C. natator samples were collected from bottom trawler catches in the southern Taiwan Strait; the fishing areas largely overlapped with 22.00−24.00° N and 117.30−120.00° E in 1994–1996 (Ye, 1999), and 22.00−24.00° N and 116.30−190.00° E in 2019 (this study).

Monthly sampling of C. natator was conducted from January to April and August to December 2019. Approximately 15–35 kg (one to two baskets) of C. natator samples were randomly collected from at least five bottom trawlers each month.

Carapace width (CW, in 0.1 cm) and body weight (BW, in 0.01 g) were measured for each individual ( Figure 1A ). Based on abdomen morphology, sex was determined ( Figures 1B, C ). Females carrying eggs on the abdominal pleopods were considered fully mature, with three color groups, namely, red, yellow, and dark-gray, indicating embryonic developmental stages from early to late ( Figures 1D, F ) (Sumpton, 1990).

The percentage of females carrying eggs each month, irrespective of egg color, was calculated as follows: the number of females carrying eggs/the total number of females × 100. The spawning season was defined as the months with females carrying eggs, and the spawning peak season was defined as the month(s) with at least 50% of females carrying eggs or matured (Sadovy, 1996). The minimum size (CW) at female maturity was determined by the smallest female collected that carried eggs. The size at 50% female maturity (CW ₅₀) was determined by fitting a logistic regression (probit link) to the percentage of females carrying eggs in 0.5 cm CW size classes using females from the spawning peak month(s).

Female fecundity, defined as the number of eggs carried by a female, was estimated empirically following standard methodologies (Lin et al., 2021a). The absolute fecundity (Fa) was calculated as follows: Fa = W × m/n, where W is the total weight of the entire egg mass for a female, m is the average weight of five egg sub-samples (approximately 0.02 g per each sub-sample), and n is the average number of eggs from the five sub-samples (Johnson et al., 2010; Soundarapandian et al., 2013). The relative fecundity (Fr) was calculated as follows: Fr = Fa/CW and Fr = Fa/BW. Egg diameters (to 1 μm) for all three aforementioned colors were measured using a Leica M165FC fluorescence stereo microscope.

Data from the mid-1990s were exclusively extracted from one publication in Chinese (Ye, 1999). Charybdis natator was collected monthly from July 1994 to June 1996, and the biological data were presented as a 12-month dataset from January to December without annual variation. Sex ratio, CW and BW ranges, and spawning seasons were obtained from the tables directly. Male and female size differences were extracted from figures using the GetData Graph Digitizer. Length (CW)–weight (BW) relationships of males and females and the minimum CW were documented in the main text. CW ₅₀, female fecundity, and egg diameters were not available.

Differences in sex ratio from 1:1 monthly and overall were analyzed using a Chi-square test for both the 1994−1996 and the 2019 datasets. Monthly and overall male and female size differences (CW) for 2019 were analyzed via one-way ANOVA or non-parametric analysis (Mann–Whitney U test). Male/female size comparisons (CW) by month and overall between the two datasets were analyzed via Student’s t-test or non-parametric analysis (Wilcoxon test). The relationships of CW–BW in males and females and Fa−CW were all analyzed via one-way ANCOVA. Statistical analyses were conducted using R (R Core Team, 2020) and Microsoft Excel 2019, setting the statistical significance at p ≤ 0.05.

The overall sex ratio of C. natator changed largely from a male bias in 1994−1996 to a female bias in 2019 ( Table 1 ). The overall male:female ratio was 1:0.76 (N = 1,810, with 1,027 males and 783 females) in 1994−1996 (Ye, 1999), which is significantly different from a 1:1 ratio (χ² = 32.89, df = 1, p < 0.01). In 2019, the overall male:female ratio was 1:1.38 (N = 1,332, with 560 males and 772 females), again, significantly different from a 1:1 ratio (χ² = 33.37, df = 1, p < 0.01). Monthly variations in sex ratios were different between the two datasets; a significant male bias was found in June, July, and December in 1994−1996 (p < 0.01), and a significant female bias was found in February and March (p < 0.01) and October (p < 0.05) in 2019.

Males were larger and heavier than females, both in 1994−1996 (data were not available for statistical tests) and in 2019 (CW, F = 27.49, df = 1330, p < 0.05; BW, F = 45.43, df = 1,330, p < 0.05) ( Table 2 ; Figure 2 ). The average CW and BW of males in 2019 were significantly smaller (t = −22.03, df = 559, p < 0.01) and lighter (t = −28.19, df = 559, p < 0.01) than those in 1994−1996, decreasing by 15.28% in CW and by 43.78% in BW. The same trends were found in females over 25 years; the average CW and BW of females in 2019 were significantly smaller (t = -20.58, df = 771, p < 0.01) and lighter (t = -25.39, df = 771, p < 0.01) than those in 1994−1996, decreasing by 9.69% in CW and by 30.57% in BW. The maximum CW and BW of C. natator in 2019 were smaller and lighter than those in 1994−1996, decreasing by approximately 7.5% in CW and more than 32% in BW ( Table 2 ). Over 25 years, the proportions of large individuals (CW > 10 cm) declined from 32.94% (1994−1996) to 10.36% (2019) for males and from 7.53% (1994−1996) to 2.33% (2019) for females. The dominant size groups declined from 8.0−9.9 cm (46.03%, 1994−1996) to 6.5−8.4 cm (55.44%, 2019) for males and from 7.5−9.9 cm (72.97%, 1994−1996) to 6.0−8.9 cm (84.45%, 2019) for females ( Figure 2 ).

Monthly CW variations of males and females were found in both datasets ( Figure 3 ). The monthly average CW of males and females in 1994−1996 were significantly larger than those in 2019 (p < 0.01), except in February, August, and September ( Figure 3 ). The length (CW)–weight (BW) relationships of males and females declined from 1994−1996 to 2019 ( Table 2 ; Figure 4 ). The BW of C. natator larger than 10.0 cm CW was reduced by approximately 10% (41.37 ± 16.90 g) in males and 5% (23.66 ± 19.10 g) in females over 25 years.

Charybdis natator females carrying eggs were found in most months of the year (except October in 1994−1996 and except August and October in 2019) ( Figure 5 ). One spawning peak season was found for each dataset: March−August in 1994−1996 with a peak in April, and February−April in 2019 with a peak in February ( Figure 5 ). The minimum sizes for females carrying eggs were 6.9 cm CW in 1994−1996 and 6.1 cm CW in 2019. The estimated size at 50% female maturity was 7.7 cm CW in 2019 ( Figure 6 ). The majority size groups for female sexual maturity were 8.0−9.0 cm CW in 1994−1996 (> 50%) and in 2019 (48.94%).

A total of 15 females carrying eggs with 6.9−10.1 cm CW (seven with red eggs, three with yellow eggs, and five with dark-gray eggs) were selected for fecundity and egg size measurements. Fa was between 287,597 and 973,417 eggs (575,349 ± 157,478 N = 15). Fr was between 41,681 and 94,398 eggs cm⁻¹ CW (66,009 ± 12,906, N = 15), and 2,933 and 5,423 eggs g⁻¹ BW (3,769 ± 650, N = 15). A typical power relationship between Fa and CW was found (p < 0.01) ( Figure 7 ). Egg diameters were 283–371 μm (326 ± 16, N = 150), 317 ± 10 μm for red eggs, 320 ± 14 μm for yellow eggs, and 338 ± 14 μm for dark-gray eggs, showing a general increase in egg diameters from early to pre-hatching stage (F = 34.84, df = 147, p < 0.01).

An increasing abundance of one sex usually occurs in specific sex-only and size-selective crustacean fisheries (Carver et al., 2005; Sato et al., 2007; Wahle et al., 2008). In this study, however, a significant change from a male- to a female-bias sex ratio was noticed in the non-selective, bottom trawling fishery in the southern Taiwan Strait over 25 years. In the area, non-selective fishing gears such as bottom trawlers and crab traps have been the main fishing gears for crab catches, and there are no sex or size regulations in action.

The impact of long-term, non-selective fishery on the crustacean sex ratio merits further discussion and investigation. First, marine crustaceans commonly have a skewed sex ratio due to environmental differences, such as salinity and temperature (Wenner, 1972; Jury et al., 2019; Koepper et al., 2021). In the southern Taiwan Strait, bottom trawling has developed since the 1970s and changed the biodiversity, biomass, and habitat of the benthic ecosystem (Xiao, 2002). The changed habitat may impact the benthic communities and the food compositions, which particularly favor C. natator females. Second, the changing climate has been reported to influence the spatial-temporal distribution, migration pattern, and sex ratio of marine crustaceans (Sato, 2011; Wahle et al., 2015; Greenan et al., 2019; Fedewa et al., 2020; Tanaka et al., 2020; Naimullah et al., 2021). For example, under the warming seawater, the American lobster females moved toward farther offshore waters, and the large males were more likely to inhabit warmer shallow waters (Tanaka et al., 2020; Koepper et al., 2021). In the southern Taiwan Strait, the surface sea temperature has increased dramatically (Belkin and Lee, 2014), and C. natator males might stay at nearshore shallow water where the non-selective bottom trawl fisheries operate. More studies are needed to understand the sex ratio patterns of C. natator under changing climate and exploration conditions in the future.

Long-term harvesting can truncate the size structure of a population or a stock (Melville-Smith, 1988; NEFSC, 1993; NEFSC, 1998). This phenomenon is not only observed in marine crustaceans (Jamieson et al., 1998; Wahle et al., 2008; Sato, 2012) but also in marine fishes and other invertebrate species (Roy et al., 2003; Harvey et al., 2006). This study also detected a reduction in the proportion of large individuals (>10 cm CW), average size, and body weight in C. natator male and female catches in the southern Taiwan Strait over a 25-year fishery. The larger males and females found in February 2019 might be relative to the spawning activity of C. natator in the southern Taiwan Strait. A similar phenomenon also occurred in the peaking spawning month (April) in 1994−1996. The significantly larger C. natator males and females observed in August and September 2019 may be due to China’s summer fishing moratorium management strategy, as they reflect the first and second months’ catches right after the restricted three-and-a-half-month fishing ban. A bottom trawl ban management regulation would significantly increase the mean weight and size and the proportion of large-sized individuals in crustaceans (Tao et al., 2018).

The relationship between average carapace width (CW) and body weight (BW) declined between 1994−1996 and 2019 datasets. The exponential value “b” showed that male C. natator shifted from positive allometry (b = 3.13398) in the 1994−1996 dataset to negative allometry (b = 2.9913) in the 2019 dataset. The same phenomenon was also recorded in Chaceon quinquedens in the northwest Atlantic after nearly 30-year commercial fishing (Weinberg and Keith, 2003). The reduction in average male body weight would reflect a negative impact on the availability or quality of the prey and the success rate of male crustaceans mating (Sato, 2012; Kolts et al., 2013). The impact of lighter male body weight in a population of C. natator needs further investigation.

A 1-month shift was found in C. natator’s spawning peak season over 25 years, changing from March−August in the 1994−1996 dataset to February−April in the 2019 dataset in the southern Taiwan Strait. Because of the national summer fishing moratorium, there was a 3-month data gap in 2019 (May−July), and it is unknown whether the spawning peak season could extend into May−July. Small juveniles (1.8−2.2 cm CW) collected in October 2019 (Liu and Lin, 2020) indicate that spawning activity is likely to occur at least in July, as fertilized C. natator eggs usually take approximately 3 months to reach 2.0 cm CW (Sumpton, 1990; Arshad et al., 2006).

The causes for the 1-month shift in C. natator’s spawning peak in the Taiwan Strait have not completely been understood. The earlier spawning season might be caused by the increased seawater temperatures, smaller maturation size, earlier maturation age, or interaction effect. The higher temperature stimulates early ovarian development and accelerates ovarian maturity to spawn early (Annala et al., 1980). It is known that the reproductive season of the spiny king crab Paralithodes brevipes in Hamanaka Bay in 2004 started 1 month earlier than in 2003, owing to the higher seawater temperature in 2004 (Sato, 2012). In the Taiwan Strait, the annual overall sea surface temperature has increased more than 1°C from 1957 to recent times, and long-term warming was more strongly enhanced in winter with the highest average warming of 3.8°C in February (Belkin and Lee, 2014). The increased seawater temperature in the southern Taiwan Strait is considered to be a possible factor promoting the earlier spawning of C. natator. On the other hand, the warmer seawater and intensive fishing pressure can reduce the size at maturity and cause mature individuals to spawn earlier in marine crustaceans (Hines, 1989; Zheng, 2008; Hines et al., 2010; Hjelset et al., 2012; Green et al., 2014). The smallest female bearing eggs decreased from 6.9 cm CW in 1994−1996 to 6.1 cm CW in 2019 (Ye, 1999; this study). Likewise, cancer crab females, including Cancer irroratus and Cancer pagurus, in warm waters matured earlier and spawned at smaller sizes than those in cool waters (Shields, 1991). The size at maturity for tanner crab Chionoecetes bairdi decreased significantly due to the exploitation (Zheng, 2008). Therefore, higher temperatures and smaller matured sizes result in earlier spawning peak season of C. natator in the southern Taiwan Strait.

The fecundity and recruitment of a population or a stock may decline as a result of reduced abundance and individual size and skewed female sex ratios (Murua et al., 2003; Hjelset et al., 2012). Male size is an important factor in mating efficiency and success rate due to their handling and guarding behavior (Paul and Paul, 1997). The decreased abundance and male size can result in sperm competition, which potentially reduced the female’s reproductive output and success rate in crustaceans (Sato et al., 2007; Sato, 2011; Ogburn et al., 2014; Pardo et al., 2015; Pardo et al., 2017). At biased sex ratios or in species for which females preferentially mate with large males, females may have reduced reproductive success because they are unable to find suitable mates due to the prevalence of smaller males (Rowe and Hutchings, 2003; Rains et al., 2018).

Although altered sex ratios related to male-only or size-selective fisheries have been observed for a variety of crustaceans, the impacts on reproductive output at the C. natator population level under non-selective bottom trawl fishery is unclear. No historical data are available on fecundity and egg size to directly understand the impacts of fishery on C. natator.

Around the world, decapod crustacean capture fisheries are growing faster than other major fisheries, and more attention needs to be paid to their scientific research and sustainable management (Boenish et al., 2022). The “3S” (legal catchable size, sex, and season) management strategy was applied to achieve sustainable use in crustacean fisheries (Otto, 1986; Kruse, 1993). In China, one national and two provincial (Zhejiang and Fujian Provinces) regulations were released in 2015 and 2018, respectively, detailing minimum catch sizes and landing proportions of juveniles for 36 commercially important species including 28 fishes, two shrimps, four crabs (excluding C. natator), and two cephalopods (http://hyyyj.fujian.gov.cn/; http://www.zj.gov.cn/; http://www.moa.gov.cn/). The implementation of these new regulations, however, is challenging. Fisheries operations in Chinese economic exclusive zones are mainly multi-species fisheries, and approximately 90% of the total marine catch is from non-selective trawlers, purse seines, gill nets, and set nets (Kang et al., 2018). A recent study showed that 62% of Monomia haanii caught in bottom trawlers operating in the southern Taiwan Strait were smaller than the legal minimum catch size (8 cm CW) (Lin et al., 2021a).

A closed season to protect juveniles and females bearing eggs would help the recovery of the recruitment and spawning stocks to achieve the maximum sustainable yield for biological and economic objectives (Johnston et al., 2011; Dichmont et al., 2014). Management measures on protecting mature female crabs and juveniles are still limited in China. Currently, Portunus trituberculatus females carrying eggs and juveniles (individuals < 70 g) are banned at port landings and in food markets in Zhejiang Province between 1 April and 16 September. This regulation seeks to prevent the capture of bearing females and small juveniles by trap vessels and even extends beyond the fishing moratorium period in Zhejiang Province (1 April−1 August) (http://nynct.zj.gov.cn/art/2021/2/26/art_1694969_58931103.html). Recent studies revealed a consistent spawning peak (February−April) in the southern Taiwan Strait for C. natator, M. haanii, and Calappa philargius, indicating that the national summer fishing moratorium cannot protect the spawning stocks of these commercially important crabs (Lin et al., 2021a; Lin et al., 2021b; this study). Because of the dominance of multi-species fisheries in China, it is not easy to amend the closure season for specific species or species groups. However, a recent bio-economic modeling analysis of M. hannii in the southern Taiwan Strait demonstrated that the current national fishing moratorium can provide better biological and economic benefits compared with closing early to protect spawning peaks (Boenish et al., 2021). In the future, samples from May−July are needed for better assessment and management improvement.

In the early 1990s in Chinese waters, the fishing pressure in terms of fishing power (kw) and the number of trawlers have increased more than twofold since the early 1990s (Kang et al., 2018). In Dongshan County, the estimated capture production of C. natator from bottom trawlers has decreased from 3,100−3,800 t/year in the early 1990s (Ye, 1999) to approximately 2,500 t in 2019 (Liu and Lin, 2020). The CPUE of M. haanii in the bottom trawl fishery has decreased by 50% from the early 1990s to 2018−2019 in the southern Taiwan Strait (Zhang, 1997; Liu and Lin, 2020). Therefore, the CPUE decline trend of C. natator in the southern Taiwan Strait was estimated to be under the same situation as M. haanii. Since the 1990s, fishery management strategies in China, including vessel buyback programs, fishermen relocation programs, closed seasons and zones, the total allowable catch system, and zero and minus growth targets, have been implemented to control the effects of fishing and catch landing to prevent the decline of fishery stocks (Cao et al., 2017). In the future, an adaptive science-based fisheries management strategy for C. natator and other fishery species should include gathering information, setting goals, predicting outcomes, and monitoring and evaluating results toward obtaining such outcomes, then reviewing what worked and what failed, in order to lead to more economically and socially optimal outcomes under changing conditions.