The plankton of the tropical western Indian Ocean as a biomass indirectly supporting surface tunas (yellowfin, Thunnus albacares and skipjack, Katsuwonus pelamis)

The biomass of available forage is a key factor in controlling the abundance and distribution of surface tropical tunas, as they have high energy demands and live in a poor environment. The direct estimate of this forage biomass is not possible with existing techniques. Thus we have investigated the lower link, i.e. the plankton organisms which are the food of fishes preyed upon by tunas. In a previous study, this fraction of the zooplankton has been identified, both by taxa and by size, by analysing the stomach contents of the fishes which are the preys of tunas. In this paper, we use 331 plankton samples from tuna fishing grounds of the tropical Indian ocean, to define the characteristics of the planktonic fraction actually participating in the tuna food-chain. Main results are as follows : (1) Only 15-27% of the total zooplanktonic biomass (> 1 mm) is actually accessible for the fishes preyed upon by surface tunas. (D'après résumé d'auteur