Background: The red swamp crayfish, Procambarus clarkii (Girard, 1852), is one of the most promising freshwater species for aquaculture in China. Understanding its reproductive pattern and population dynamics is crucial for sustainable management, but there is currently a lack of fundamental knowledge of commercial P. clarkii populations. Therefore, the purpose of this study was to investigate the reproductive pattern and population dynamics of commercial P. clarkii throughout the yearly cycle. Methods: A total of 2,051 crayfish (1,012 females and 1,039 males) were collected from March 2016 to February 2017 in the area of Selection and Reproduction Center of Crayfish. The reproductive pattern was evaluated by the gonadosomatic index (GSI), hepatosomatic index (HSI), ovarian development and fecundity. Growth, mortality rates and exploitation rate were estimated by electronic length frequency analysis by R package “TropFishR” based on data of cephalothorax length (CTL). Results: Our results demonstrated that spawning activities of P. clarkii took place from September to November, with a mean fecundity of 429 ± 9 eggs per female. There were two recruitments yearly, a major one from October to November and a minor one from March to May. With respect to population growth, five growth cohorts were identified for both females and males. Crayfish grew faster but attained smaller asymptotic maximum CTL as indicated by higher growth coefficient (K), growth parameter index (Ø′) and lower asymptotic CTL (L_inf). The estimates of total mortality rate (Z), natural mortality rate (M) and fishing mortality rate (F) were 1.93, 1.02, 0.91 year⁻¹ for females and 2.32, 0.93, 1.39 year⁻¹ for males, which showed that the mortality of male crayfish was mainly caused by fishing. The estimates of exploitation rate (E) indicated that male crayfish were overexploited, with the values of 0.47 and 0.60 year⁻¹ for females and males, respectively. Discussion: P. clarkii spawned from September to November while two recruitments were observed yearly. We inferred that some eggs, prevented from hatching by low water temperature in winter, were more likely to hatch in the next spring. Moreover, the fishing mortality rate was relatively high for males, which might be related to the males-directed selection during the reproductive period. The higher values of exploitation rate in our study confirmed that males P. clarkii were overexploited and were under high fishing pressure. We thus suggest reducing fishing intensity on immature crayfish and avoid sex selection during the reproductive period to improve the overall sustainability of commercial P. clarkii populations.

Mandarin fish Siniperca chuatsi (Basilewsky) is a commercially important aquaculture species, mainly cultured in Chinese ponds and only fed live fish. We evaluated the effects of artificial vegetation on consumption and growth of mandarin fish foraging on live prey Cirrhinus mrigala (Hamilton) in outdoor pools. In comparison to the 0% vegetation treatment, mandarin fish captured more prey in all treatments where vegetation was added. However, prey consumption did not vary between 15, 30, 45 and 60% vegetation additions. Weight gain of mandarin fish in the 0% vegetation treatment was significantly lower than those in the four vegetated treatments, with the highest weight gain recorded for the 30% vegetation treatment. Similar differences in specific growth rate were observed. The specific growth rate increased with increased vegetation coverage, but then declined slowly as vegetation coverage continued to increase, with a peak in the 30% vegetation treatment. These results suggest that transplanting submerged vegetation into culture ponds in an effort to improve water quality would not affect or even improve prey consumption and growth of mandarin fish, and that suitable vegetation coverage would range between 30% and 60% in terms of growth rate.

The Danjiangkou Reservoir (DJKR) serves as the water source for the world’s biggest water diversion project, the Middle Route of the South-to-North Water Diversion Project (MR-SNWDP) in China, and this project concerns the water security of tens of millions of people in northern China. Hence, the maintenance of ecosystem health and optimization of management necessitate studies to assess the composition and dynamics of key aquatic living resources. Zooplankton represent a critical component of the reservoir ecosystem and are sensitive to environmental changes and anthropogenic disturbances. In this study, the zooplankton compositions in DJKR were quantified and compared in May, August, and November 2017. Simultaneously, the effects of water trophic states on the zooplankton community structure were analyzed at three levels (overall, taxonomic, and functional groups). A total of 65 zooplankton taxa were recorded, with the taxonomic richness of Rotifera (28 taxa) being the highest among taxonomic groups, which were further classified into 10 functional groups. The community was characterized by low diversity and high evenness. Compared with historical studies, the biomass had increased remarkably, while the abundance showed a decreasing trend in DJKR, and there were more large-bodied zooplankton in this study. The multivariate analysis revealed that zooplankton compositions changed significantly among the three sampling months without distinguishable spatial variations. Moreover, the zooplankton compositions at all three levels correlated significantly with total nitrogen, water transparency, and permanganate index in most situations, as verified by db-RDA and Mantel’s test. However, the contributions of chlorophyll a and total phosphorus were only significant for the LCF group, implying that the bottom-up effects of phytoplankton on zooplankton were weak in DJKR. Therefore, analysis based on functional groups may reflect a more accurate snapshot of the relationships. Our findings will contribute to enriching the long-term fundamental ecological knowledge of the DJKR and the MR-SNWDP, as well as provide key taxonomic information for ecosystem assessment and management.

Physicochemical water quality parameters in typical rice-crayfish integrated systems (RCIS) in China