7. DEMERSAL FISH

The analysis of demersal fish is based on catches from the shrimp trawl.

As in previous surveys, the Sofala Bank was divided in four subareas: Angoche-Moebase (16°20’-16°50’), Quelimane II (17°20’-17°50’): Quelimane I (17°50’-16°50’) and Moebase-Beira (18°50’-21 00’). Angoche-Moebase and Machese-Beira were grouped and considered as two subareas due to the low number of trawl stations.

Catch composition and distribution

Fig. 20 shows that the demersal fish occurs all over the surveyed area with the best catches in Quelimane I and Beira.

Fig. 20. Distribution of demersal fish.

Table 8 gives the composition of the catches, splitted by depth and by subarea. Pelagic fish usually dominated, the catches except for the catches in the Angoche and Moebase areas where demersal fish constitute approximately 65% of the catch. At depths shallower than 25 meters, the Sciaenidae is the dominant group in Quelimane II, Quelimane I and Beira. In Machese the family Trichiuridae dominates. Besides this family, Polynemidae made significant contribution in Quelimane II, Trichiuridae in Quelimane I and Pomadasyidae in Beira, Mullidae was the most important family in Angoche-Moebase together with Polynemidae.

Table 8. Catch composition (kg/h) by depth and subarea.

At depths of more than 25 meters, Mullidae dominated in Quelimane I and Machese while Serranidae was the most important family in Quelimane II. In general, the main families in shallow waters (above 25 m) are Sciaenidae, Mullidae and Trichiuridae and in deeper waters (below 25 m), Mullidae is the dominant family.

Sciaenidae

Among the demersal fish this family had the highest yields in waters shallower than 25 m and the best catches were obtained at Quelimane I and Beira (Fig. 21).

Fig. 21. A - Distribution of family Sciaenidae (catch per hour). B - Distribution of Johnius belengerii (catch per hour).

Mainly three species were represented: J. belengerii, O. ruber and J. dussumierii.

Fig. 22. Distribution of Otolithes ruber (catch per hour).

In table 9 it can be seen that J. belengerii occurs from Quelimane II to Beira subarea, with the best catches in Quelimane I. In Beira, one trawl station had a catch rate of more than 100 kg/h (Fig. 21B). O. ruber occurs from Moebase to Beira, with best yields in Quelimane I and Beira. In Quelimane II, one trawl station had a catch rate of more than 50 kg/h (Fig. 22).

Table 9. Species composition from family Sciaenidae splitted by species.

Species composition	Ang + Moebase	Quelimane II		Quelimane I		Machese		Beira	Sofala Bank
	<25	<25	>25	<25	>25	<25	>25	<25	<25	>25
J. belengerii	-	6.69	-	19.20	0.27	6.41	-	20.84	11.47	-
O. ruber	1.77	15.23	-	12.11	8.0	5.32	-	20.83	10.91	-
J. dussumierii	1.26	5.11	-	-	-	0.13	-	5.86	2.05	-

Johnius dussumierii is present in Angoche-Moebase, Quelimane II, Machese and Beira. The first subarea was not included as the catches were small. Looking at table 9, it can be seen that the best catches were found in Quelimane II and Beira.

Pomadasyidae

This family occurs from Moebase to Beira with the best catch rates in Beira (328 kg was caught at one trawl station) (Fig. 23A). Three species of this family occurred in this survey: P. maculatus, P. hasta and R. stridens; of which the first is the most important. Table 10 gives the catch composition of the family Pomadasyidae. Pomadasys maculatus is present from Moebase to Beira with the best catches in Beira. Looking at table 10 Rhonciscus stridens occurred only in Angoche-Moebase with a catch rate of up to 352 kg/h.

Fig. 23. A - Distribution of family Pomadasyidae (catch/hour). B - Distribution of Pomadasys maculatus (catch/hour)

Table 10. Species composition from family Pomadasyidae splitted by species.

Species composition	Ang + Moebase	Quelimane II		Quelimane I		Machese		Beira	Sofala Bank
	<25	<25	>25	<25	>25	<25	>25	<25	<25	>25
P. maculatus	4.95	1.57	-	0.14	-	2.24	-	33.44	7.56	-
P. hasta	0.30	5.64	-	2.07	-	0.24	-	0.24	1.29	-
R. stridens	3.52	-	-	-	-		-	-	0.52	-

Mullidae

This family is present from Moebase to Machese-Beira with the best catches in Moebase (Fig. 24A) at 9-16 meters.

Fig. 24. A - Distribution of family Mullidae (catch/hour). B - Distribution of Upeneus vittatus (catch/hour).

Three species of this family occurred: U. vittatus, U. bensasi and U. sulphureus. The first one was the most important. Table 11 gives catch composition of the family Mullidae. U. vittatus occurs from Moebase to Machese Beira with best catches in Moebase (137.37 kg) (Fig. 24B). U. bensasi occurs from Moebase to Quelimane I. The catch rates were insignificant.

Table 11. Species composition from family Mullidae splitted by species.

Species composition	Ang + Moebase	Quelimane II		Quelimane I		Machese		Beira	Sofala Bank
	<25	<25	>25	<25	>25	<25	>25	<25	<25	>25
U. vittatus	137.37	1.66	-	12.64	0.07	10.17	0.24	5.6	27.49	0.46
V. sulphureus	0.02	0.15	-	-	-	0.2	-	-	0.08	-
Others	2.45	-	0.35	0.01	14.53	-	-	0.09	1.36	0.5

Synodontidae

This family occurs from Moebase to Machese-Beira with catch rates up to 10 kg/h, excepting two trawl stations, in Quelimane II and Machese at depths of more than 25 meters (Fig. 25A) Three species occurred in this family: Saurida undosquamis, Saurida tumbil and Trachinocephalus myops. Looking at table 12, Saurida undosquamis occurred in all subareas with the best catches in Machese.

Fig. 25. A - Distribution of family Synodontidae (catch/hour) B - Distribution of family Trichiuridae (catch/hour)

Table 12. Species composition from family Synodontidae splitted by species.

Species composition	Ang + Moebase	Quelimane II		Quelimane I		Machese		Beira	Sofala Bank
	<25	<25	>25	<25	>25	<25	>25	<25	<25	>25
S. undosquamis	0.48	0.05	-	0.27	0.32	2.47	-	0.95	0.92	0.1
S. tumbil	0.73	0.85	0.14	1.45	-	0.33	-	-	0.72	0.05
T. myops	0.02	-	0.05	0.001	-	-	-	-	+	0.02

Trichiuridae

This family occurred from Quelimane II to Machese Beira with the best catch rates (11.4-46.0 kg/h) in Quelimane I at 25 meters depth (Fig. 25B).

Polynemidae

This family occurs from Moebase to Machese Beira. Generally the catch rates were less than 10 kg/h (Fig. 26).

Fig. 26. Distribution of family Polynemidae (catch/hour).

Cynoglossidae

In table 8, it can be seen that best catch rates were found at Quelimane I, in waters shallower than 25 meters.

Theraponidae

Table 8 shows that the best catches per hour of this family were found at Beira at depths lower than 25 meters.

Abundance

A crude estimate for the demersal stock size can be obtained by the “swept area” method. The average catch rate for waters shallower than 25 m was found to be 99.98 kg/h.

The area of this zone is 14502 km². The average catch rate for waters deeper than 25 m was found to be 25.32 kg/h. The area of 2 this zone is 3688 km². For the calculation of the swept area the following values were considered: horizontal opening of the net 18 meters and trawling speed 2.8 knots. The following table shows the biomass estimates obtained, considering three different values of the efficiency coefficient, q, of the trawl. For a covered area of 18170 km², a minimum biomass estimate of 16519 tonnes was obtained (with q = 1) and a maximum estimate of 55.064 tonnes was obtained (with q = 0.3).

Table 13. Calculation of mean biomass using different efficiency coefficient.

q	Mean biomass (tonnes)
	25 m <	25 m >	Total
0.3	51750	3314	55064
0.5	31050	2000	33050
1	15525	994	16519

Biological characteristics of the main species Johnius belengerii

Total length range from 4-20 cm with most of the individuals (79.87%) being between 7-14 cm. Fig. 27 shows that the mean length varies in the three subareas, and the biggest specimens are found in Machese-Beira. The presence of the smallest individuals in Quelimane I, suggests that this is a recruitment area. The same result was observed in the SEBASTO-POLSKY RYBAK samples (TIMOCHIN, SOUSA et al., 1983).

Fig. 27. Length frequency distribution of J. belengerii.

Sofala Bank

Machese + Beira

Quelimane I

Quelimane II

The length-weight relationship was studied for females and males and the functional regression was applied (Fig. 28).

Fig. 28. Length-weight relationship of J. belengerii.

The results are as follows:

Females:	n = 182
	r = 0.975385
log	W = -1.955772 + 3.090012 log L
Males:	n = 193
	r = 0.982327
log	W = -1.996015 + 3.104683 log L

At the Sofala bank, females generally were at maturity stages 3 and 4; while males were at stages 2-3 and 4, excepting Quelimane II subarea, where 58.5% were at stage 2 (Table 14). Females were found at spawning in Quelimane I and Machese, and males only in the last subarea.

Table 14. Relative frequencies (%) of maturity stages of J. belengerii by subarea.

Stages Maturity	Females									Males
	Juv	2	2-3	3	3-4	4	5	6-3	n	2	2-5	5	3-4	4	5	n
Subareas
Quelimane II % (in nº)	8.3	32.8	5.7	20.0	22.9	18.6	-	-	70	58.5	17.1	24.4	-	-	-	41
Quelimane I % (in nº)	2.5	30.0	3.1	22.3	12.9	30.5	1.2	-	256	36.4	12.0	26.7	0.9	24.0	-	217
Machese % (in nº)	4.8	41.7	-	19.4	-	27.8	8.3	2.8	36	44.0	16.7	17.8	-	13.1	8.3	84
Beira % (in nº)	0.8	19.7	3.0	48.5	3.0	25.8	-	-	66	3.2	14.5	66.2	-	16.1	-	62
Total % (in nº)	3.5	29.9	3.3	25.7	11.9	27.6	1.4	0.2	428	35.1	13.9	30.7	0.5	18.1	1.7	404

Gonadosomatic index was estimated for both females and males and the results were:

Females:

n = 64

Males:

n = 36

Otolithes ruber

Total length varies from 5-38 cm () with most of the individuals (54.24%), being between 15-22 cm. The length varies in all subareas, and the biggest specimens were found in Machese-Beira (Fig. 29).

For the same reason as for Johnius belengerii, Quelimane I, seems to be a recruitment area. The same result was observed with S. RYBAK (TIMOCHIN and SOUSA et al., 1983).

Fig. 29. Length frequency distribution of Otolithes ruber.

Machese-Beira

Moebase

Quelimane I

Quelimane II

Sofala Bank

The length-weight relationship was studied for females and males (Fig. 30). The equations were:

Females:	n = 115
	r = 0.934980
log	W = -1.971848 + 3.001492 log L
Males:	n = 138
	r = 0.954251
log	W = -2.117650 + 3.101417 log L

Fig. 30. Length-weight relationship of O. ruber.

Generally, females were at maturity stages 3 and 4, and males at stages 2-3 and 3, excepting Quelimane I and Machese sub-areas, where the major number of specimens (males and females) were at stage 2 (Table 15).

Gonadosomatic index was estimated for males and females.

Females:

n = 107

n = 91

Maturity Stages	Females								Males
	2	2-3	3	3-4	4	4-5	5	n	2	2-3	3	3-4	4	4-5	5	n	Juv
Subareas
Moebase (% in nº)	29.2	12.5	33.3	20.8	4.2	-	-	24	7.2	21.4	35.7	35.7	-	-	-	14	9.5
Quelimane II (% in nº)	19.7	16.9	38.0	14.1	11.3	-		71	30.0	22.0	36.0	2.0	10.0	-	-	50	11.0
Quelimane I (% in nº)	59.3	3.6	21.0	3.6	12.0	-	0.6	167	50.9	6.8	17.4	0.6	21.1	-	3.1	161	5.7
Machese (% in nº)	53.6	-	14.3	-	30.3	-	1.8	56	43.9	2.4	19.6	-	31.7	-	2.4	41	-
Beira (% in nº)	2.2	6.7	37.8	13.3	37.8	-	2.2	45	2.5	5.0	32.5	-	55.0	-	5.0	40	-
Total (% in nº)	41.6	6.6	26.2	7.4	17.4	-	0.8	363	38.2	9.2	23.5	2.3	24.2	-	2.6	306	5.8

Johnius dussumierii

Total length varies from 8-21 cm , with most of the individuals (73.04%) being between 12-16 cm (Fig. 31). The comparison between subareas is not possible as samples are quite different.

Fig. 31. Length frequency distribution of Johnius dussumierii.

Quelimane II

Machese + Beira

Sofala Bank

In Quelimane II, both sexes were at maturity stages 2, 3 and 4 but the majority of females (38.4%) and males (53.7%) were at stage 3. In Machese-Beira, both sexes were at stages 2, 3 and 4, however, the majority of females (43.4%) were at stage 4, while males (48.3%) were at stage 3.

Gonadosomatic index was estimated for males and females.

Females:

n = 46

n = 46

Pomadasys maculatus

Total length range from 5-21 cm , with most of the individuals (79.11%) being between 9-15 cm. In Fig. 32 it can be seen that mean length is more or less similar in all subareas, except in Quelimane I, where the mean length is reduced. It seems that this is a recruitment area for this species. The same situation was found for other species.

Fig. 32. Length frequency distribution of Pomadasys maculatus.

Angoche + Moebase

Subarea Quelimane II

Quelimane I

Machese Beira

Sofala Bank

Fig. 33. Length-weight relationship of P. maculatus.

The length-weight relationship was studied for males and females (Fig. 33) the equation follows:

Females:	n = 91
	r = 0.99-1174
log	W = -1.774875 + 2.992047 log L
Males:	n = 152
	r = 0.988976
log	W = -1.670804 + 2.898801 log L

Generally for Sofala bank, females were at maturity stage 2. In Quelimane II, however, the majority of the females were at stages 2-3 and 3-4. In Angoche-Moebase and Quelimane I, males were at stage 2, while in Machese-Beira and Quelimane II they were at stages 3 and 4 (Table 16).

Table 16. Relative frequencies (%) of maturity stages of Pomadasys maculatus.

Maturity Stages	Females									Males
	Juv	2	2-3	3	3-4	4	4-5	5	n	2	2-3	3	3-4	4	4-5	5	n
Subareas
Angoche + Moebase % (in nº)	13.5	53.3	2.2	20.0	14.1	8.2	1.5	0.7	135	50.0	4.1	21.2	15.8	8.9	-	-	146
Quelimane II % (in nº)	5.0	36.3	18.2	18.2	27.3	-	-	-	11	25.9	11.1	59.3	-	3.7	-	-	27
Quelimane I % (in nº)	9.2	89.2	2.7	8.1	-	-	-	-	37	78.6	-	19.0	2.4	-	-	-	42
Machese + Beira % (in nº)	4.4	54.9	6.2	27.4	1.8	6.2	-	3.5	113	42.5	2.8	26.5	2.8	21.5	-	3.9	181
Total % (in nº)	8.5	57.7	4.4	21.3	8.1	6.1	0.7	1.7	296	48.0	3.5	26.0	7.1	13.6	-	1.8	396

In Machese-Beira, both sexes were at spawning. Gonadosomatic index was estimated for males and females. The results follows:

Females:

n = 45

Males:

n = 59

Rhonciscus stridens

Total length ranged from 6-16 cm, and the mean length was 12.27 cm (Fig. 34).

Fig. 34. Length frequency distribution of Rhonciscus stridens.

Females and males were at maturity stages 2, 3 and 4. However, the majority of females (71.8%) were at stage 3, while males (75.9%) were at stage 2 (Table 17).

Table 17. Relative frequencies (%) of maturity stages of Rhonciscus stridens by subarea.

Maturity Stages		Females						Males
		2	2-3	3	3-4	4	n	2	2-5	3	5-4	4	n	Juv
Subareas
Angoche + Moebase (% in nº)		28.2	25.6	46.2			39	75.9	6.9	10.4	3.4	3.4	29	1.4

Upeneus vittatus

Total length varies from 7.5-19.0 cm with most individuals (70.76%) being between 11.5-13.0 cm. It seems that the mean length decreases from north to south.

The length-weight relationship was studied for males and females (Fig. 36). The equations follows:

Females:	n = 192
	r = 0.9446
log	W = -1.888943 + 3.041261 log L
Males:	n = 178
	r = 0.9490
log	W = -2.177613 + 3.299889 log L

In Angoche-Moebase, Quelimane II and Machese-Beira the majority of females were at maturity stages 3 and 4, while males were at stage 2 (Table 18).

In Quelimane I males and females were at spawning. Gonadosomatic index was estimated for females and males.

Females:

n = 78

n = 27

Fig. 35. Length frequency distribution of U. vittatus.

Sofala Bank

Machese + Beira

Quelimane I

Quelimane II

Angoche + Moebase

Fig. 36. Length-weight relationship of U. vittatus.

Table 18. Relative frequencies (%) of maturity stages of U. vittatus by subarea.

Maturity Stages	Females									Males
	Juv	2	2-3	3	3-4	4	4-5	5	n	2	2-3	3	3-4	4	4-5	5	n
Subarea
Angoche (% in nº)		31.5	1.1	57.3	2.2	7.9			89	66.7	10.5	22.8					57
Moebase (% in nº)	1.1	21.0	7.4	37.1	11.1	22.2		1.2	81	45.4	5.7	31.2	8.5	9.4	0.9	0.9	106
Quelimane II (% in nº)		17.9	6.0	25.4	28.3	20.9	1.5	-	67	66.7	11.1	11.1	-	11.1	-	-	9
Quelimane I (% in nº)		16.7	27.8	25.9	0.9	22.2		6.5	108	24.1	14.5	32.5	10.9	9.6	-	8.4	83
Machese + Beira (% in nº)	0.3	39.1	12.0	41.2	-	7.7			233	54.2	8.5	21.1	0.7	14.8	-	0.7	142
Total	0.3	28.7	11.9	38.4	5.4	14.0	0.2	1.4	578	47.1	9.3	26.2	4.8	10.1	0.2	2.3	397

Total length varies from 9.0-16.5 cm and there is no significant variation in mean length for the various subareas (Fig. 37).

Saurida undosquamis

Total length range from 15-30 cm, the mean length being 22.29 cm, (Annex 4)

Polynemidae

Total length varies from 6-20 cm. The major specimens appeared in Quelimane I + Beira (Annex 4).

Fig. 37. Length frequency distribution of U. bensasi.

Moebase

Quelimane I