Quantification of waste feed and fish faeces in sediments beneath yellowtail pens and possibility to reduce waste loading by co-culturing with sea cucumbers: an isotopic study

摘要

To evaluate environmental impacts of yellowtail culture and to examine the efficiency of the sea cucumber Apostichopus japonicus in reduction of waste loading, surface sediments were sampled and A. japonicus were cultured at a yellowtail farm in Owase Bay, Japan. Waste feed- and faeces-derived organic matter (WOM, FOM) in sedimentary organic matter (SOM) was estimated based on the C-13 and N-15 of the fish feed (-21.7 parts per thousand, 9.2 parts per thousand), yellowtail faeces (-20.6 parts per thousand, 6.2 parts per thousand) and SOM reference (-24.4 parts per thousand, 4.4 parts per thousand). Small WOM (0-22% in SOM) but substantial FOM (28-61%) loadings in the fish farm area and high sulphide concentrations in the sediments (1.0 mg g(-1)) suggest that reduction in the fish stocks or mitigation of the faeces should be considered. A. japonicus juveniles were cultured in three cages deployed below a pen and growth was assessed after three different periods (62, 107, 181days). A. japonicus grew well during the first 107days (daily specific growth rate, 3.7%) and their survivorship was high (80-90%). Growth ceased after 107days, probably due to fouling on the cages. The C-13 and N-15 of their hypothetical diet (-19.7 parts per thousand, 5.5 parts per thousand) were close to the FOM values, suggesting assimilation of FOM.