Fisheries Research 110 (2011) 268–276 Contents lists available at ScienceDirect Fisheries Research journal homepage: www.elsevier.com/locate/fishres Integrating community structure and stable isotope analysis to assess a heavily exploited coastal marine ecosystem off Central Vietnam Nguyen Thanh Nam a , Todd William Miller b,∗ , Nguyen Xuan Huan a , Vu Trung Tang a , Koji Omori b a b Faculty of Biology, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai Street, Thanh Xuan District, Hanoi, Viet Nam Global Center of Excellence, Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan a r t i c l e i n f o Article history: Received 15 February 2011 Received in revised form 18 April 2011 Accepted 19 April 2011 Keywords: Community structure Vietnam Fisheries Trophic structure Stable isotopes Benthic–pelagic coupling Food web Phan Thieˆ´ t Shelf Marine a b s t r a c t We examined the community and trophic structure of the heavily exploited benthic-demersal community off Phan Thieˆ´ t 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 ␦15 N to examine trophic level (TL) and size-specific ontogenetic shifts, and ␦13 C 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 ␦15 N 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 ␦15 N-based trophic shift was expressed in taxa when the proportional size range ( Lmax ) obtained for analysis was at least 40% of the largest reported size for the species (Lmax ) From ␦13 C, 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 © 2011 Elsevier B.V All rights reserved Introduction About 65% of Vietnam’s fisheries production is derived from its coastal waters 0.10) and between each other (p = 0.36) Using the Placopecten-based TL estimates, of the 19 species of fish and invertebrates analyzed, 15 were between TL 3.0 and 3.8; the fish Inegocia japonica was the only fish to exhibit a TL below 3.0 (mean 2.8), and the overall highest was the squid L japonica (TL 3.8), with highest Osteichthyes being T myops and C puncticeps both at TL 3.7 (Fig 3) Of 19 taxa analyzed for size-specific ontogenetic shifts in ␦15 N, nine showed a significant positive relationship Fish taxa expressing this trend had a greater proportional size range of the Lmax measured (mean and maximum Lmax = 52 and 40%, respectively) compared to those that failed to show a trend (mean Lmax = 33%) With the exception of pandalid shrimp and the turbine snail, all taxa which displayed significant size-specific shifts in ␦15 N also expressed shifts in ␦13 C (Table 4) The relationship between ␦13 C and ␦15 N was significant in 11 of the 19 taxa (Table 4); of these, with the exception of shrimp, all had significant ontogenetic shifts in ␦15 N Table Summary results from stable isotope analysis and estimated trophic positions of fish and invertebrate species off Phan Thieˆ´ t Bay (Vietnam) Trophic position was based on the mean ␦15 N of Placopecten sp (placo) as trophic position 2, with subsequent trophic positions every 3.4‰ increase in ␦15 N Zooplankton (zoo) values are from Loick et al (2007) Species abbreviations are in Table Taxa (Figs and label) Invertebrates Placopecten sp (plco) Argopecten sp (argo) Octopus sp., juv (octo) Cuttlefish (cutt) Phalium glaucum (gast) Pandalid shrimp (shmp) Loligo japonica (lsqd) Fishes Inegocia japonica (ineg) Paramonacanthus choirocephalus (pcho) Parapercis alboguttata (palb) Callionymus filamentosus (call) Nemipterus furcosus (nemi) Priacanthus macracanthus (pria) Upeneus sundaicus (upen) Leiognathus berbis (leio) Pseudorhombus arsius (pseu) Pristotis obtusirostris (prist) Dysomma sp (dyso) Apogon kallopterus (apog) Cynoglossus puncticeps (cyn) Trachinocephalus myops (trac) n ␦15 N Ave (SD) ␦13 C Ave (SD) C:N Ave (SD) Length Ave (cm) TL 30 41 13 30 13 49 10 4.7 (1.4) 6.4 (0.9) 7.3 (0.6) 8.3 (0.5) 8.9 (1.6) 9.7 (2.4) 11.1(0.7) −18.3 (1.1) −14.9 (1.6) −15.2 (1.6) −14.7 (0.3) −12.7 (1.7) −16.2 (0.9) −18.0 (0.4) 3.3 (0.1) 3.7 (0.3) 3.5 (0.1) 3.7 (0.1) 3.5 (0.1) 3.3 (0.1) 3.4 (