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The commercially important pink ear emperor fish, Lethrinus lentjan, often occurs as a juvenile in subtropical and tropical interlinked mangrove and seagrass ecosystems, but little is known about its feeding habits and habitat use. Here, we used gut contents and stable isotopic (δ13C and δ15N) ratios to determine temporal changes in food sources and foraging habits of juvenile and sub-adult fish collected in mangrove forests and seagrass beds in the Setiu Lagoon. Gut content examination identified the main food sources as crustaceans, gastropods, bivalves, and annelids. Stable isotope analysis of food sources showed marked differences between the mangroves (δ13C = −26.8 ± 2.0‰; δ15N = 4.3 ± 1.7‰) and the seagrasses (mean ± S.D. δ13C = −20.5 ± 5.5‰; δ15N = 5.8 ± 1.2‰). The isotopic composition of L. lentjan revealed that it mainly utilized seagrass-based food sources. Rainfall and the semi-diurnal tidal regimes may affect the foraging habitats of fish in the lagoon. A significant depletion of 13C related to body size was observed, suggesting that mangroves provided some benefits to sub-adult fish. In contrast, trophic position increased with fish growth, although this increase was negligible. A stable isotope mixing model confirmed that the seagrass bed constituted the main carbon source for the fish, but with an increasing contribution of mangrove prey, such as sesarmid crabs, related to fish growth. This study provided novel information on seasonal variations in feeding areas and flexible habitat use in L. lentjan in the Setiu Lagoon, which will help optimize management strategies for sustainable use and wildlife conservation.

The combined analyses of total mercury (Hg) and stable isotopic ( δ 13 C and δ 15 N) ratios were conducted to describe the food web pathways of dietary Hg, from basal food sources to benthic invertebrates and higher trophic level fish, and to determine if biomagnification of Hg is a persistent process in the food web in a mangrove creek in Setiu Lagoon. The study showed that Hg concentrations were relatively low in mangrove litter and sediment, but elevated gradually in higher trophic level consumers. Based on δ 13 C values, the variation of gastropod Hg concentrations are likely correspond with local dietary sources of Hg in sediments, while variations in bivalve Hg reflect their exposure to low Hg concentrations in the water body. The combination of depleted δ 13 C values and high Hg concentration in gastropods suggest that microbially produced Hg sources in mangrove sediments play an important role in benthic biotransfer pathways. The isotopic compositions of crustaceans demonstrate the importance of feeding behaviour in Hg bioaccumulation. High bioaccumulation of Hg occurred consistently in carnivorous fish species, particularly piscivorous Caranx ignobilis. The enriched δ 13 C of fish species reflects a small contribution of mangrove-derived organic carbon to the fish food web in the mangrove creek, accordingly the fish community may intake dietary sources of Hg via trophic relay or bioadvection, however further studies are needed to elucidate such factors. A positive relationship was found between Hg concentration and trophic level (derived from δ 15 N, trophic magnification factor of 1.5) even at low Hg concentration in the base of the food web, providing evidence for Hg biomagnification in the mangrove food web of Setiu lagoon. Whilst Hg concentrations in fish and commercial crabs did not present a risk for human consumption, the Hg concentration of Caranx ignobilis approached the official permitted level. In the future, there is a need for Hg biomonitoring designed to assess carnivorous fish in order to comprehensively assess the potential effects of human activities and land use around the upper reaches of the Setiu ecosystem.

Seasonal variations in total mercury concentrations [Hg] and trophic transfer through the food web were assessed using stable isotopic tracers for the Setiu Wetlands, Terengganu. The [Hg] measured in surface sediments and biota varied inversely between wet and dry seasons. Increased rainfall and water disturbance during the wet season are suggested as the main factors releasing Hg from surface sediments and enhancing the bioavailability of Hg to biota. The elevated Hg levels associated with the leaf stage suggested that litterfall and atmospheric deposition may be the main Hg inputs into mangrove food webs. The positive relationships between log [Hg] and δ15N provided evidence for Hg biomagnification, however low trophic magnification slopes in both seasons indicated that the ecological risk of Hg in the wetland would be negligible. The [Hg] in fish and commercial crabs were below the permitted limits for human consumption.

In order to understand the metal concentrations in Japanese eel Anguilla japonica, nine elements were analyzed in the livers of different migratory types of eels collected from Tokushima region (south Japan). Migratory types were defined by examining the Sr:Ca ratio in otoliths. The results showed that there were significant differences in V, Cr, Cd, and Pb concentrations among the migratory types. Mature-sea-eels show a higher risk of metal accumulation than other migratory types of eels, and the concentrations of Mn, Cu, and Zn in mature eels were significantly higher than those in immature eels. The study suggests that the eel liver is a valuable bioindicator for trace metals; however, when using the eel as a bioindicator to reveal the pollutants in aquatic systems, life history analysis should be carried out for accurate interpretation of the results.