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Despite being the most dominant and widespread small fish species in the lakes along the middle and lower reaches of the Yangtze River basin, Toxabramis swinhonis has been paid little attention by fisheries scientists and little is known about its population characteristics. For this reason, we estimated age, growth, mortality and recruitment of this species based on three shallow lakes, Biandantang Lake, Shengjin Lake and Kuilei Lake (BDT, SJH and KLH, respectively) in this region. A total of 13,585 (8,818 in BDT, 2,207 in SJH and 2,560 in KLH) individuals were collected during monthly sampling from July 2016 to September 2017. The results revealed that the age structures of T. swinhonis consisted of four age groups (0+–3+), with 0+–1+ year old fish comprising more than 98% of the samples. Allometric growth patterns were displayed by fish from all sampling sites and the von Bertalanffy growth functions estimated were Lt = 173.25 (1 – e−1.20 (t + 1.09)): BDT; Lt = 162.75 (1 – e−1.20 (t + 1.08)): SJH and Lt = 215.25 (1 – e−1.20 (t + 1.12)): KLH, respectively. The rates of total mortality (Z), natural mortality (M) and fishing mortality (F) at BDT, SJH and KLH were computed as 5.82, 5.50 and 4.55 year−1; 1.89, 1.87 and 1.75 year−1; 3.93, 3.63 and 2.80 year−1, respectively. Meanwhile, growth performance indices (φ′) were 0.68 (in BDT), 0.66 (in SJH) and 0.62 (in KLH), which indicated that T. swinhonis were overfished slightly in all study areas. Area-specific recruitment patterns were similar to each other, displaying evidence of batch spawning, with major peaks in April and August, accounting for 92.21% (BDT), 88.21% (SJH) and 88.73% (KLH) of total recruitment, respectively. These results showed that brief generation-time, fast growth rate, relatively high natural mortality rate and strong reproductive capacity (r-strategies) are reasons why this species became the most dominant species in many lakes of China.

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The reproductive biology of Toxabramis swinhonis, one of the most abundant bycatch species in freshwater fishery in China, was first reported based on 783 specimens collected in Biandantang Lake, central China from October 2016 to September 2017. The overall sex ratio (female/male) was 1.03:1, not significantly different from the expected value 1:1. Nevertheless, sex ratio varied with seasons: 1.32:1 in the early stage of the spawning season (March–April), while 0.46:1 in the later stage (July–August). The monthly succession of the average gonadosomatic index (GSI) and percentages of mature individuals co-indicated that T. swinhonis was a spring-summer (March–August) spawner. Further, the bimodal distribution of the size of eggs from one gravid ovary and histological analysis of mature gonads suggested that T. swinhonis was a multiple spawner with indeterminate fecundity and high degree of spawning asynchronicity. The sizes and ages at first maturity for females and males were 84.47 and 81.86 mm, and 1.62 and 1.56 yr, respectively. The batch fecundity (FB) of this species varied in a wide range from 2006 to 73592 eggs per fish with a mean value of 26487.15 ± 2675.61 (S.E.) eggs and increased with the increasing of the gonad weight (WG), eviscerated weight (WE), total length (LT) and age (A). Overall, T. swinhonis in Biandantang (BDT) Lake is characterized by high fecundity and prolonged spawning season, resulting in easy population explosion. Thus, to remove T. swinhonis moderately or release predator fish discreetly to control its population is proposed considering its reproductive characteristics.