30 cm 6-7 months after spawning.

Synodontidae

The dominant species in this group, the brush-tooth lizard fish (Saurida undosquamis), had a length between 12 and 46 cm. The majority were between 17 and 27 cm. They seemed to be widely distributed over most of the shelf and slope area and were found at depths between 40 and 400 m. The length distribution seemed to be different between males and females. There also appeared to be an increase in length with depth.

Pre-spawning or spawning females were never caught at depths shallower than 200 m, and spent or recovering individuals were always below 300 m. This indicates a vertical migration associated with the reproduction cycle. The ripe and spent fish were found in November and January, suggesting a peak in sexual activity in September to November.

These observations are in reasonable good accordance with the investigations into S. tumbil in Philippine waters where the temperature conditions are approximately the same as along the Mozambican coast TIEWS, MINES and RONQUILLO, 1968).

6.3. Abundance

If the average fish density is known for a given area, the size of the stock may then be calculated. Table 6.9 gives the result of such calculations split into areas, depth zones and seasons. The fish density is obtained from the mean catch rate and the area swept by the trawl. The latter is the distance between the trawl wings multiplied by the length of the tow. All the trawl hauls from the surveys of the research vessels “Aelita”, “Dr. Fridtjof Nansen”, “Kattegat” and “Professor Mesyatsev” were applied. For each vessel the demersal fish density was calculated according to the specification of her trawl net, trawl speed and catch rate. The average density was obtained as a weighted mean for all the vessels. The efficiency coefficient of the trawls was taken as 1, i.e. all the fish ahead of the trawl were caught. The areas of the different depth zones are found in Table 3.1.

In order to reach a reliable result by these “swept area” calculations, the following conditions must be fulfilled:

- The estimation of the efficiency coefficient of the trawl must be correct.
- Random chosen fishing localities should be used.

Table 6.9. Mean density of demersal fish and standing stock size in thousand tonnes estimated by using the catch rates of bottom trawls. Numbers in brackets are extrapolated values.

As seen in Table 6.9, the demersal fish stock during summer was 103 700 tonnes and in winter 97 500 tonnes. In winter the stock at Sofala Bank decreased, and increases along the rest of the coast. BIRKETT (1978) presents similar calculations based on the data from “Professor Mesyatsev” alone (Table 1.5). Except for Delagoa Bay in winter, when he reached a stock size less than half of our calculations, there is an astonishing degree of correspondance.

Table 6.10 shows the acoustic abundance estimates of demersal fish from “Dr. Fridtjof Nansen”. During Cruise II in November the coverage was not as good as on the other cruises, due to lack of time. As a result of this, the acoustic estimate is probably too high, due to overvaluation of the area of the acoustic recordings. The most likely acoustic estimate of the demersal resources will be 250-300 000 tonnes.

Table 6.10. Acoustic abundance estimates of demersal fish from “Dr. Fridtjof Nansen” (thousand tonnes).

	Inhaca	Delagoa	Bazaruto	Sofala	Northern	St. Lazarus	Total
September-October 1977	36	2	31	194	19	9	291
November 1977	-	111	29	260	5	9	414
April-June 1978	3	12	52	166	4	11	248

An acoustic estimate of demersal fish will usually be an underestimate, due to the limitation of the echo sounder and integrator in making recordings very close to the bottom. An investigation by BURCZYNSKI (1976) at Sofala Bank indicates that an average catch rate of 100 kg/trawl hour could be expected even if there are no recordings on the echo sounder. This catch rate is equivalent to about 100 000 tonnes of demersal fish for the whole shelf area of Mozambique. If we add this to the acoustic estimate the total demersal stock will be 350-400 000 tonnes.

The discrepancy between of the two different abundance estimates of demersal fish, 100 000 versus 350-400 000 tonnes, seems to be unreasonably large. If the last figure is correct, it follows that the efficiency coefficient of the bottom trawls used is about 0.3, which appears to be suspiciously low for demersal species. A reasonable assumption is then that the conversion factor between the integrator reading and the density of fish per unit area is too high. Therefore, as a first approximation of the abundance of the demersal stock off Mozambique, 200 000 tonnes is suggested. This means that the efficiency coefficient of the trawls will be approximately 0.5.

The formula of GULLAND (1970), , can be used to estimate the potential annual yield. M is the natural mortality rate and B_O the virgin or unexploited stock. Considerable differences in natural mortality are to be expected between species. There is a lack of estimates of M for the fish of the region. Investigations from other areas, however, suggest that M = 0.5 is reasonable for the demersal stock as a whole (ANON, 1979, PAULY, 1978). For the snapper at St. Lazarus Bank this value probably is too high, and therefore M = 0.2 has been applied to this species. A value of 0.2-0.3 seems to be common for the larger slow-growing species (MUNRO, 1969-1973, PAULY, 1978). Using these figures, the potential annual yield of the demersal stocks is about 50 000 tonnes. For the St. Lazarus Bank the annual yield will be about 1000 tonnes.

The by-catch of demersal fish in the shrimp fishery at Sofala Bank is probably about 20 000 tonnes. As the confidence interval of the abundance estimate is so large, this catch, as well as the catch from the Soviet trawlers, has not been included in the calculations.

There is lack of information on the fishery going on in the coral reef areas as well as in more inshore waters. The coast between Cabo Delgado and Angoche is just such a typical reef area. Along the southern part of the Tanzania coast the trap fishery from the reef area has a current annual yield of 4.8 tonnes/km². Investigations from other areas in the region suggest that the maximum yield from the coral reefs is about 5 tonnes/km² (ANON, 1979). The area of the reefs along the northern Mozambican coast is of the order 1-2000 km², which gives an annual potential production of 5-10 000 tonnes.

By inshore fishery we mean fishing at depths less than 10 m. The area of the inshore waters is 2-5000 km², depending of the method of calculation. GULLAND (1970) suggests a potential annual yield of 2.5 tonnes/km² for areas of intermediate productivity in the Indian Ocean. Under these assumptions the expected potential yield of the inshore waters will probably be of the same order as that of the reef fishery.