Commercial bottom trawl and trap vessels from Dongshan County (Fujian Province, China) operate exclusively in the fishing grounds of the southern Taiwan Strait (21.50–24.50° N, 116.00–119.00° E) (see Figure 2 in Lin et al., 2021). Monthly sampling of whole body crabs was conducted during January–April 2019 and August–December 2019 at the landing ports of Dongshan County. Owing to the national annual summer fishing moratorium regulation (i.e., no trawl fishing permitted from May 1st to August 15th, and no trap fishing from May 1st to August 1st) in China, no crabs were available for sampling spanning May–July 2019.

Approximate 15–35 kg (about 1–2 baskets) C. philargius samples were randomly collected from at least five bottom trawl vessels in each sampling month (Figure 1A). About 1 kg of “feed-grade fish” (see definition in Zhang et al., 2020) from the same trawl vessels were also collected to obtain smaller C. philargius samples. All samples were transferred to Dongshan Swire Marine Station (Xiamen University) for further analysis.

No C. philargius samples were collected from trap vessels because they typically only land the large first-paired claws (Figure 1B); the declawed live hard bodies were discarded at sea.

For every whole body crab collected, carapace width (CW, to 0.1 cm), carapace length (CL, to 0.1 cm), and body weight (BW, wet weight, to 0.01 g) were measured (Figure 2A). Based on abdomen morphology, sexes were determined, and berried females were considered fully mature (Figures 2B,C). Consistent with literature on other similar crabs, embryonic developmental stages from early to late were noted by the yellow, orange-red and brown coloration, respectively (Arshad et al., 2006; Hisam et al., 2018; Lin et al., 2021).

Female fecundity, defined as the number of eggs carried by a female, was estimated empirically following standard methodologies (Lin et al., 2021). The absolute fecundity (Fa) was calculated as: Fa = W ^∗ m/n, where W is the total weight of the whole egg mass for a female, m is the average weight of the five egg sub-samples (approximate 0.02 g per 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: Fr = Fa/CW and Fr = Fa/BW. Egg diameters (to 1 μm) from different colors (5 individuals for yellow eggs, 5 individuals for orange-red eggs, and 4 individuals for brown eggs) were randomly measured using a Leica M165 FC fluorescence stereo microscope (Figures 2D–F).

Information on the total capture production (kg/vessel), C. philargius production (kg/vessel), and the number of fishing days at sea (days/vessel) was collected in the landing ports from the same trawl vessels during monthly sampling. Data collection was through captain and crew interviews, landing port estimation directly (i.e., landing volumes by species or species group level), and trade notebooks (i.e., captain’s wholesale information including weight and price on the per-trip level documented for revenue purposes).

Information collected from the trap vessels was comparable to that of the trawl vessels. However, due to the claw-only characteristics in trap vessel landings, C. philargius capture production (kg/vessel) was back-calculated from claw production estimation (Figure 1B). Briefly, both female and male whole body crabs were randomly selected after measurement and examination above. The claw yield (%) per crab was calculated as: W_claws/BW ^∗ 100, where W_claws is the wet weight of the paired claws, which are typically broken off proximal of the merus. Then, the average claw yield (%) of C. philargius was calculated. Finally, whole crab capture production of C. philargius from trap vessels was estimated by dividing the total claw landings (kg) by the claw yield percentage.

The percentage of berried females each month, irrespective of egg coloration, was calculated as: the number of berried females/the total number of females ^∗ 100. The spawning season was defined as the months with berried females, and the peak spawning season was defined as the month(s) with at least 50% of berried females (Sadovy, 1996).

We estimated the size at 50% female maturity (CW₅₀) by fitting a logistic regression (probity link) to the percentage of berried females in 0.5 cm CW size bins using females from the peak spawning month determined above.

Differences in sex ratio from 1: 1 monthly and overall were analyzed by student’s t-test. Differences in sizes (CW) of females and males monthly and overall were analyzed via one-way ANOVA and non-parametric analysis (Mann-Whitney U-test). The relationships of CW—BW, CW—CL, Fa—CW, and Fa—BW were all analyzed via one-way ANCOVA. The statistical analyses were conducted using R (R Core Team, 2020), SPSS 13.1 and Microsoft Excel 2019 software, by setting statistical significance at the level of p ≤ 0.05.

A total of 1,009 whole body crabs with 408 males and 601 females were collected from trawl vessels; 58–183 crabs each month (Table 1). The overall male: female ratio was 1: 1.47 (χ² = 36.54, df = 1, p < 0.01). Significant monthly differences (p < 0.01) from a 1: 1 ratio were observed from January, February, August, October, and November. For the rest of the sampling months there were no significant differences from an equal sex ratio (p > 0.05). Data showed a significant male-bias in August 2019, and a significant female-bias in January, February, October, and November 2019 (Figure 3). CWs in the trawl landings ranged from 1.8 to 15.7 cm (12.12 ± 1.66 cm CW, mean ± SD, N = 408) for males, and 1.2–16.0 cm (11.60 ± 1.63 cm, N = 601) for females (Figure 4). Males were mainly landed in size classes between 11.0 and 14.9 cm CW (80.15%) and females between 9.0 and 13.9 cm CW (91.18%), determined by the size frequencies > 10% (Figure 4). Males were significantly larger than females in CW (t = 4.23, df = 1,007, p < 0.01). Monthly variations in sizes of males (F = 4.80, df = 407, p < 0.01) and females (F = 6.95, df = 600, p < 0.01) were also significant (Table 1). Smaller crabs < 5 cm CW were found in feed-grade fish samples in August (1.2 cm CW, N = 1), October (3.2–4.5 cm CW, N = 3), and December (1.8 cm CW, N = 1).

The sex specific CW—BW relationships were not different significantly (F = 0.776, df = 1, p > 0.05). The regression coefficient b of C. philargius differed significantly (p < 0.01) from isometric growth (b = 3), with both males and females exhibiting negative growth allometry (Figure 5A and Table 2). The CW—CL relationships between females and males were significantly different (F = 3.875, df = 1, p < 0.05) (Figure 5B). The CW—CL relationships were highly correlated for males (R² = 0.8329, p < 0.01) and females (R² = 0.7158, p < 0.01).

A total of 112 berried females were collected (Figure 6A). Two spawning seasons were determined, in January–April and in August–October, with the peak spawning month occurring in February (Figure 6B). It is unclear to what extent spawning occurs in May–July due to lack of data.

The minimum and maximum sizes for berried females were 9.9 cm CW (7.1 cm CL) found in February and 15.1 cm CW (11.8 cm CL) in October, respectively (Table 3). The CW₅₀ for females was estimated as 11.47 cm CW (8.7 cm CL) based on females sampled in the peak spawning month, February (Figure 7). The dominant size classes for berried females were 11.0–13.9 cm CW (86.61%). Small berried females (< 11.0 cm CW) were mainly caught in February.

From the sampled ovigerous females, 14 were selected to estimate fecundity (Fa) (10.0–12.4 cm CW), 5 with yellow eggs, 5 with orange-red eggs, and 4 with brown eggs. Fa ranged between 441,742 and 1,025,607 eggs (734,947 ± 181,397, N = 14), and Fr ranged between 42,071 and 82,710 eggs cm^–1 CW (63,417 ± 12,486, N = 14), and between 2,227 and 4,232 eggs g^–1 BW (3,159 ± 534, N = 14). A significant power relationship was found between Fa and CW (R² = 0.83, p < 0.01) (Figure 8).

Egg diameters were 210–418 μm (332 ± 29, N = 145), with 312 ± 23 μm for yellow eggs (N = 49), 327 ± 22 μm for orange-red eggs (N = 53) and 362 ± 18 μm for brown eggs (N = 43). Egg diameters were significantly different among three embryonic developmental stages, increased with the increase of embryonic development (F = 64.50, df = 142, p < 0.01).

All 95 C. philargius samples collected in January 2019, i.e., 71 females (8.3–13.6 cm CW) and 24 males (7.9–14.2 cm CW), were used for the claw yield assessment. The claw yield ranged between 28.45% and 44.60% of the total wet body weight (36.28 ± 3.07%, N = 95), and showed no significant difference between sexes (t = 3.63, df = 93, p > 0.5). The claw yield also showed no difference with CW (F = 0.044, df = 93, p > 0.5). The average claw yield of 36.28% was used to estimate the C. philargius capture production in the trap fishery below.

From January to December in 2019, we surveyed a total of 127 vessels (77 trawl vessels and 50 trap vessels) at the landing ports of Dongshan County, all operating in the southern Taiwan Strait (Tables 4, 5). The estimated total capture production of C. philargius was 1,652 t; 63.65% from the 77 trawl vessels and 36.35% from the 50 trap vessels. Trawl vessels usually spent 3–12 days at sea per trip, while trap vessels operated longer at sea, usually 20–30 days per trip. However, trap vessel catches were delivered to transfer vessels approximately every 1–11 days. The CPUEs (kg day^–1) of C. philargius were significantly higher in the trap fishery than the trawl fishery (Z = 2.782, p < 0.01). In trawl fishery, the CPUEs were mainly < 10 kg day^–1, while in trap fishery, the CPUE averaged > 25 kg day^–1 in January, April and September–December. C. philargius was less significant in the trawl fishery in terms of capture volume (wet weight, kg), all < 4% and mainly < 1%. C. philargius made up a substantial component of the total capture production in the trap fishery, e.g., about 13.7% in January and 25.6% in April.

Based on captain, crew, and trader interviews at the landing ports of Dongshan County, the Chinese C. philargius claw market is largely domestic and has developed in less than one decade. In Dongshan County, the wholesale price for whole C. philargius caught from the trawl fishery is commonly around 6 RMB kg^–1, which is typically further processed into claws (Figure 1A). The wholesale price of claws (processed on board) from the trap fishery is typically around 14 RMB kg^–1 (Figure 1B). The claw price is nearly double in the wet and night food markets.

Since 2019, the domestic consumption market for C. philargius has continued to expand. Hard cleaned bodies of C. philargius are delivered to inland tourism cities of China (Figure 1C), and commonly cooked as spicy cuisine. Live whole crabs are also becoming popular in local provincial markets, particularly for dining on the ripe ovaries. The wholesale price of live whole crabs is around 10 RMB kg^–1.

Marine crustaceans commonly have a skewed sex-ratio, and the existence of sex-specific habit segregation and single-sex migration may explain the phenomenon (Wenner, 1972). The overall sex ratio of C. philargius habiting in the southern Taiwan Strait showed a strong predominance of females (p < 0.01), collected by trawlers; monthly variation from the 1: 1 was also observed (p < 0.01) in January, February, August, October, and November. The same overall female-bias trend was also reported for C. philargius in Thailand, collected by gill nets (Wisespongpand et al., 2016). In C. philargius, a strong female-bias occurred in the peak spawning month (February), suggesting that C. philargius females may also migrate from deeper waters to spawn nearshore. A recent study on M. haanii from the same fishing grounds showed a similar pattern, i.e., a strong female-bias in peak spawning months (February and March) (Lin et al., 2021).

Crabs often show sexual size dimorphism. Males of C. philargius were significantly larger than females in CW (p < 0.01). The pattern of sexual size dimorphism, however, is largely species specific. For example, Callinectes danae, C. sapidus, C. philargius, Chaceon macphersoni, Chaceon affinis, Chionoecetes tanneri, Cancer borealis Johngarthia lagostoma, M. haanii, Platyxanthus crenulatus, and Ranina ranina (Krajangdara and Watanabe, 2005; Hartnoll et al., 2009; Sforza et al., 2010; Keller et al., 2012; Groeneveld et al., 2013; Farias et al., 2014; Biscoito et al., 2015; Jivoff et al., 2017; Truesdale, 2018; Lin et al., 2021; this study) are female-biased in terms of sexual size dimorphism whereas Cancer pagurus, P. sanguinolentus, and P. trituberculatus (Oh, 2011; Yang et al., 2014; Öndes et al., 2017) are male-biased. Meanwhile, sexual size dimorphism could also exhibit spatial variation within the same species. For instance, Scylla olivacea, S. paramamosain, and S. tranquebarica (Ikhwanuddin et al., 2011; Waiho et al., 2016, 2021; Fazhan et al., 2021; Rouf et al., 2021). Therefore, sexual size dimorphism in crabs merits further investigation.

Sexual selection in decapod crustacean fisheries can influence both sex ratio and sexual size dimorphism in the population (Orensanz et al., 1998; Fenberg and Roy, 2008; Alborés et al., 2019). For instance, the earlier fishery regulation for larger C. affinis males in the Northeast Atlantic eventually resulted in the change of sex ratio (female: male) from 1.24: 1 to 2.08: 1 and variation shifting from males being larger, to females being larger after 30 years of heavy exploitation (Wahle et al., 2008). For the claw-only C. philargius fishery, large males are preferred in trade due to large claws and quality of meat. As a newly developing claw-only fishery, C. philargius males are significantly larger than females in CW with female-bias sex ratio (female: male = 1.47: 1) in the southern Taiwan Strait. Intensive and continuous fishing pressure may be changing the population structure of C. philargius in southern Taiwan Strait. Long term population monitoring for C. philargius is needed to evaluate the sex ratio and size variation pattern by sex under sexual selection fishery that is largely driven by specific claw-only market demand.

The sex specific CW—BW relationships were consistent (p > 0.05); however, we found sex-specific CW—CL relationships were significantly different (p < 0.05), i.e., females were typified by a more rounded and less oval carapace shape (Figure 5B). Size difference by sexes may reflect their difference in growth rates during different developmental stages or be associated with female fecundity. In C. philargius, females and males showed an intersection at 8.6 cm CW, where at that point females may input their energy in growing for maturation, particularly for increasing CL, as a larger carapace size is able to accommodate more eggs (Oh, 2011; Yang et al., 2014).

Studies in the southern Chinese waters suggest there is a common spawning season in February–April for many crabs, e.g., C. philargius (this study), the red swimming crab Monomia haanii (Lin et al., 2021), the blue swimming crab Portunus pelagicus (Liu et al., 2014), and the three-spot swimming crab P. sanguinolentus (Yang et al., 2014). However, it is unclear to what extent spawning takes place into May–July for all the aforementioned species, because the national annual summer fishing moratorium regulation typically disallows fishing and biological sampling. Some crabs demonstrate long spawning seasons, e.g., the ghost crab Ocypode retundata with one spawning season from May to October in the Persian Gulf (Naderi et al., 2018), and P. pelagicus from November through July in southeastern Australia (Johnson et al., 2010). For C. philargius, there could be a continuous spring–summer–autumn spawning period (Figure 6). Several small C. philargius juveniles (1.2–4.5 cm CW) were found in feed-grade fish samples in August, October and December, suggesting they were hatched in the summer closure period (May–July). On the other hand, no small juveniles found in January to April, supporting that C. philargius stops spawning activities in winter (November and December) (Figure 6). An extended spawning season in C. philargius might increase the number of breeding clutches to enhance the population recruitment. In the future, C. philargius samples from the missing months (May –July), and gonad examination before egg ovulation are needed to further understand this developing claw-only fishery in China.

The logistic estimation of CW₅₀ for C. philargius showed an imperfect curve without 100% mature groups in larger female sizes. Two possible factors may lead to this result. First, some berried females may be discarded at sea because of their light meat weight and poor meat quality. Second, the logistic estimation does not extend to those mature females with fully developed ovaries inside the carapace which have not yet released eggs and attached them to their pleopods.

Fecundities of decapod crustaceans often have positive linear or power function correlation with body size or body weight (Erdman and Blake, 1988; Hisam et al., 2018; Lin et al., 2021). Fa had a strong power correlation with CW (p < 0.01) in C. philargius, similar to M. haanii (Lin et al., 2021), O. punctatus (Jiang and Yu, 2012), O. rotundata (Naderi et al., 2018), and P. sanguinolentus (Yang et al., 2014; Figure 9). For Fr, C. philargius has higher Fr than some swimming crabs, such as M. haanii (Lin et al., 2021), P. sanquinolentus (Yang et al., 2014), and O. rotundata (Naderi et al., 2018), but smaller than O. punctatus (Jiang and Yu, 2012). C. philargius females carry egg mass on their narrow pleopods ventrally, and the elongated pleopod appendages result in the capacity to bear more eggs in one clutch (Figure 2B). Fa of the same species may show large variation in different regions. For example, Fa of C. philargius caught in the southern Taiwan Strait (734,946 ± 181,396 eggs) is higher than that in Thailand (average 307,714 eggs) (Wisespongpand et al., 2016).

There are some debates on the biological merits of claw-only fishery development. On one hand, a claw-only fishery can sustain crab capture production while causing less mortality than traditional crustacean fisheries. It is believed that at least some declawed crabs do survive and grow after their return to sea, and therefore regain possible breeding opportunities and re-enter into the fisheries. Clearly, declawed females expend energy on claw regeneration, which may negatively impact reproductive success and shorter spawning seasons (Savage and Sullivan, 1978; Hogan and Griffen, 2007). For males, a declawed status may negatively affect breeding success through reducing an individual’s competitive prowess as well as by compromising defensive ability (higher natural mortality) (Duermit et al., 2015).

In practice, only 7–13% regenerated claws entered the M. mercenaria market fishery in Florida, the United States (Simonson and Hochberg, 1986; Wilber, 1995; Muller et al., 2011). The mortality rate associated with two-claw removal is high in M. mercenaria and C. borealis, ranging from 28 to 100% and 70%, respectively, and higher with increasing temperatures under laboratory conditions (Davis et al., 1978; Simonson and Hochberg, 1986; Gandy et al., 2016; Goldstein and Carloni, 2021). The recent developing one-claw fishing regulations on M. mercenaria and C. borealis showed an around 35–50% reduction in mortality rate when compared to two-claw removal in the fishery (Davis et al., 1978; Duermit et al., 2017; Goldstein and Carloni, 2021). However, the application of one-claw fishing strategy in practice is difficult to monitor and highly depends on voluntary adherence (Duermit et al., 2017). The impacts of one-claw and two-claw removal fisheries of C. philargius merit further investigation on feeding ability, survival rate, claw regeneration, mating success, and rates of re-entrance into a fishery for further management consideration and policy-making.

Obtaining the claw yield (%) contributes significantly to estimate the C. philargius capture productions in the field surveys. The average claw yield for C. philargius was 36.28%, which is higher than that of C. affinis (15% in Robinson, 2008), but lower than those of U. tangeri (40% in Crane, 1975) and M. mercenaria (50% in Davis et al., 1978). Capture productions of C. philargius in the southern Taiwan Strait varies by fishing gear and month. The production proportions of C. philargius were generally less than 1% of the total catches in trawl fishery, but made up a considerable portion (> 10%) of the trap fishery landings (Tables 4, 5).

Higher C. philargius catches occurred in September and October in the trawl fishery, and in January, April and December for the trap fishery. One possible explanation is the differences in fishing zones in the southern Taiwan Strait by fishing gears over time and potential migration behavior in C. philargius during their life history. Among the C. philargius trawl catches in the southern Taiwan Strait, about 95% individuals were ≥ 9.0 cm CW, and about 75% individuals ≥ 11.0 cm CW. Individuals < 9.0 cm CW are usually discarded at sea in both trawl and trap fisheries based on interviews because of the low prices offered for small claws. Based on our estimated CW₅₀ (11.47 cm CW and 8.7 cm CL), at least 25% of our sampled C. philargius catch was sexually immature.

To date, only one national and two provincial (Zhejiang and Fujian Provinces) regulations have been implemented in China on the minimum catch sizes and landing proportions of juveniles. Collectively, these cover 36 commercially important species including 28 fin fishes, two shrimps, four crabs, and two cephalopods.¹, ², ³ Currently, there are landing prohibitions on P. trituberculatus juveniles (individuals < 70 g) and berried females as well as bans on sale in food markets in Zhejiang Province between April 1st and September 16th, i.e., throughout the entire national summer fishing moratorium period.⁴ This management strategy could also be suggested to apply to other crab species in China for protecting the spawning populations during the peak spawning seasons.

As a newly developing claw-only fishery for C. philargius, no management measures are set for the species except the general national summer fishing moratorium. However, the critical peak spawning months (February and April) for C. philargius fall outside of the national summer fishing moratorium. About 60% of C. philargius catches in the trawl fishery are females, and moreover, high female trawl catches occurred in the highest peak spawning month, February, indicating high exploitation of spawning female stocks. Recently, a seasonal spawning closure has been proposed in lieu of the one claw fishing regulation based on biological and practical tradeoffs between strategies (Duermit et al., 2017). For C. philargius, February needs to be considered in the spawning seasonal closure measure.

A recent bioeconomic models reviewed that simple regulatory measure adjustments to the M. haanii fishery in the southern Taiwan Strait, the same fishing areas as C. philargius, can provide multiple biological and economic benefits (Boenish et al., 2021). Furthermore, a total allowable catch (TAC) pilot project for the P. trituberculatus fishery in Zhejiang Province is evaluating the utility of e-logbooks to improve response time and monitoring accuracy for Chinese crab fisheries (Zhu et al., 2021).