Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). VI. Conclusions sur le rôle des euphausiacés dans les circuits trophiques de l'océan Pacifique intertropical

The role of euphausiids in the food webs of the Intertropical Pacific Ocean is defined through analysis of their nutrition, vertical distributions and migrations, and their utilization by pelagic predators. It is suggested that the abundance of the group, the extensive vertical migrations of many species and the fact that feeding takes place mainly in subsurface layers, result in a leading role of euphausiids in energy transfer between different bathymetric levels. For night-time feeding predators, they represent a noticeable food source only in the 0 to 300 m water layer, as 97% of the euphausiid biomass concentrates in this layer at night. In the daytime, only the smaller specimens (chiefly genus Stylocheiron), accounting for 10 to 15% of the whole biomass of the group, remain available for epipelagic (0 to 400 m) predators, larger individuals dwelling deeper. Euphausiids account for 8 to 10% of the food ingested by micronektonic fishes, but the species are not the same for different categories of fishes. Migrating fishes caught by pelagic trawls, more or less connected with the deep scattering layer, feed on migrating species in subsurface layers at night as well as in deeper layers during the daytime, and on non-migrating species inhabiting shallower and intermediate layers. On the other hand, fishes which comprise the prey of large long-line tunas, which are not caught by trawls because they are fast swimmers, feed almost solely on species which remain above 400 m in the daytime. These results suggest a certain degree of independance between the trophic webs which concern, on the one hand, epipelagic ichthyofauna (including tuna), and, on the other hand, migrating and deep-living faunas. Migrating populations are able to feed at night upon subsurface organisms, a part of this resource being then transmitted during the day to the deep-living fauna; but the epipelagic ichthyofauna, with a feeding activity restricted to light hours, has few possibilities to benefit from the migrating or deepliving biomass. Therefore, energy transfers seem to be intense only from subsurface (0 to 400 m) to deeper layers. From a more general point of view, these investigations suggest that, in the pelagic system, vertical distributions and migrations, and feeding rhythms, are the main factors determining the structure of the food webs.