Eels as natural samplers highlight spatial heterogeneity in energy flow in an estuary

Abstract

The high spatiotemporal heterogeneity makes it difficult to ensure the representation of samples in estuarine studies. Using Japanese eels as natural samplers (n = 881), we revealed the spatial heterogeneity of energy flow among three successive estuarine segments (upper, middle, and lower estuaries) of the Hamana Lake−Miyakoda River system on the Pacific side of central Honshu, Japan. In the upper estuary which serves as the nursery ground for eels, the predominant small eels (body weight <30 g) fed mainly on polychaetes (58.8% IRI [index of relative importance]), mainly Hediste atoka. This polychaete species depends on the detritus food chain driven by terrestrial inputs, and the detritus food chain appears to be predominant in the upper estuary. However, large eels (body weight >150 g) fed directly on terrestrial subsidies in the form of fallen insects (49.0% IRI) and earthworms (20.2% IRI), indicating that the detritus food chain does not directly affect the diet of large eels. Terrestrial subsidies in the form of living organisms do not contribute to the diet of eels in the adjacent middle estuary, which is the main growth habitat for eels within an estuary. However, small eels depend on terrestrial subsidies by feeding on oysters (71.6% IRI), which assimilate terrestrial particulate organic matter. The predominant medium eels were highly dependent on the high diversity and biomass of prey groups, as indicated by the high % IRI values of several groups, such as infaunal bivalves (31.7% IRI), oysters (19.0% IRI), amphipods (22.6% IRI), and polychaetes (16.4% IRI). Eelgrass beds usually serve as shelters and foraging areas for medium and large eels in the lower estuary. Despite the high species diversity around seagrass meadows, the eels concentrated on infaunal bivalves (85.4% IRI) with high biomass, predominantly Anadara sp. In addition to being tools for monitoring fauna, eels as natural samplers provide insights into the energy flow in ecosystems.