10. DEEP-WATER CRUSTACEANS

10.1. Characteristics of the main species
10.2 Abundance

10.1. Characteristics of the main species

By ‘deep-water crustaceans’ the present authors mean to indicate crustaceans living mainly below 200 m depth. Table 10.1 gives the main species in the crustacean catches from “Aelita”, “Kattegat” and “Dr. Fridtjof Nansen” from Delagoa Bay during summer. The data are not comprehensive enough to identify any significant differences in the species composition between areas and seasons. As seen, the spiny lobster (Palinurus delagoa) was dominant at depths down to about 350 m. At 350-400 m the crayfish (Nephrops andamanica) contributed approximately equally to the catches. There was also a minor contribution from the deep-water shrimp (Haliporoides triarthus). This species was dominant in the crustacean catches below 400 m.

Table 10.1. Composition of the crustacean catches below 200 m in Delagoa Bay during summer (% weight).

Depth zones (m)	Palinurus delagoa	Haliporoides triarthrus	Nephrops andamanica
201 - 250	89
251 - 300	98
301 - 350	85		15
351 - 400	43	10	46
401 - 500	1	67	32
500		100

In addition to these dominant species, some other crustaceans such as the shovel-nosed lobster (Scyllaridae) and crabs made a minor contribution to the catches. Table 10.2 lists the deep-water shrimps identified during the investigation.

Spiny lobster

The distribution area of Palinurus delagoa as deduced by the trawl catches from all the research vessels appears in Figs. 10.1 and 10.2. North of 21°S this species was very scattered and occurred rather seldom in deep-water trawl catches. In the Delagoa Bay two distinct distribution areas seemed to occur. There was a western ground off the Limpopo River and an eastern one at the south-eastern slope of the Boa-Paz Bank. The stock off the southernmost coast seemed to have its northern border just north-east of Inhaca Island. There were some gaps in the distribution from the Bazaruto Island to the Boa-Paz Bank. It is uncertain if these gaps in the distribution were real or only reflecting patchiness in abundance. The stock seemed to be most abundant during summer and there was usually a pronounced decrease in the catches during April.

Fig. 10.1. Recordings of deep-water crustaceans during winter.

Fig. 10.2. Recordings of deep-water crustaceans during summer.

The spiny lobster was usually caught at depths between 150 and 400 m. Occasionally it was recorded in deeper water. During summer the spiny lobster was most abundant between 200 and 300 m, while in winter 250-350 seemed to be its preferred depth.

Fig. 10.3. Modal length of the spiny lobster.

I) Inhaca, D) Delagoa, B) Bazaruto, E) Egg-bearing females.

The reproductive cycle is also indicated in Fig. 10.3. This is in reasonable accordance with the results of the investigation carried out along the coast of Natal (BERRY, 1973). The main hatching of the larvae probably took place in May, though egg-bearing females were observed until July. The breeding started mainly in September and the main moulting seemed to occur from July to September. A few specimens have also been observed moulting at other times of the year. It is likely, however, that these are a continuation of the sub-adult moulting cycle in which several moultings probably occur per year.

There seemed in general to be a decrease in length with depth. Off Inhaca a decrease in length was observed in the upper part of the distribution area from June to July. This indicates a vertical migration to shallower water during the time of moulting.

Crayfish

The species Nephrops andamanica was recorded in the Bazaruto area and on the western slope of the Boa-Paz Bank, as seen in Figs. 10.1-10.2. Some catches were also obtained over the slope off Sofala Bank. Crayfish were recorded at depths between 300 and 500 m but seem to be most abundant between 350 and 450 m. In the Bazaruto area a few catches were also taken below 500 m.

Crayfish seem mostly to occur at isolated localities. This apparent distribution might be due to the habit of the Nephrops sp, to form burrows in mud and remain there during the night. This habit reduces the possibilities of catching them with the usual bottom trawls. Only on the western slope of the Boa Paz Bank was a larger coherent distribution area recorded. The available data are too sparse to draw any firm conclusion on the distribution area of this species.

The reproductive cycle of the crayfish seems to follow that of the spiny lobster. Although crayfish have a widely extended hatching and spawning period, there are definite peaks of activity (BERRY, 1969). The peak of spawning activity occurs in August, and hatching takes place nine or ten months later with a peak in May. The sexually mature females probably moult annually with a peak of activity during the months of May to July.

Shovel-nose lobster

Species belonging to the Scyllaridae family were occasionally found along most of the coast at depths between 30 and 300 m. They seemed to be most abundant at 150-200 m. Maximum catch in the trawl did not exceed 6 kg per trawling hour. Rather good catches were obtained in pots off the Limpopo River and south of Inhambane in May.

Crabs

The most interesting of the very few catches of crabs were some good pot catches of the deep-water crab Geryon quinquedens. Up to 72 individuals or 30 kg per pot were obtained in Delagoa Bay at depths of about 350 m. A recent publication deals with this potential resource. From this investigation it seems that 350 m is close to the upper limit of Geryon sp. distribution area. This crab was recorded at depths more than 1000 m, and the best catches off the coast of the Congo were obtained at depths between 500 and 700 m. The sex ratio changes with depth, as the number of males increased with increasing depth (CAYRE, LE LOEUFF and INTES, 1979).

In our sample of 72 individuals there was only four males which confirms this observation. The males are the most commercially important as they are bigger. Further investigation should be carried out on this potential resource.

Deep-water shrimps

This group consists of several species listed in Table 10.2, of which Haliporoides triarthrus is the dominant one. The localities where this species were observed appear in Fig. 10.1. It was caught at depths from 350 m down to more than 500 m. Data are too scarce to establish with any certainty the real distribution area. The locality of highest abundance was found off Ponta da Barra Falsa. The deep-water shrimp is most likely also distributed along the slope of Sofala Bank, but no data are available from this area.

Table 10.2 Species of deep-water shrimp identified.

x) The commercially most important.

10.2 Abundance

Tables 10.3 and 10.4 show the catch rates and percentages of crustaceans in total catches for summer and winter respectively. As the different vessels were using different gear the data for each of them were put into the tables separately. “Dr. Fridtjof Nansen” and “Kattegat” used bottom and shrimp trawls with a horizontal opening of 15-20 m while “Aelita” used a special lobster trawl with a 55 m horizontal opening. As expected, the catches of the first two vessels were in general lower than those of “Aelita”. There seemed to be a minimum both in percentage of crustaceans and in catch rate at 300-350 m.

Table 10.3 Deep-water crustacean catches during winter. N = no. of hauls, C ^kg/_h = catch rate of crustaceans, C % = percentage of crustaceans in total catch.

Table 10.4 Deep-water crustacean catches during summer. N = no. of hauls, C ^kg/_h = catch rate of crustaceans, C % = percentage of crustaceans in total catch.

The abundance of the spiny lobster and deep-water shrimp were calculated using the “swept area” method. For the calculations the distribution areas in the three regions were regarded as continuous. The catch rates used were mostly from “Aelita”. Only for the western ground of Delagoa Bay during summer were the data from “Kattegat” applied. The efficiency coefficient was taken as 1.0 which seems to be reasonable estimate for crustaceans.

Table 10.5. Sampling area and stock size (tonnes) of spiny lobster. A = area km² B_S = stock size (summer). B_W = stock size (winter).

Depth (m)	Bazaruto			Delagoa			Inhaca
	A	BS	BW	A	BS	BW	A	BS	BW
201-250	1030	262	31	1125	81	14
251-300	1030	125	48	1190	96	47	330	62	29
301-350	1030	63	23	700	1	2	110	19	12
351-400	1030		16
Total		450	118		178	63		81	41

Table 10.6 Sampling area and stock size (tonnes) of deep-water shrimp during summer. A = area km², B_S = stock size.

Depth (m)	Bazaruto		Delagoa
	A	BS	A	BS
400 - 500			450	73
500	450	170	150	32
Total		170		105

The results of the calculations are presented in Tables 10.5 and 10.6. The stock of spiny lobster seemed to be much smaller in winter than in summer. The total estimate for summer was 709 tonnes and for winter 225 tonnes. This apparent seasonal variation could be a result of insufficient data. BUDNICHENKO et al. (1977) calculated the stock for summer using the “Aelita” data alone. Even considering a smaller distribution area he reached a total estimate of 1850 tonnes for the stock size of the spiny lobster. The main reason for this discrepancy between the two estimates is that Budnichenko was using an efficiency coefficient for the trawl of 0.3. This figure seems to be rather small for crustaceans and a value between 0.5 and 1.0 seems more likely.

The data available are insufficient to establish either the size of the area or the average density with the desired degree of certainty. The estimates presented should therefore be used with caution. As a first approximation a stock size of 1000 tonnes is suggested for the spiny lobster. This is probably an underestimate.

Longhurst in GULLAND (1970) suggests a stock size of the spiny lobster along the coast of Natal and Mozambique of about 2500 tonnes and a potential annual production of 600-700 tonnes. It we apply the same ratio between the standing stock and the potential annual yield for the coast of Mozambique, an annual production of spiny lobster of about 300 tonnes is to be expected. However, yield estimates calculated from the South African fishery in the area during the late sixties and the early seventies suggest a higher value for the annual production.

The calculations carried out for the deep-water shrimp are even more dubious as these species seem to be more patchily distributed and the number of trawl hauls within the distribution area was very small. The slope of Sofala Bank has not been included in the calculations due to lack of data, but catches confirm that this resource also occurs there. Most likely, this area is the most important for deep-water shrimp and the stock there is at present exploited by a Spanish fleet which is believed to catch some few thousand tonnes annually. As a preliminary estimate of the stock size of deep-water shrimps south of Bazaruto Island we suggest 500 to 1000 tonnes. Due to lack of data from Sofala Bank this stock size is probably seriously underestimated.

For the crayfish the available data give some support to a “guesstimate” of a few hundred tonnes.