Analysis of fish diet from the Murray Estuary using C, N, and S stable isotopes - a preliminary assessment

[摘要] Executive Summary: The Coorong food web is not well understood.Fish populations in this region have been declining in recent years and a decreased food availability brought by low Murray-Darling River inflows is thought to be partially responsible.A stable isotope analysis was undertaken to gain some insights into the current structure of the food web. It was hypothesised that there is a pelagic (based primarily on zooplankon) and a benthic component to the Coorong food web (based on amphipods, polychaetes and a range of other invertebrates associated with the substrate).The goals of the study were to: 1) Determine if there is a distinct isotopic signature between the pelagic and the benthic components of the food web, and if so 2) quantify the proportion of fish diet derived from each component.These questions were investigated by collecting a range of aquatic plant, invertebrate, and fish species in the North Lagoon of the Coorong (between Pelican Point and Mark Point) in March 2005 and analysing them for C, N and S stable isotope ratios. Organisms collected The abundance and diversity of invertebrates was low at the time of study and some of the targeted invertebrate (zooplankton, snails, isopods) and fish taxa (black bream) could not be collected. However, a range of organisms expected to represent the isotopic signature of the pelagic and benthic components of the food web were collected.The potential indicators of the signature of the pelagic component were filter-feeding polychaetes (Ficopomatus sp.) and bivalves (Notospisula sp.).A range of other invertebrates at the primary and secondary consumer levels were obtained from the benthic zone, including amphipods (Melita sp.), polychaetes (Phylodoce sp.) and crabs (Paragrapsis sp.).Both prey fish (small mouthed hardyheads, gobies and juvenile yellow-eye mullet), an omnivorous fish (adult yellow-eye mulletAldrichetta forsteri), and a predatory fish (mullowayArgyrosomus japonicus) were also collected. C and S isotopic trends There was a difference in d13C between filter-feeding organisms (19.2 to 18.6‰) and the other invertebrates (17.4 to 14.5‰) collected from the benthic zone.However, invertebrates had a fairly narrow range in d34S (13.5 to 16.4‰), more depleted than the signature expected from the use of marine SO42 (21‰).The difference in d13C signatures between filter-feeders and the other invertebrates suggested that organic matter produced in the pelagic and benthic zones have different isotopic signatures. There was a greater range in d13C and d34S signatures in fish than in invertebrates. Gobies and 1+ year-old hardyheads had d13C within the range found in benthic invertebrates,\ while mullet and 0+ year-old hardyheads were more enriched than all the invertebrates sampled (14.1 to 10.7‰).Mulloway had a large range in d13C (17.7 to 13.1‰), that partially overlapped with the benthic invertebrate values. However, there was a marked contrast in d34S signatures between fish and invertebrates.All fish were more 34S depleted than the invertebrates, especially juvenile mullet (2.5 to 5.1‰), adult mullet (6.0 to 8.4‰), and mulloway (6.7 to 9.5‰).N isotopic trends The enrichment in d15N between predator and prey (~3.4‰) can be used to estimate the number of trophic levels in a food web.At the study site, d15N values ranged from 4.0 to 8.1‰ in primary consumers to values up to 15.8‰ in mulloway.However, the trends in d15N were obscured by negative relationships between d13C and d15N within many taxa.Thus, it was not possible to accurately estimate the number of trophic levels in the food web because the d15N signature of primary consumers was not constant. The baseline d15N signature could not be standardised using d13C because of the limited overlap in d13C signature between fish and invertebrates. Conclusions and Recommendations This preliminary stable isotope analysis could not unambiguously identify fish diet in the Coorong because the isotopic signatures in most fish were outside the ones of any of their prey. A number of possibilities are presented to account for this discrepancy. However, the most probable one is that the isotopic signature of prey at the study area was not representative for the whole ecosystem.It is proposed that variability in the biogeochemical cycling of C, N and S along the Coorong salinity gradient imposes a substantial spatial variability in the isotopic signature of the food web. A number of challenges must be addressed before stable isotope analysis can be used to investigate food web processes in the Coorong.In particular, it will be critical to understand the causes for the shifts in baseline isotopic signature across the ecosystem. Salinity, depth gradients, and shifts in plant communities are among the factors that could influence isotopic signatures in the food web.Due to limitations in the application of the stable isotope analysis approach in the Coorong, it is recommended that the assessment of fish diet in the system be made using a range of techniques.

[发布机构] CSIRO

[学科分类] 地球科学(综合)