Contribution of the natural biota associated with substrates to the nutritional requirements of the post-larval shrimp, Penaeus esculentus (Haswell), inhigh-density rearing systems

Abstract

The contribution of epiphytes associated with physical substrates to the nutritional requirements of post-larval shrimp, Penaeus esculentus Haswell, was determined in high-density rearing systems (3000, 6000 and 11 000 m⁻³). Stable isotope signatures of epiphytes on polyethylene mesh substrate, AquaMats™ and tank walls were compared with shrimp signatures. Two methods were used: the determination of carbon and nitrogen natural abundance ratios; and ¹⁵N-nitrogen enrichment ratios after the addition of ¹⁵N-ammonium to tanks. Using the natural abundance technique and a simple mixing model, epiphytes were found to contribute substantially to the carbon requirements of post-larval shrimp (39–53%). This was despite the addition of formulated feed at satiation levels. There was no indication of a reduced contribution of carbon from epiphytes to shrimp nutrition at higher shrimp densities. The lack of a difference in the ¹⁵N/¹⁴N ratios of the two food sources meant that mixing models could not be used to calculate the contribution of nitrogen from epiphytes vs. artificial feed to shrimp nutrition. Using the ¹⁵N-nitrogenenrichment method, the amount of nitrogen contributed by epiphytes to shrimp nutrition over 24 h could be determined. This method showed that nitrogen from epiphytes was assimilated by shrimp. ¹⁵N-enrichment methods provided a more accurate alternative to natural abundance techniques, particularly when the stable isotope signals ofthe food sources are similar. This experiment hasshown the benefits in providing substrates for P.esculentus in high-density rearing systems to provide an additional food source for shrimp.