Age, growth, and mortality of silver carp Hypophthalmichthys molitrix (Valenciennes, 1844) in the middle and lower reaches of the Pearl River, and implications for management and conservation

This study aimed to determine the age, growth, mortality, and population structure of the economically important cyprinid silver carp Hypophthalmichthys molitrix (Valenciennes, 1844) in the middle and lower reaches of the Pearl River. A total of 297 silver carp were sampled quarterly from the catches of gillnet fishermen, at six sites, between June 2019 and September 2020. Standard length of the specimens ranged from 130 to 585 mm, and body weight ranged from 45.5 to 3930 g. The length–weight relationship parameter b values reached 3.015. Age was determined through examination and measurements of fish scales, and the age composition of the sampled silver carp varied from 0+ to 4+. Fitting the new data to the von Bertalanffy growth model, we obtained an asymptotic size (L∞) of 1107 mm, k of 0.135, and t0 of −0.666 for silver carp in this stretch of the river. The calculated growth performance index ϕ and estimated longevity tmax were 5.22 and 21.56, respectively. The rates of total mortality, natural mortality, and fishing mortality were calculated as 0.4997, 0.1621, and 0.3377, respectively, while the exploitation ratio was evaluated as 0.6757. The overall results confirm overexploitation of this resource in the middle and lower reaches of the Pearl River. It was concluded that this species should be protected from capture until at least 790 mm in standard length, representing an optimal minimum size for capture to benefit conservation of the species and to sustainably develop this valuable fishery.