The workplan for the project, prepared by the Institute, was based upon a review of information available with regard to the region, a summary of which was included in the workplan. For the purposes of the survey the important features of the surveyed region are:
1. its sharing the Indo-Pacific fauna, of which the large number of species, adapted to tropical and sub-tropical habitats, is a distinctive characteristic;2. the two-monsoon climatic regime and the diverse physical, chemical and biotic consequences of this regime;
3. the several up-welling situations; and
4. the high levels of primary production.
Obviously these are only separately specified manifestations of the structure and behaviour of what is a vastly complex system; nevertheless, interconnected though they must be, they separately had strong significance in the planning of the survey operations, as in progressive revision of those plans, and have to be kept in mind in analysing the results.
The higher levels of the biota were, of course, the immediate target of the work and from feature 1 the survey planners had to prepare for a considerable task of identification in working on catches to be taken; moreover it had to be expected that interpretation of echo-traces would be made difficult by the diversity of species. It was clear, however, that many if not the majority of the species would be of little importance and that special attention should be directed upon a relatively short list of species which were already of economic importance in the Indian Ocean; the species thought to be most abundant were the following:
|
Sardines: |
|
Indian oil sardine: |
Sardinella longiceps |
Other sardines: |
Sardinella sindensis |
|
|
Sardinella fimbriata |
|
|
Sardinella dayi |
Rainbow sardine: |
Dussumieria acuta |
|
Mackerels: |
|
Indian mackerel: |
Rastrelliger kanagurta |
Mackerel: |
Scomber japonicus |
|
Horse Mackerels, jacks and scads: |
|
|
|
|
Scad: |
Decapterus ruselli |
Jack: |
Caranx spp. |
|
Anchovies: |
Stolephorus spp. |
|
Other species: |
|
Frigate mackerel: |
Auxis thazard |
Horse mackerel: |
Caranx spp. |
|
Tunas: |
|
Yellowfin tuna: |
Thunnus albacares |
Skipjack: |
Katsuwonus pelamis |
Little tunny: |
Euthynnus affinis |
The Indian Mackerel (Rastrelliger kanagurta) was known to be widely distributed in the Arabian Sea along the coast from Africa to India, although the only substantial fishery at that time was on the southwest coast of India (mostly by beach-seining). Mackerel schools had been seen (not frequently) at the surface off Arabia (Aden-Mukalla). On one occasion the fish had been recorded as a scattering layer at a depth of 20-80 fathoms just above the bottom for 10-15 nautical miles. The largest schools had been estimated to be of 10-20 tons. Horse mackerels had been caught in both the western and eastern parts of the ocean.
A substantial body of information on the biology of the important species listed above, and on others, was available from local research, published especially in the Indian Journal of Fisheries (see Weber, 19), and is being drawn upon in the current analysis.
The climatic regime with its strongly contrasted monsoon periods (feature 2) is a major determinant of the periodicity and intensity of biotic processes in this region, and the incidence of strong winds, especially near the middle of the SW Monsoon, had to be allowed for in deciding upon the timing of the cruises. Of immediate relevance to the survey was the probability that the distribution of species of interest would be found to be related in various ways to this regime and that, insofar as the timing of each monsoon and the intensity of its winds might vary so it might be found that the distribution of some species varied, even markedly. These considerations had a bearing on search tactics and, as is discussed later in this report, have a bearing on an interpretation of the acoustic evidence on the distribution and abundance of the stocks.
Of longer range importance is the role of the climatic regime, and of variations in that regime, especially through its generation of upwelling, in the primary and secondary production processes, (features 3 and 4) and through them in determining the abundance of the species of interest, both at their average levels and at other levels they take in particular seasons and from year to year. The survey programme could not include an analysis of the relationship in which the stocks stand to their habitat or, more particularly of the role of the climatic regime, but in the interpretation of results some assumptions must be made as to the relations that prevail in that relationship.
In particular, it was to be supposed that major concentrations of some stocks would be found to be associated in some way with the upwelling sites, and that any differences of abundance of stocks which might be found between sectors < the surveyed region would be connected with differences of primary production. Cushing (1971) and Alverson (1971, a) discussed the distribution of productivity of the Indian Ocean in relation to resource survey priorities Observations of primary production from the Indian Ocean Expedition had shown very high production in the western part of the Arabian Sea during the SW Monsoon period extending far (500 miles) seaward. Pronounced upwelling along the coasts of Somalia and Arabia during the SW Monsoon (May through September) nourishes high primary production in this sector and during the NE Monsoon a narrow coastal zone of fairly high production is maintained along the coast of Kenya, Somalia and in the Gulf of Aden. Unfortunately, no observations of basic production in the north-eastern part of the Arabian Sea had been reported, but estimates of plankton biomass, from measurement of displacement volume, had shown high values off the Malabar coast and confirmed that production was high fairly far offshore in the western Arabian Sea. That tertiary production in general followed the same pattern had been demonstrated by the distribution of fish eggs and larvae (Nellen 1973); again, high concentrations of these stages seemed to extend far seaward in the western part of the ocean, but no information on species distribution was available. It was not unlikely that the distribution of pelagic coastal species would be found to be continuous from the Gulf of Oman along the Pakistan coast to the productive zone off the Malabar coast, and these sectors were also expected to harbour considerable resources of pelagic fish.
Finally, several particular oceanographic features were noted which could affect the acoustic observations and fishing operations and whose effects had perhaps to be taken into account in interpreting the results. The entire area is tropical with strong heating from radiation, and high evaporation, resulting in high temperature and high salinity in the surface layer. There is a bimodal temperature cycle nearly within the whole area, with maxima in April-May and in October. The summer decrease is explained as a result of wind intensity.
The surface currents change with the monsoon. During the NE Monsoon there is a drift, mainly westward, but modified by the direction of coast lines, particularly towards southwest along the Somali coast. The current system is more regular and also stronger during the SW Monsoon period, at that time forming an eastbound drift also modified by the direction of coast lines, as a northeastward rather strong and stable current along the Somali and Arabian coasts and a southeastern flow along the Indian coast. Within the central region there is an eastward flow.
The westward flow during the NW Monsoon is rather weak. It is strongest along the western coasts particularly south of 6°N where it exceeds 100 cm/s. During the SW Monsoon the currents are stronger. The northeast bound current outside the Somali coast is reported to be between 2 and 5 knots with maximum in July. In the Gulf of Aden there is a weak flow into the Gulf during the NE Monsoon and a flow out during the SW Monsoon. Along the Indian coast there is a weak and variable mostly northward current during the NE Monsoon and a stronger southward flow during the SW Monsoon.
The vertical temperature profile show a strong decrease down to about 200 m thereunder a much slower decrease with depth. Wyrtki (1971) has given some vertical curves which show temperature declines of up to 15-18°C from the surface down to 200 metres.
Of great importance is also the oxygen minimum at intermediate depths with values mostly between 0 and 1 ml/L. As a consequence of this it was expected that fish would be located generally in upper layers.
