Investigations on the Troph~c Position of a Group o/ Pelagi~ Organisms (Euphausiacea). I. Trophic Levels o~ ~pecies
Trophio relationships are the determining factor of biologieal equilibrium, as existence and abundance of any population depend on the ratio nutrition-predation. This paper is the first of a series devoted to the study of the trophie position of euphausiid crustaceans in the equatorial and south-tropical Pacific Ocean; it defines the role of a pelagic group in its biotope, suggests methods in trophie studies, and shows that analytical research on limited subjects is a reliable way to reveal more general features applying to the whole pelagic world. Until now, the literature dealing with nutrition of euph~usiids in most cases has provided only qualitative lists of items found in the stomachs; the drawbacks of this kind of investigation are that only a small part of the s~omaeh content is recognizable in this way and, moreover, that the remains identified belong only to the food consumed which had the most resistant structures. In order to study the trophic levels of species, it has been necessary to take into account the whole stomach content, to establish if its origin was plant or animal or both. ]~inoenlar examination of the dissected stomachs made it possible to discriminate between phyto-and zooplankton. More than 5000 specimens have been examined in this way, each stomach being named Y if more than 80 % of its content is phy~oplankton, A if this same percentage is animal food, VA ff phyto-and zooplankton are of comparable importance; the trophie level of each species has been defined as the percentage Z [A + (VA/2)], which measures the part of animal materi~l among the total food. Although zooplankton appears on the whole to be more important than phytoplankton as food for tropical euphausiids, all the trophic levels are observed among the 16 species studied, from phytephagous to strictly carnivorous.