2. MATERIALS AND METHODS

“Dr. Fridtjof Nansen” was working off Mozambique from 24 August 1977 to 20 June 1978. The whole coast was covered four times. Details on tracks and the work carried out are fully described in the cruise reports (ANON. 1977 b, 1978 a, 1978 b, 1978 c) and will not be repeated here. As an example of the survey grid, Fig. 2.1 is presented. This shows the vessel’s track and stations along the southern part of the coast during cruise no. 4.

Table 2.1. Number of fishing stations on the different surveys.

GEAR	Aelita	Professor Mesyatsev	Kattegat	Dr. Fridtjof Nansen
Bottom/Shrimp trawl	556	164	169	96
Pelagic trawl			18	76
Long line	29			24
Gill-net				7
Hand line				12
Pots				55

In addition to data from the “Dr. Fridtjof Nansen”, trawl data from the following expeditions have also been included in the present report: “Aelita”, “Kattegat” and “Professor Mesyatsev”. Table 2.1 reviews the number of fishing stations on each survey. The surveys of “Aelita” and “Kattegat” were mainly trawl surveys while that of “Professor Mesyatsev” was a combined trawl and acoustic survey. On the survey of “Dr. Fridtjof Nansen” the main emphasis was put on acoustic surveying. Some data from the Soviet commercial trawler have also been included in the report, together with data on fisheries statistics for 1977 from SIP (Servico de Investigacoes Pesqueiras) in Maputo.

Vessel and gear

R/V “Dr. Fridtjof Nansen” is a 150-foot combined stern trawler and purse seiner. The main engine of 1500 Hp gives a maximum speed of 13 knots. There is accommodation for 28 men. All winches are hydraulic. The boat carries two pelagic trawls, one bottom trawl and one purse seine, gill-nets, long-lines, hand lines and pots. A satellite navigator allows very precise determination of position.

Fig. 2.1. Survey routes and stations - southern part during cruise no. 4.

1) Hydrographic station 2) Bathythermograph station 3) Pelagic trawl 4) Bottom trawl 5) Handline 6) Pots 7) Long-line 8) Gill net.

The acoustic equipment consists of three scientific sounders (120, 50 and 38 kHz), two echo integrators, each of two channels, one sonar (18 kHz) and one netsonde (50 kHz). The two echo integrators were coupled to the 38 kHz sounder.

Echo integrator values were read at each nautical mile and averaged over five nautical miles. Continuous watch was kept on the acoustic instruments and fishing carried out whenever the echo recordings changed characteristics. The acoustic data were scrutinized once a day. Integrator contributions from false bottom, wakes etc. were deleted and the readings were split into four categories: small pelagic fish, demersal fish, plankton and fish larvae, and mesopelagic fish. The values within each group were plotted on charts. The integrator readings on the charts were split into strata, depending on the value. By drawing isolines, areas of different fish density were obtained. The size of the areas was calculated by mean of a planimeter. These values multiplied with the average integrator readings gave relative abundance indices.

Following FORBES AND NAKKEN (1972) the output of an echo integrator is proportional to the fish density:

(1)

where P_A is the fish density expressed in weight per unit area, M is the integrator reading and C_W a conversion coefficient depending on the fish species and size, as well as on the characteristics of the sounder and integrator used. It can be shown that C_W is proportional to the length of the fish (NAKKEN 1975, NAKKEN and OLSEN 1977).

The numerical value of C_W applied to the “Dr. Fridtjof Nansen” data was 10.5 tonnes/mm per nautical mile · (n. mile)². This figure was established for a mixture of mainly pelagic species with an average length of about 17 cm (ANON 1977 a). The density coefficients, C_W used in the present study will then be:

(2)

where l is the mean length in cm of the observed species.

The total fish abundance, B, can then be calculated using the equation:

(3)

where is the average integrator reading and A the corresponding area.

The value of C_W, equation (2), is believed to be reliable for small pelagic species. The distribution area of pelagic fish was split into subareas where the species composition and the average length of the different species were determined by means of the trawl catches. The calculation was than carried out according to equations (2) and (3). The same equations were also applied for mesopelagic fish, assuming that the scattering properties of these fish are similar to those of pelagic fish.

For demersal species, however, the acoustic estimate will be less reliable, as there are no observations of C_W for these fish. Due to lack of other information on the sound reflecting properties of demersal fish from the region, a C_W = 10.5 tonnes/mm per nautical mile · (n.mile)² was chosen. For the snapper (Lutjanus sp.) at St. Lazarus Bank the value of C_W was doubled.

Due to a misunderstanding the acoustic investigations of Cruise no. 3 were not carried out the same way as on the other three cruises. The abundance estimates from this cruise have therefore been deleted from the present report.

Fishing gear

The pelagic trawl in use had dimensions of 16 × 16 fathoms around the trawl mouth, usually corresponding to a height of 17 m when fishing. It was operated with superkrub doors and 120 m bridles, and was always used together with a net sonde.

The bottom trawl covered a track of about 60 m between the doors when fishing using 40 m bridles. The footrope was equipped with 0.5 m bobbins, and the effective vertical opening of the net was 6.5-7.0 m.

Three types of pots were used:

TYPE RF. This pot was originally constructed for fishing black cod (Anoplopoma fimbria) on the west coast of USA and Canada (HIPKINS, 1974). It is a metal frame pot with webbing cover of 60 mm mesh size. The pots used in this investigation were slightly smaller than the original and had the dimensions 0.75 × 0.75 × 2.25 m.

TYPE RK. This pot was designed for fishing crayfish (Nephrops norvegicus) in Norwegian waters. It is also a metal frame pot with webbing cover of 28 mm mesh size. It has four entrances.

TYPE RQ. This pot construction is used for fishing Queencrab (Chionocetes sp.) in Canadian waters. It is a cone-shape pot with largest and smallest diameter of 1.20 m and 0.75 m respectively. The height is 0.65 m and it has only one entrance.

Fish biology

The literature used for identification is listed in the references. The nomenclature in the FAO Species Identification Sheets for Fishery Purposes (Fishing Areas 57/71) was used when there were any conflicts of nomenclature in the literature. For sharks the nomenclature of the Oceanographic Research Institute in Durban was adopted.

Catches were sorted by species and measurement of total length, weight, sex and maturity stage were carried out for the most abundant species. The total length of small specimens was measured to the nearest 0.5 cm below, and large ones to the nearest 1 cm below. The following maturity scale was used:

Maturity stage	Stage no.
Immature	1
Mature unripe	2
Mature ripening	3
Mature nearly ripe	4
Mature ripe (non spawning)	5
Mature ripe running (spawning)	6
Mature spent	7

Plankton

Plankton samples were obtained at each hydrographic station by a vertical haul 100-0 m with a 36 cm diameter Juday net of 500 µ mesh size. Wet displacement volume of the samples was measured on board. The contribution to the echo integrator readings from plankton was plotted on separate maps.

Hydrography

Six fixed hydrographic sections were carried out along the coast. In addition, four sections at Sofala Bank were worked in order to follow the fresh water outflow from the Zambezi River from Cruise no. 2 onwards. The hydrographic sections were repeated during each of the four coverages. The locations of the sections appear in Fig. 2.2.

At the hydrographic stations temperature and salinity were observed at the following standard depths: 0-10-20-30-50-75-100-125-150-200-250-300-400-500 m. Samples for oxygen titration-were taken from 0-10-20-30-50-75-100-150-200-300-500 m. At SECTION V samples were taken at standard depths to the bottom during cruises 1 and 4.

The salinity and oxygen samples were analyzed on board. At approximately each 30 nautical miles, the vertical distribution of the temperature down to 250 m or bottom was observed by means of a bathythermograph. At these stations the surface salinity was also observed. The surface current was mapped by observing the drift of the vessel.

Data on wind from Aug. 1977 to June 1978 and the average fresh water run off were obtained from SIP in Maputo. The locations of the stations used appear in Fig. 2.2.

Trawl bottom

The quality of the trawl bottom as judged from the echo recordings was characterized at depths shallower than 250 m according to the following scale:

Impossible to use bottom trawl	1
Possible with caution	2
Good trawl bottom	3

Fig. 2.2. Hydrographic sections, meteorological stations and freshwater runoff gauges.