Journal of the Marine Biological Association of India

Volume 16 Issue 1

Aspects ecologiques due plancton et du micronecton dans l'est de l'ocean Indien

M. Legand, P. Bourret, J. A. Gueredrat, A. Micheal, R. Repelin and C. Roger

The CSIRO Division of Fisheries and Oceanography undertook a series of six cruises at two-months intervals, along the 110°E and between 32° and9°S, of which the principal aim was to assess the effects of changes in the hydrological environment on main biological components. The present paper deals with data from 93 hauls with a 5 ft. pelagic Isaacs Kidd midwater trawl (mesh width of the cod-end 0.366 mm), and represent an effort to combine partial results, already published by the Centre ORSTOM of Noumea, into a coherent whole.

A quantitative analysis was carried out when organisms from the samples had been separated into four categories: plankton, macroplankton, micronekton and gelatinous organisms. Results display succession in time of peak abundances of those trophic levels originating with the June-July phytoplankton spring. Analyses of diflFerent groups on a species level show that distributions of certain organisms and characteristics of water masses and surface currents as evidenced by Rochford (1969), in that region are related.

The Stomatopod larvae and the epipelagic Copepods apparently follow the movements of water masses of the South Equatorial Current and the West Australian Current which are the more noticeable features of the surface (0-50 m) circulation. Most of the Copepods, Phronimidae, and some Fishes Uke Vinciguenia nimbaria appear to be distributed in relation with subsurface (100-200 m) low oxygen tropical and subtropical waters. Two species of deep-water Copepods supersede each other at \1°S which is an approximative latitudinal limit for two subsurface (200-500 m) waters. The distribution of Euphausiid species according to the different regions of the Temperature-Oxygen diagram shows their relationships with the hydrological properties along the 110° E meridian.

In some instances no clearcut connection with the upper 500 m waters could be established : such species probably undertake dirunal migrations of a large amplitude and their distributions could be under the influence of deeper water.

The seasonal fluctuations of abundance are often related to the reproductive cycles of the different species and different somewhat in the North and the South of the prospected zone. In the northern part of that zone, the main reproductive period extends from September to December and is sometimes followed by a secondary peak of reproduction; however some groups may have more than two maxima of sexual activity and some may even' reproduce all the year round. In the south there is only one maximum which frequently occurred later than in the north.

That type of survey could afford some evidence on latitudinal distributions, since one must keep in mind that it is difficult to assign the observed variations to fluctuations of the same population rather than to possible migrations across the merdian of various communities drifting with east-west surface and subsurface currents.

Date : 30-04-1974