Primary carbon sources for juvenile penaeid shrimps in a mangrove-fringed bay of Inhaca island, Mozambique

Abstract

A study to estimate the relative importance of mangrove primary carbon and nitrogen sources to five commercial penaeid shrimps species was done at Saco da Inhaca, a non-estuarine mangrove-fringed bay on Inhaca Island, southern Mozambique. Carbon and nitrogen stable isotope ratios were determined in a variety of primary producers (mangroves, epiphytes, phytoplankton and seagrasses), sediments and in five penaeid shrimp species (Penaeus (Fenneropenaeus) indicus, P. japonicus, P. semisulcatus, Metapenaeus monoceros and M. stebbingi), collected within the bay in different habitats and during two different periods. The penaeid shrimps showed ∂ 13C values ranging from –13 to –19 ‰, (average of –15.6 ± 0.4 ‰, n=19) which is highly enriched compared to the mean value for mangrove leaves (average - 27.6± 3.6 ‰, n=3) which varied from –20 to –32‰. The results shows that some shrimps may derive their carbon either from detritus, plankton remains or from benthic organisms. Overall, the carbon isotopic signal shifted as the shrimps got bigger, suggesting a change of diet with growth. No significant differences were found between ∂ 15N isotope values among the shrimps studied, indicating that they may belong in the same trophic position (except P. semisulcatus, which occupies a higher level). There is some evidence that sampling period influences the carbon isotope ratios for sediment and shrimps. Keywords: stable isotopes, penaied shrimps, mangrove bay, Inhaca island, Mozambique West Indian Ocean Journal of Marine Science Vol. 3 (2) 2004: pp. 151-162