Integrating community structure and stable isotope analysis to assess a heavily exploited coastal marine ecosystem off Central Vietnam

We examined the community and trophic structure of the heavily exploited benthic-demersal community off Phan Thiết Bay, Central Vietnam. Using cluster analysis and non-metric multidimensional scaling (NMS) of catch data from a trawl survey, we examined the nekton community structure pertaining to bottom substrate type and depth. For dominant fish and invertebrate taxa we applied stable isotope analysis, using δ15N to examine trophic level (TL) and size-specific ontogenetic shifts, and δ13C as a measure of contribution by benthic secondary production. Based on trawl data small fishes and cephalopods were the numerically and biomass-dominant taxa. Community structure analysis showed many of the sample sites shared the same species composition, but that there was significant heterogeneity related to substrate types of sand and gravelly muddy sand. Results from δ15N showed 70% of the species were between TL 3.3 and 3.8, with no species indicating true piscivory (TL 4); highest TL was from the squid Loligo japonica (TL 3.8). Size-specific δ15N-based trophic shift was expressed in taxa when the proportional size range (ΔLmax) obtained for analysis was at least 40% of the largest reported size for the species (Lmax). From δ13C, nekton expressed between 35 and 77% dependence on benthic secondary production. Evidence of over-exploitation from our study included the relatively low TL's of dominant taxa, small size distribution of specimens collected, and the dominance of taxa with very high growth rates such as cephalopods (juvenile Octopus sp. and cuttlefish) and small fishes. The effectiveness of stable isotopes as an indicator of over-exploitation is discussed.

► We assessed community and trophic structure of exploited benthic-demersal nekton of Vietnam. ► Community structure was related to sediment type in some taxa. ► Nitrogen stable isotopes indicated all taxa were below trophic level 3.8. ► The level of benthic secondary production was assessed using δ13C. ► Biomass and trophic results indicated a heavily exploited food web.