C3 and C4 source incorporation in Amazonian fish: contribution to dietary ecology studies

Abstract

Tambaqui ( Colossoma macropomum ), a native Amazonian species, is one of the principal fish harvested in the region and is of great economical importance in the Central Amazon. In recent years, however, it has suffered a decrease in fish landings, most likely due to overfishing (lsAAc & RuFINO 1996). The primary autotrophic energy sources for tambaqui in its natural habitats have been investigated using the isotopic method, allowing for new perspectives regarding the trophic ecology of this species (FoRSBERG et al. 1993, ARAÚJO-LIMA et al. 1998, BENEDITO-CECILIO et al. 2000, ÜLIVEIRA et al. 2006). Tambaqui is an omnivorous fish species with a diet composed of a variety of food items originating from c4 and c3 plant sources, including phytoplankton, whose importance to maintaining the biomass of fish stocks varies in relation to the hydrological cycle (ÜLIVEIRA et al. 2006). The results of previous studies looking at the autotrophic energy sources o f tambaqui were interpreted from isotopic ecology research of animals from temperate ecosystems that utilize aquatic habitats with different characteristics and complexity than that ofthe Amazon. Until now, no laboratory study has ever been done with tambaqui to evaluate possible differences in the isotopic values present in its tissues due to physiological effects, and specifically with respect to diet changes (HoBSON et al. 1993). Because the carbon in animal tissues is constantly being renewed, it is of fundamental importance to establish the timeline necessary for incorporation and replacement of varying carbon sources in the tissues of any given species. The period in which the ratio of C/C in the tissues reflects the isotopic value of a diet depends, in part, on the animal 's rate of movement (HoBSON & CLARK 1992). The isotopic change, defined as isotopic tumover, is due to the growth rate and metabo1ic replacement within the tissues, which is associated with diet change (MAcAvov et al. 2001). A fast or slow tumover can be characteristic o f the animal 's general growth rate or the speed of metabolic replacement within individual tissues (FRY & ARNOLD 1982, HESSLEIN et al. 1993). For tambaqui, the isotopic tumover rate in muscle tissue or how different carbon sources are distributed and stored in this type of tissue is unknown. To try and clarify these questions, the present work investigated the incorporation dynamics of carbon (C) and dietary nitrogen (N) in the muscle tissue of tambaqui fingerlings through calculation of a complete e and N tumover of C3 and C4 sources, which ultimately contributes to a better understanding of the importance of these sources in maintaining tambaqui stocks.