Distribution and nitrogen isotope ratios of fish scales in surface sediments from the upwelling area off Namibia

Abstract

We investigated the amount and origin of fi sh scales in 20 surface sediment stations from the diatomaceous mud belt on the shelf offshore Namibia (19°-24° S). Most abundant were scales of four different fispecies: Sardine ( Sardinops ocellatus), anchovy (Engraulis capensis), hake (Merluccius capensis/paradoxus), and jack mackerel (Trachurus capensis), albeit in strongly variable amounts. The total number of scales was highest in samples from approximately 70 m water depth. In total scales of sardines dominate the assemblage, which we attribute to both stability of the scale architecture and high abundance of sardine in the coastal upwelling ecosystem. The peak abundance of sardine- and total scales corresponds to an area of high biological productivity northwest off Walfi shbay. The stable nitrogen isotope composition (δ 15 N) in fi sh scales clearly refl ects the trophic level inhabited by the four species. δ 15 N of sardine scales suggests an intermediate position in the food chain between a consumer of fi rst and second order. The average δ 15 N signature of anchovy scales suggests a higher position (second order consumer) in the food web. δ 15 N of hake and jack mackerel indicate that they are in even higher trophic positions.