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MATERIAL AND METHODS

Fishing gear

The pelagic trawl in use had dimensions of 16 x 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 demersal trawl covered a track of about 60 m between the doors when fishing using 40 m bridles. The footrope was equiped with 0.5 m bobbins, and the effective vertical opening of the net was 6.5-7 m.

The dimensions of the iron framed traps were 75 x 75 x 200 cm. The frames were covered with 30 mm nylon net and the traps baited with chopped fish. They were set 50 m apart in gangs of six.

The bottom long line consisted of 200 hooks, size no 7, baited with available fish pieces.

Hydrography and plankton

Three east-west hydrographical sections across the bank were carried out. At all stations temperature, salinity and oxygen were observed at the following standard depths: 0-10-20-30-50-75-100-125-150-200-250-300-400-500 meters or to the bottom in the shallower areas. The salinity and oxygen samples were analysed on board.

At approximately 30 nautical mile intervals the vertical distribution of the temperature down to 250 m was observed by means of a bathythermograph.

The surface current was mapped by observing the drift of the vessel utilising a Magnavox Satellite navigator. A vertical haul from 50 m to the surface was obtained at each hydrographic station by a 180 µ Juday net with a diameter of 36 cm, and the wet displacement volume of the plankton was measured.

Fish biology

The literature which has been 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.

In particular, the nomenclature of the three Decapterus species caught during the cruise is confused. They have been identified as

a) Decapterus maruadsi (Temmink + Schlegel) which is probably equivalent to Decapterus kiliche (C + V) of Nekrasov (1968), but which may yet prove to be Decapterus dayi (Wakiya), an East African coastal species.

b) Decapterus macrosoma (Bleeker), synonyms of which appear to be Decapterus macarellus (C + V) of Nekrasov (1968) and Decapterus lajang (Bleeker) of Smith (1950).

c) Decapterus kurroides listed as Decapterus russelli (Rüppel) in Nekrasov (1968).

Other references to and identifications of Decapterus species are too vague to be of use.

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


Acoustics

Two of the scientific sounders (38 kHz and 50 kHz) were damaged before the cruise started. The scientific sounder in use (120 kHz) was coupled to the integrator. To get a better picture of the bottom echorecordings the echo magnifier was used in the six meters range on the 120 kHz echosounder. The depth ranges of the echointegrator were 4-50 and 50- 100 m. The values were read for each nautical mile and averaged over five nautical miles. Continous watch was kept on the acoustic instruments, and fishing carried out whenever the echorecording changed its characteristics. The acoustic data were scrutinized every day and the integrator readings were split into three categories: pelagic fish (including juvenile fish), demersal fish and plankton with fish larvae. The echo abundance within each category was classified as scattered, dense and very dense.

Trawl bottom

Every five nautical miles the quality of the trawl bottom was judged from the echo recordings and characterized according to the following scale:

Good trawl bottom

1

Trawling possible with caution

2

Impossible to use bottom trawl

3


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