11. BY-CATCH IN THE CRUSTACEAN FISHERY

11.1 Shallow-water shrimp
11.2 Deep-water crustaceans

11.1 Shallow-water shrimp

Magnitude of the by-catch

The main fishing areas for shallow-water shrimp are found at the Sofala Bank between Angoche and the delta of the Zambezi River at depths between 10 and 40 m. The total catch of the national fleet in the area is probably 6-7000 tonnes annually. In addition there is a licensed foreign fleet, mainly from Spain, which has an estimated yearly catch of at least 5000 tonnes (ULLTANG et al., 1979). This gives a total of about 12 000 tonnes of shrimps from Sofala Bank.

Table 11.1. Catch rates and percentages of shrimps in catches from the research vessels at Sofala Bank.

VESSEL - GEAR		N	NC	Shrimp catches
				P	PC	CCkg/h
Summer (Oct-Mar)
“Aelita” (Oct-Dec 1976)
	Bottom trawl	34	-	6.0	-	6*
“Kattegat” (Oct-Dec 1977)
	Shrimp trawl 22/26-33	13	12	6.9	7.4	14.2
	Shrimp trawl 22/26-Mod	9	7	18.6	20.3	43.0
	Shrimp trawl 26/32-33	5	4	12.5	12.6	51.6
	Bottom trawl RG-140	31	17	0.6	1.8	3.2
“Dr. Fridtjof Nansen”
	Bottom trawl
Cruise II (Oct-Dec 1977)		6	4	3.2	3.6	9.5
Cruise III (Jan-Mar 1978)		5	4	0.5	0.5	5.8
Winter (Apr-Sep)
“Aelita” (Jun-Aug 1976)
	Bottom trawl	59	-	13.6	-	8.9*
“Dr. Fridtjof Nansen”
	Bottom trawl
Cruise I (Aug-Sep 1977)		5	3	3.8	16.8	11.0
Cruise II (Apr-Jun 1978)		14	14	3.8	3.8	9.2

N = total number of hauls, N_C = number of hauls containing shrimps, P = % of shrimps in all hauls, P_C = % of shrimps in positive hauls, C_C = catch rate of shrimps in positive hauls.

* Catch rate calculated for all hauls.

Table 11.1 summarizes the percentage of shrimps in the total catch from the research vessel surveys at Sofala Bank at depths shallower than 40 m. As can be seen, the average percentage of shrimps in catches where shrimps occure may vary between 0.5% to 20.3%. Is the percentage of shrimp in the bottom trawl catches representative for that of the commercial shrimp trawlers? The most likely answer to this question is “no”. The ratio shrimp catch/fish catch in a typical bottom trawl is expected to be less than in a typical shrimp trawl for the following reasons:

- the shrimp trawl will catch a large proportion of the shrimps ahead of the trawl due to the lack of bobbins and the use of a tickler chain.

- on a shrimp trawl the trawl doors are usually attached close to the net wings, while on a bottom trawl long bridles are used. The shepherding effect of the bridles will result in a decrease of the shrimp catch/fish catch ratio for the bottom trawl.

The small semi-industrial trawlers of size less than 11 GRT in Maputo Bay and less than 16 GRT in Beira Bay usually land the whole catch of both fish and crustaceans. Table 11.2 gives the percentage of shrimps in the landings and the catch rate of shrimps for 1977. As seen, the percentage of shrimps varied between 12 and 33% in Maputo Bay with an annual mean of 20.1%. In Beira Bay the variation was between 8 and 50% with an annual mean of 18.0%. The high values of the percentage in July, August and October in Beira Bay were associated with high catch rates. These percentages are most likely overestimated as the fishermen probably did not land the total catch during days with high catch rate of shrimps.

Fig. 11.1. Average catch rate for the commercial shrimp trawlers.

A reasonable conclusion to this discussion seems to be that the percentage of shrimps for the commercial shrimp fleet will be about 20% of the total catch. This estimate must of course be considered as a first approximation. Only sampling of the commercial catches can improve this figure.

A by-catch of 80% means that the shrimp trawlers are catching about 60 000 tonnes of fish annually at Sofala Bank of which probably only about 1000 tonnes are landed.

Table 11.2. Shrimp percentages in landings and shrimp catch rates (kg/fishing day) made by the smaller trawlers at Maputo and Beira Bays in 1977 (BRINCA, 1979. Pers. comm.).

Month	Maputo Bay		Beira Bay
	% shrimp	Catch rate	% shrimp	Catch rate
January	12.2	32.2	11.4	88.7
February	21.7	62.2	11.1	89.4
March	33.0	99.4	16.6	135.8
April	24.9	93.9	16.9	121.6
May	27.5	89.3	10.0	60.0
June	25.4	67.5	17.4	134.5
July	18.4	57.0	50.6	382.4
August	19.0	48.0	47.1	194.6
September	14.9	50.3	23.0	89.2
October	17.5	67.1	32.5	185.7
November	12.6	40.8	7.7	48.5
December	12.4	40.2	9.6	68.8
Annual mean	20.1	64.2	18.0	124.9

Species composition of the by-catch

The species composition of the research vessel surveys is shown in Table 11.3. As can be seen, about 50% consisted of pelagic species, while demersal species contributed about 30%. Of the crustaceans, the main part was, of course, shrimps, but swimming crabs may contribute during summer, and also the inshore spiny lobster (Panulirus spp.) can be present.

The main species of ponyfish present were Leiognathus equula, Secutor insidiator and Gazza minuta. In the sardine group Pellona ditchella and Thryssa vitrirostris were dominant. Besides the Trachurus sp. and Decapterus spp. the scad group also included Megalaspis cordyla.

Demersal fish present in the by-catch were mainly small bream, such as Pagellus natalensis and Nemipterus delagoa. The group “Others” consisted mainly of jellyfish in summer, while during winter hairtails Trichurus lepturus and barracudas Sphyraena spp. were frequent. In general, all of the fish present in the by-catch were of some commercial value. It is uncertain to which degree the species composition of the research vessels were representative for the commercial trawlers.

Table 11.3. Composition of the catch from the research vessels at the shrimp ground at Sofala Bank (% weight).

Species		Summer	Winter	Total
CRUSTACEANS		4.0	8.0	5.1
PONYFISH Leiognathidae		20.6	1.7	15.6
SARDINES Clupeidae - Engraulidae		8.7	29.3	14.1
JACKS Carangoides spp		8.0	1.9	7.3
SCADS Decapterus spp		6.8	0.8	6.1
MACKEREL Scomberomorus spp - Rastrelliger spp		2.6	2.5	2.6
SHARKS Carcharhinidae		3.9	8.5	5.1
	TOTAL PELAGIC	49.5	46.1	48.6
BREAMS Sparidae - Nemipteridae		9.3	1.7	7.3
GRUNTERS Pomadasyidae		6.6	4.8	6.1
CROAKERS Scianidae		4.8	7.8	5.6
GOATFISH Mullidae		3.5	5.1	3.9
LIZARD FISH Synodontidae		3.5	4.0	3.6
	TOTAL DEMERSAL	27.7	23.4	26.5
	OTHERS	18.8	22.5	19.8
TOTAL CATCH RATE (kg/h)		247	116	190

Seasonal variations

Table 11.2 indicates that the catch rate of shrimps and the percentage of shrimps in the catch were higher during winter (Apr.-Sep.) than during summer. Fig. 11.1 shows a maximum catch rate at Sofala Bank in April-June. From Table 11.3 sardines seemed to be most abundant in winter while the other pelagic species mainly occurred more frequently in the catches during summer (Oct.-Mar.). For the demersal species it is difficult to see any seasonal variation at all.

11.2 Deep-water crustaceans

The by-catch of fish when fishing for deep-water crustaceans at depths below 200 m comprised usually between 80 and 90% of the total catch. Off Inhaca at depths between 300-350 m and off Bazaruto at 200-250 m the contribution from fish in the catches may decrease to about 50%. During winter there were usually lower total catch rates and percentages of crustaceans than in winter, as can be seen from Table 11.4.

Table 11.4. Percentages of deep-water crustaceans (% weight) and total catch rates from the research vessels surveys.

Area	Summer (Oct.-Mar.)			Winter (Apr.-Sep.)
Depth	No. of hauls	% Crustaceans	Catch rate kg/h	No. of hauls	% Crustaceans	Catch rate kg/h
BAZARUTO
201 - 250	10	46.8	164	23	10.5	83
251 - 300	9	37.1	98	18	10.8	117
301 - 350	4	10.4	179	9	5.3	128
351 - 400	-	-	-	4	17.4	63
401 - 500	1	22.5	130	-	-	-
> 500	9	60.5	105	1	42.9	23
DELAGOA
201 - 250	18	9.6	155	10	1.6	135
251 - 300	32	14.9	138	34	15.3	87
301 - 350	9	4.6	89	9	11.6	86
351 - 400	18	15.5	96	-
401 - 500	14	22.5	136	-
> 500	2	66.3	48	-
INHACA
251 - 300	12	45.5	143	15	20.0	110
301 - 350	24	52.2	102	9	28.1	97

The by-catch usually consisted of species of little or no commercial value. Quantitatively, the most important groups were the typical mesopelagic fish, such as lanternfish Myctophidae, snake mackerel Gempylidae, man-of-war fish Cubiceps natalansis and Scombropidae.

The rest of the fish catch in the upper 300 m consisted mainly of the spiny dogfish Squalus acanthias, the lizard fish Synodontidae, rat tail Macrouridae, robins Triglidae and crocodile fish Peristediidae, of which the two first species contribute most.

In the Inhaca area squid and cuttlefish (Loligindae and Sepiidae) sometimes contributed significantly, up to 15% of the catch. Molluscs were more abundant during summer and occurred in catches mostly above 350 m. At depths between 200 and 250 m pelagic species, such as horse mackerel, Indian mackerel and barracudas, were sometimes frequent in the by-catches.

The percentage of fish in the catches seemed to be lower in summer when the total catch rates were higher. The available data give no indications of a pronounced seasonal variation in the species composition.