C-13/C-12 AND N-15/N-14 ISOTOPE RATIOS IN THE SOUTHERN BENGUELA ECOSYSTEM - INDICATORS OF FOOD WEB RELATIONSHIPS AMONG DIFFERENT SIZE-CLASSES OF PLANKTON AND PELAGIC FISH - DIFFERENCES BETWEEN FISH MUSCLE AND BONE-COLLAGEN TISSUES

Measurements of the stable isotope ratios of carbon and nitrogen were obtained for 3 size-fractions of plankton and 2 species of pelagic fish, Engraulis capensis Gilchrist and Etrumeus white-headi Wongratana, from the Benguela current system on the southwest coast of Africa. There was a trend towards enrichment in C-13 and N-15 with increasing plankton size. The data are consistent with the hypothesis that larger plankton feed further up the food web than smaller plankton. Fish muscle and bone collagen tissues tended to be more enriched than the plankton. There was depletion of the heavier isotopes with increasing fish length, in contrast to the opposite tendency with increasing plankton size and towards the trophic position of fish. The fractionation of C-13/C-12 and N-15/N-14 ratios during the formation of muscle tissue was different from that for bone collagen. Bone collagen was richer in C-13 than muscle tissue, whereas muscle was richer than bone collagen in N-15. For both C-13 and N-15, the less enriched tissue displayed the stronger negative correlation with fish length.