Food sources of the sergestid crustacean, Acetes sibogae, in shrimp ponds

A combination of stable isotope measurements and gut contents analysis was used to determine the major food sources of the sergestid crustacean Acetes sibogae, in commercial shrimp ponds at two farms in southeast Queensland, Australia. Slight differences were observed between farms but overall the results were consistent. Although gut contents analysis gave a good indication of the range and temporal occurrence of food items consumed by Acetes, it was difficult to ascertain the contribution each item made to the diet. This was mainly due to the large proportion of unidentifiable material in the guts. All specimens examined contained unidentifiable material and about half the Acetes from both farms contained nothing identifiable. This unidentifiable material may be the result of processing by the Acetes gastric mill or the consumption of detritus, sediment or processed material from shrimp pellets. Only resilient items such as crustacean remains, diatoms and tinntinnids were commonly identified from the guts, and although present over the majority of the sampling period, FOCs were never greater than 25%.Stable isotope signals were measured for Acetes and likely food sources including pelleted shrimp feed, zooplankton and macroalgae. The pattern of changes in isotopic signals of Acetes across the season showed that zooplankton was a primary food source. Changes in the signals of zooplankton were reflected by changes in Acetes, but the changes in Acetes signal were less pronounced. At both farms, Acetes were more enriched in 13C and 15N (− 15‰ to − 20‰ and 12‰ to 13.8‰) than the zooplankton (− 18.9‰ to − 23.7‰ and 5‰ to 13.1‰), during the whole season. The absolute difference between the δ13C values of Acetes and zooplankton were more consistent than for δ15N, but both were greater than might be expected based on fractionation over a single trophic level. Furthermore, laboratory feeding trials showed that fractionation could not explain the greater than expected enrichment of the Acetes signal compared to that measured for zooplankton in the ponds. This, together with evidence from gut content analysis, showed that a food source other than zooplankton must also be important to Acetes. Macroalgae are the most likely additional source, although some minor contribution of pellets or microalgae cannot be ruled out entirely. There was no evidence from either gut contents or stable isotope signatures of dramatic dietary changes for Acetes either through a season or as they grew. It would appear unlikely that Acetes would have a great effect on shrimp production in ponds unless they were extremely abundant early in the season when the postlarvae are also feeding on zooplankton.