Changes in trophic linkages to shortfin eels (Anguilla australis) since the collapse of submerged macrophytes in Lake Ellesmere, New Zealand

Abstract

Lake Ellesmere (Te Waihora) is a nationally important coastal brackish lake in New Zealand, however degradation in water quality and loss of submerged macrophytes over past decades have raised concerns in regards to the declining status of the lake’s commercial and customary fisheries, predominantly targeted at shortfin eels (Anguilla australis). We investigated foodweb dynamics and trophic linkages to shortfin eels in Lake Ellesmere using a combination of abundance assessments, dietary studies, and stable isotope analyses. Data from our study are compared with historical data sets on benthic invertebrate community composition and shortfin eel diets to trace changes in the trophic linkages to top predators that have occurred since the late 1960s. Stable isotope analyses indicate that the foodweb is predominantly driven by epipelic and phytoplankton derived carbon sources, although it was difficult to discriminate between these two carbon pools because of wind-driven resuspension of lake sediments. Comparison of our survey results with historical data sets indicates a clear shift in benthic biota from being dominated by phytofaunal species such as Potamopyrgus antipodarum (comprising 90% of total invertebrate biomass) during the 1960s, to now being almost entirely comprised of subterranean species such as Chironomus zealandicus and oligochaetes (together comprising 82% of total invertebrate biomass). This shift in benthic communities has resulted in significant changes in the size-specific diet of juvenile shortfin eels ( 30% of total biomass). This shift towards foraging on smaller sediment-dwelling species could have implications for juvenile eel bioenergetics, and may help explain why juvenile shortfin growth rates have significantly decreased in past decades. Juvenile shortfins now appear to switch to foraging on preyfish (mainly common bullies, Gobiomorphus cotidianus) at a smaller size (≈400 mm) than historically recorded (>500 mm). Dietary and stable isotope signatures indicated that small shortfins (100–299 mm) have considerable overlap in trophic position (δ13C = −20.4‰, δ15N = 13.6‰) with common bullies (δ13C = −20.5‰, δ15N = 13.7‰), the dominant fish in Lake Ellesmere (92% of total abundance CPUE), potentially indicating that these two species may directly compete for food resources. These findings again highlighted the importance of C. zealandicus in sustaining the fish populations of the lake.