Stocking and Harvesting Patterns Influence Age Structure, Growth, and Body Condition of Bighead Carp Populations From Two Large Subtropical Reservoirs

Abstract

Understanding the density-dependent effects is essential for sustainable fishery management. Stocking and harvesting activities directly influence fish abundance and associated density-dependent responses. In this study, we compared the body size, age structure, growth patterns, and body condition of bighead carp (Hypophthalmichthys nobilis) from two large subtropical reservoirs in China: Shanmei Reservoir (SMR) (n = 161) and Liuxihe Reservoir (LXHR) (n = 257), which have different stocking and harvesting patterns. In SMR, annual stocking and harvesting maintain a lower bighead carp biomass (catch per unit effort, 0.62 ± 0.03 g/m²/24 h), whereas in LXHR, annual stocking without harvesting results in higher fish biomass (0.84 ± 0.01 g/m²/24 h). Plankton biomass and abundance, particularly zooplankton, were significantly higher in SMR than in LXHR. The SMR population exhibited a more stable age structure, faster growth, and better body condition, whereas the LXHR population showed the opposite trends under higher fish density. Both populations exhibited isometric growth and included individuals aged one to five, but dominant age groups differed. The SMR population had a larger inflection age and greater standard length (SMR: 4.7 years and 591 mm; LXHR: 3.5 years and 456 mm). In addition, total mortality, fishing mortality, and exploitation rates were higher in SMR than in LXHR. These findings highlighted the role of stocking and harvesting in shaping fish density and hence inducing density-dependent effects. A balanced fisheries strategy integrating both practices is crucial for sustainable fish population management.