Enhanced silver carp to bighead carp ratios provide greater regulatory benefits for water quality in shallow subtropical lakes

Abstract

Nontraditional biomanipulations using filter-feeding silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) represent a promising strategy to mitigate the severity of lake eutrophication problems. However, the optimal stocking ratios between these two species remain inadequately determined. This study conducted a controlled mesocosm experiment to investigate how varying silver carp (L) to bighead carp (Y) stocking ratios-L/Y (0:4), L/Y (1:3), L/Y (2:2), L/Y (3:1), and L/Y (4:0)-influence water quality variables and plankton community structure. Results demonstrated that treatments with higher silver carp to lower bighead carp ratios [L/Y (2:2), L/Y (3:1), L/Y (4:0)] led to the realization of improved water quality conditions compared to treatments dominated by bighead carp [L/Y (0:4), L/Y (1:3)]. The L/Y (3:1) treatment exhibited optimal performance, displaying the lowest concentrations of total nitrogen, total phosphorus, ammonia nitrogen, chemical oxygen demand, and trophic level index values. Both phytoplankton and zooplankton abundance and biomass were significantly reduced in all treatments introducing filter-feeding fish compared to the control treatment without filter-feeding fish introduction. The ratio of zooplankton to phytoplankton biomass reached maximum values in control treatments, while the ratio of chlorophyll a to total nitrogen was minimized. Gut content analyses revealed that silver carp demonstrated superior filtration capacity for phytoplankton, whereas bighead carp preferentially consumed zooplankton. Structural equation modeling corroborated that silver carp exerted statistically distinct negative effects on phytoplankton, while bighead carp primarily impacted zooplankton communities. These findings have important implications for lake biomanipulation programs, indicating that filter-feeding fish stocking effectively reduces algal biomass and improves water quality, with optimal regulatory effects achieved under a 3:1 silver carp to bighead carp ratio.

Keywords: Bighead carp; Biomanipulation; Phytoplankton; Silver carp; Stocking ratios; water quality.