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RESULTS

The cruise track, fishing stations and hydrographic stations are shown in Fig. 1. Table 1 gives details of the fishing stations and Table 2 describes the length distributions and maturity stages of the most important species. The appendix contains a complete list of all species recorded with names in Latin, in English and where possible, in Creole. A list of stations recording current direction and speed is also included in the appendix.

It should be stressed that there was very often a discrepancy between the charted position of the edge of the plateau and the position as recorded by the Magnavox Satellite navigator. This was also often true for the charted depths of the plateau and for the position of some of the reefs.

Oceanography

The survey took place during the height of the S.E. monsoon period. The current (Fig. 2) appeared to follow the edges of the plateau, whilst in the more central areas of the bank the dominating current direction was southerly. The effect of the main group of islands was to make the direction more southwesterly. During the observed period, the wind direction was constant from the southeast with a force of 4 - 5 on the Beaufort scale. At about 5°20’S on the plateau, there appeared to be a clockwise circulation in the surface current.

East of Mahé, near the edge of the plateau the current was between one and two knots, whilst over the rest of the area it was less than one knot.

The hydrographical sections are shown in Fig. 3, 4 and 5. In all the sections the temperature of the surface layer was about 26°C, as also shown in Fig. 6. A common feature to all sections was that cold water of low oxygen content was found at shallower depths on the west side of the plateau than on the east side. This water mass also appeared in the deeper parts within the plateau as demonstrated by the depth of the thermocline in Fig. 7. In the shallow areas in northeast no thermocline was observed. The distribution of cold water masses poor in oxygen was consequently limited by the bottom topography. Below about 150 m the temperature appeared to be the same on both sides of the plateau.

Fishing operation and trawl bottom

As observed during earlier surveys, the best trawling grounds were in the southeastern plateau region with some grounds of limited extent to the southwest of Mane (Fig. 8). Within the untrawlable zone, small areas also appeared which are available to trawling by small sized vessels. The region to the northwest of Mane could possibly contain such small areas with trawlable ground, but this region was not visited by the vessel due to the inadequate soundings and the many reefs indicated on the available chart.

On the untrawlable grounds, traps and bottom long lines could possibly be useful gears as there are usually demersal fish registrations in those parts of the plateau. The catches from the present survey indicate yields of up to 7.3 kg/trap and 20 kg/100 hooks. The major contributors to the trap catches were Lutjanus sanguineus, Lutjanus sebae, Lethrinella miniatus and Lethrinella variegatus, all being valuable commercial species. Lutjanus sebae and Lutjanus sanguineus were the only species taken by long lines besides several different species of sharks.

The sharks will possible present some problem to a long line fishery unless the lines are frequently overhauled.

Plankton and fish larvae

The distribution of echo-abundance is shown in Fig. 9. These recordings will also include minor contributions from especially small juvenile fish, as they sometimes are difficult to separate from plankton. During the night, mesopelagic fish could have made some contribution to the plankton values. They are, however, thought to be of little importance as they only were caught once on the plateau and in small quantity.

Both squids and jellyfish were presumed to make a significant contribution to the plankton recordings, as they were commonly found in the pelagic trawl catches.

The highest concentrations of plankton were found in the southern part of the Mahé plateau, and along its outer edges. In the northern area, north of 4°40’S, the concentrations were rather low.

The wet displacement volumes of plankton from the hydro-graphical sections, which never exeeded 10 ml, are shown in Fig. 10. No special difference between night and day catches in the Juday net were registered. There appeared to be a positive correlation between the plankton volumes and the integrator values along the three sections. From the integrator values and the plankton volumes there was an apparent plankton minimum in the clockwise circulation noted above at about 5°20’S.

Demersal fish

Most of the demersal fish concentrations appeared to be on the plateau south of 4°40’S as shown in Fig. 7. Demersal fish registrations were very scarce on the whole plateau. The integrator values of demersal fish are, however, subject to large errors and they must be considered as underestimates. Due to the deadzone of the echosounder, near-bottom fish will not be detected.

Except at station 225 and 219 with catches of 1106 kg/hour and 350 kg/hour, all the other demersal trawl hauls were poor with catches of less than 200 kg/hour. Catches of that size were obtained in areas both with and without demersal echo registrations. The most common species were Lutjanus sebae, Lutjanus lineolatus, Pseudupeneus seychellensis, Saurida undosquamis, Nemipterus peroni, Abalistes stellaris and Loxodon macrorhinus.

In Fig. 11 the total length frequency distribution of Lutjanus lineolatus, Psedupeneus seychellensis and Saurida undosquamis are shown. All three distributions appear to reflect length distributions of different yearclasses. The figure shows, however, that there is a difference between length of males and females of Pseudupeneus seychellensis and Saurida undosquamis. This should be borne in mind when making length distributions of samples as a total length distribution could mask the real distribution of the yearclasses.

Pelagic fish

Surface schools of large fish, mostly bonito (Euthynnus affinis) were frequently observed throughout the area. They are, however, not included in Fig. 6 as they were close to the surface and therefore not recorded by the echosounder. As can be seen from the figure, the recordings were very patchy on the plateau. Only a few good registrations were obtained, all southeastwards of Mahé. In that area indicated by an “S” in the figure, many dense schools of juvenile Decapterus sp. were observed. They were just in the surface layer and therefore not recorded on the echosounder, moving too quickly to be caught by the pelagic trawl. West of this position, about 10 nautical miles outside the edge of the plateau dense schools of juvenile Decapterus sp. were recorded at 10 - 20 m depth. They gave a catchrate of about 1800 kg/hour of fish in the range of 3-9 cm. The area outside the edge of the plateau was not well covered mainly due to the fact that the echo-sounder in use was only able to adequately cover depths to about 100 m. Therefore the survey was confined to the plateau. Another dense concentration of pelagic fish was recorded west of Mahé. This appeared also to be juvenile Decapterus sp. with a length range of 7-10 cm, and gave a catchrate of 1188 kg/hour. A fairly good catch of juvenile Decapterus sp. of about 260 kg/hour with a length range of 5.5 - 11.5 cm was obtained in the area to the south of Mahé. Smaller catches of juvenile Decapterus were also obtained in both pelagic and bottom hauls, and the length frequency distributions of all juvenile catches, including St.no. 234 are shown in Fig. 12. The smallest specimens were found south of Mahé at stations 220 and 231. In the same general area, further to the west, less dense concentrations of pelagic fish yielded a mixture of species including some adults of Decapterus maruadsi, Decapterus macrosoma and adults of Selar crumenophthalmus together with juveniles of Decapterus spp. (3,5 - 15,0 cm), Sardinella sp. (3,5 - 15,0 cm), Rastrelliger kanagurta (8,0 - 12,5 cm) and Auxis thazard (7,5 - 12,0 cm).

According to the echoregistrations and trawl catches it seems obvious that the Mahé plateau is nursery area, particularly for Decapterus spp., but also for other pelagic species. Only minor concentrations of adult Decapterus spp. were recorded during this season in contrast to the previous survey in October 1977 with the R/V “Professor Mesyatsev”, when substantial concentrations were found with catches up to 8000 kg/hour.

The length frequencies of the three species of Decapterus caught on the plateau are shown in Fig. 13.

About half of the adults of Decapterus macrosoma were in maturity stage 5, and a further 10% were observed to be in stage 6. Only about 20% of the adults of Decapterus maruadsi were in maturity stages 5 and 6.


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