Development and Evaluation of an Escape Vent for Undersized Swimming Crab (Portunus trituberculatus) Bycatch Reduction in Pots

Abstract

We sought to develop and evaluate an escape vent designed for undersized swimming crabs (Portunus trituberculatus) to reduce bycatch, contributing to the preservation of marine resources. To this end, we conducted aquarium experiments and selectivity analysis to determine the appropriate size of the escape vent that would allow only undersized crabs with a carapace length of 64 mm or less to escape. The optimal dimensions for the escape vent were approximately 34.1 mm in height and 69.1 mm in width. During the sea trial, the average bycatch rates for undersized crabs per pot were 57.2%, 15.0%, and 22.3% for the control, basic, and door types, respectively. Regarding legal-sized crab catch per pot, averages of 1.40, 1.72, and 1.62 individuals were obtained for the three pot types. To our knowledge, this study is the first to assess the optimal size for an escape vent capable of reducing the bycatch of undersized crabs while maintaining legal-sized swimming crabs capture.

1. Introduction

The swimming crab (Portunus trituberculatus) serves as a crucial fishery resource, yet its population is in decline; hence, effective resource management is necessary [1,2,3,4,5]. The incidental catch of undersized crabs contributes to declining swimming crab stocks [2,3]. The swimming crab primarily inhabits the coastal waters of Japan, Korea, Southeast Asia, the Yellow Sea, the East Sea, and the Bohai Sea of China [6]. This species exhibits a preference for substrates rich in sand and mud, and it is frequently observed in estuarine and coastal environments [6].

In South Korea, the capture of undersized swimming crabs with a carapace length of 64 mm or less is legally prohibited to protect crab resources. The primary fishing gear utilized to catch swimming crabs comprises gill nets and pots. Gill nets facilitate the selective capture of individuals of specific sizes by adjusting the mesh size [7,8,9,10]. The crab pots also have selectivity determined by mesh size. However, the legal minimum mesh size for crab pots (35 mm) is insufficient to exclude undersized crabs, given the 64 mm minimum legal carapace length, rendering mesh size selectivity ineffective for their protection. The predominant crab pot in South Korea is a cylindrical design with three funnel-shaped entrances per side [3,11]. Kim and Lee (2014) [11] reported a 63% bycatch rate of undersized crabs using these pots. With 4683 vessels operating in the South Korean pot fishery in 2023 [12], the annual discard of undersized crabs is estimated to be significant, highlighting the need for selective capture methods targeting legal-sized crabs.

Reducing the bycatch of undersized individuals positively impacts population stability. Undersized and not yet contributing to reproduction, they are crucial to population dynamics; their capture reduces reproductive capacity, potentially depleting resources long term. Protecting undersized individuals to mature increases reproductively viable individuals, supporting sustainability [13]. Economically, reducing bycatch may lower short-term catch but enhance long-term fishery sustainability, maximizing benefits. Releasing undersized individuals increases the harvest of mature, valuable individuals. Furthermore, preventing resource depletion mitigates the risk of fishery collapse, thereby ensuring the livelihood stability of fishers. Additionally, this approach reduces the time and effort required to sort bycatch individuals [6].

Strategies to reduce bycatch in pot fisheries include depth-specific pot placement based on species distribution [14], mesh size adjustments [15,16], and the attachment of escape vents [2,3,17,18,19,20,21,22]. Among these, escape vents offer a practical and efficient solution without requiring the structural redesign of existing gear [2,3]. Despite these benefits, no studies have investigated the optimal escape vent size for undersized swimming crabs (Portunus trituberculatus) with a carapace width of 64 mm or less, caught in cylindrical pots used along the coast of South Korean, underscoring the need for such research. Furthermore, Korean crab pot fishers often resist adopting escape vents, believing that commercially valuable crabs may also escape through these openings. This misconception among fishers, coupled with the need to inform policymakers of the necessity of escape vents, highlights the urgent requirement for scientific evidence to address these concerns and support informed decision making.

Thus, we performed aquarium experiments to investigate the optimal escape vent size that allows only undersized crabs with a carapace length of 64 mm or less, captured off the South Korean coast, to selectively escape. The associated efficacy in reducing undersized crab bycatch was confirmed through subsequent sea trials.

2. Materials and Methods

2.1. Aquarium Experiments

The aquarium experiments took place from 14 May to 12 June 2019, at the Fish Behavior Laboratory of the West Sea Fisheries Research Institute, a branch of the National Institute of Fisheries Science. During this time, the water temperature within the experimental tank fluctuated between 15.1 °C and 19.6 °C. Seventy swimming crabs were procured for the experiment via a stow net on 9 May 2019, from the coastal waters of Seocheon-gun, South Korea. Subsequently, 50 individuals were selected for further experimentation (

Table 1. Number of experimental swimming crabs in each carapace length class.

Carapace Length Class (mm)	Number
45–48	1
48–51	0
51–54	6
54–57	6
57–60	9
60–63	13
63–66	5
66–69	2
69–72	2
72–75	2
75–78	2
78–81	1
81–84	1
Total	50

). All crabs were handled swiftly and gently to reduce exposure to air and physical manipulation. Immediately following capture in the gear, crabs were placed in aerated containers, ensuring consistent restriction of air exposure across individuals. Crabs were maintained in aerated seawater tanks at ambient temperature (15–20 °C, consistent with fishing ground conditions) during the holding period prior to experiments, thereby minimizing thermal and hypoxic stress. To prevent cumulative stress effects, crabs used in the experiments were transferred to holding tanks and allowed to rest for at least 24 h prior to testing. Prior to their use in the experiment, the crabs underwent a 5-day acclimation period within a W1000 × L1000 × D800 tank [23]. Throughout the acclimation and rearing phases, a daily provision of approximately 10 pieces of mackerel weighing approximately 10 g each was provided as feed at 05:30 PM.

An empty acrylic cylinder was used for the experiment to determine the optimal escape vent size (Figure 1). This cylinder is separated into an upper and lower part. The upper part is simply a hollow cylinder (diameter: 600 mm, height: 300 mm), and the lower part is a cylinder with several escape vents of the same size. We prepared several lower parts according to the height or width size of escape vent to be tested (See the right images of Figure 1a and Figure 1b respectively). Considering the carapace shape and escape behavior of swimming crabs [17], the escape vent was rectangular.

The experiment comprised two stages [23]. In the first stage, the optimal height of the escape vent was determined for undersized crabs with a carapace length of 64 mm or less, while in the second stage, the optimal width of the escape vent was established.

In the first stage, the experimental pot was placed in a tank (W1000 × L1800 × D900) to determine the optimal escape vent height for undersized crabs. Crabs were introduced to the pot, and their escape behavior was observed (Figure 2). The experiments were conducted for five different heights (32, 35, 38, 41, and 44 mm). The right image of Figure 1a illustrates the five types of lower parts attached under the upper experimental pot, each equipped with three escape vents. The width (528 mm) of each vent was set as large as possible, to minimize the impact of vent width.

In the second stage, to determine the optimal escape vent width, the height of the vents remained constant based on the results from the first stage of the experiment. Similar to the first-stage experiment, this experiment also tested whether swimming crabs could escape through five different widths (32, 35, 38, 41, and 44 mm). The right image of Figure 1b depicts the five types of lower parts attached beneath the upper experimental pot, each equipped with 16 escape vents.

At the initiation of the experiment, five swimming crabs were introduced into the experimental pot; new crabs were added from the rearing tank whenever an escape occurred. Crabs failing to escape from the experimental pot within a maximum period of 3 h were categorized as non-escapees and were removed from the experimental pot [17]. A total of 50 swimming crabs were introduced into the experimental pot for each experiment. This procedure was consistently applied across all experiments.

To distinguish between crabs participating in the aquarium experiment and those that managed to escape, the size parameters (carapace length, width, height, weight, and sex) of each crab were measured prior to the experiments. Subsequently, a unique number was inscribed on the carapace of each crab using a permanent marker pen after removing the moisture from the carapace with a dry cloth. The relationship between carapace length and height is represented in Figure 3.

2.2. Analysis of Optimal Escape Vent Size

The optimal escape vent size (height or width) was determined by calculating the selection probability (crab’s retaining ratio in experimental pot) according to relative carapace length (carapace length/escape vent size) of crabs. This size was established as the escape vent size at which the selection probability for the 64 mm (undersized crab’s maximum carapace length) equals 50%. The SELECT model [24,25,26,27,28] was employed to determine the selection probability of the escape vent. The selection probability curve as a function of escape vent size was estimated utilizing the logistic function [25] as represented by Equation (1):

$$\mathrm{s}\left({\mathrm{R}}_{\mathrm{i}\mathrm{j}}\right)={\displaystyle \frac{\exp \left(\mathrm{a}+{\mathrm{b}\mathrm{R}}_{\mathrm{i}\mathrm{j}}\right)}{1+\exp \left(\mathrm{a}+{\mathrm{b}\mathrm{R}}_{\mathrm{i}\mathrm{j}}\right)}}$$

(1)

where R represents the ratio of the crab’s carapace length to the escape vent size, that is, the relative carapace length; a and b are the estimated coefficients of the logistic function estimated using the maximum likelihood method [17,18].

2.3. Sea Trial

An escape vent was fabricated based on the size data obtained from the aquarium experiments. Subsequently, a sea trial was conducted by affixing the vent to a crab pot to assess its efficacy in reducing undersized swimming crab (carapace length: ≤64 mm) bycatch. Three types of pots were used: pots lacking an escape vent (i.e., existing type or control; Figure 4a), pots equipped with a basic escape vent (i.e., basic type; Figure 4b), and pots with a covered escape vent (i.e., door type; Figure 4c). The basic escape vent consists of a rectangular panel (W110 × H65 mm) with a rectangular aperture, the width and height of which were determined through aquarium experiments. The covered escape vent is an enhanced version of this, with additional hinged doors attached to both the upper and lower sections. The door attached to the covered escape vent is designed with a mechanism that opens when undersized crabs move from inside to outside, while remaining closed in the opposite direction, thereby restricting abnormal ingress of undersized crabs through the escape vent.

Sixty-nine pots were used in one set of test fishing gear (n = 23 per pot type), arranged in the sequence of control, basic, and door types (Figure 5). The one set of test fishing gear specifications were as follows: branch line spacing and length: 8 m and 4 m, buoy line length: ≈35 m, anchor line length: ≈4.5 m, buoy capacity: ≈21 L, anchor weight: ≈11.5 kg.

The sample size was determined through a power analysis for ANOVA. The effect size was set to f = 0.1, the most conservative condition among Cohen’s three standardized effect sizes, with a statistical power (1-β) of 0.80 and a significance level (α) of 0.05 [29,30]. Calculations performed using the pwr package in R software (R Foundation, Vienna, Austria; version 4.4.3) indicated a required sample size of 323 per group (total N = 969). Accounting for an approximate 10% efficiency loss associated with the nonparametric Kruskal–Wallis test, the sample size was adjusted to 356 per group (total N = 1068) [31]. Given that each set comprised 69 samples (69 pots), a minimum of 16 sets (totaling 1104 samples) was deemed necessary. To achieve this, experiments were conducted with 4 sets per day over a period of 4 days, resulting in the collection of 16 sets (368 samples per group, total N = 1104).

To compare the mean catch of undersized and legal-sized crabs per pot across different pot types, statistical analysis was performed. Normality was tested using the Shapiro–Wilk test, and variance homogeneity was assessed with Levene’s test. Depending on these outcomes, either a one-way ANOVA for parametric data or Kruskal–Wallis test for nonparametric data was applied, followed by a Dunn’s post hoc test for significant results (p < 0.05). Analyses were conducted in R software (R Foundation, Vienna, Austria; version 4.4.3).

The sea trial comprised four sessions, each utilizing four sets of test gear. The bait was frozen mackerel divided into three portions (average weight: ≈60 g/piece). The trials were conducted on October 13, 14, 28, and 29 at a swimming crab pot fishing ground (36°34′38.2″ N, 126°17′13.8″ E), off the coast of Baeksajang Port (average water depth: 12.5 m) in Taean-gun, Chungcheongnam-do, on the West coast of Korea. Each test gear set was deployed in the fishing ground for approximately 24 h (Figure 6).

3. Results

3.1. Analysis of Optimal Escape Vent Size

In the first stage of the aquarium experiment, the number of crabs escaping increased with the height of the escape vent (

Table 2. Experimental results for deriving the appropriate escape vent height.

Carapace Length Rank (mm)	No. of Crabs Added to Pots	No. of Crabs Retained in Pots
		Vent Height
		32 mm	35 mm	38 mm	41 mm	44 mm
45–48	1	E	E	E	E	E
51–54	6	E	E	E	E	E
54–57	6	E	E	E	E	E
57–60	9	3	E	E	E	E
60–63	13	6	2	E	E	E
63–66	5	5	1	E	E	E
66–69	2	2	2	E	E	E
69–72	2	2	2	E	E	E
72–75	2	2	2	2	2	E
75–78	2	2	2	2	E	E
78–81	1	1	1	1	1	E
81–84	1	1	1	1	1	1

E: All crabs placed in the test pots escaped.

). Notably, relatively small individuals escape through the vent almost immediately upon being introduced into the experimental pot.

The selection probability curve for the carapace length of the crab was depicted by the height of the escape vent (Figure 7). Using this curve, it can be determined that the 50% selection carapace lengths corresponding to escape vent heights of 32, 35, 38, 41, and 44 mm are 60.1, 65.8, 71.4, 77.1, and 82.7 mm, respectively. The selection probability curve with respect to relative carapace length was presented in Figure 8. According to the curve, the relative carapace length corresponding to a 50% selection probability was 1.8796; the appropriate escape vent height for crabs with a carapace length of 64 mm or less was 34.1 mm.

In the second stage of the aquarium experiment, the number of crabs escaping also increased with the escape vent width (

Table 3. Experimental results for deriving the appropriate escape vent width.

Carapace Length Rank (mm)	No. of Crabs Put into Pots	No. of Crabs Retained in Pots
		Vent Width
		60 mm	65 mm	70 mm	75 mm	80 mm
54–57	9	E	E	E	E	E
57–60	9	6	E	E	E	E
60–63	3	3	E	3	E	E
63–66	6	6	6	3	E	E
66–69	9	9	9	3	E	E
69–72	6	6	6	6	6	E
72–75	4	4	4	4	4	4
78–81	4	4	4	4	4	4

E: All crabs placed in the test pots escaped.

). The curve generated for the selection probability for the carapace length relative to the escape vent width revealed that the 50% selection carapace lengths for escape vent widths of 60, 65, 70, 75, and 80 mm were 55.6, 60.2, 64.9, 69.5, and 74.1 mm, respectively (Figure 9). The relative carapace length corresponding to a 50% selection probability was 0.9269 (Figure 10). The appropriate escape vent width for crabs with a carapace length of 64 mm or less was 69.1 mm. Thus, the optimal escape vent size for undersized crabs was 69.1 mm in width and 34.1 mm in height.

3.2. Sea Trial

Figure 11 illustrates the population distribution of swimming crabs captured in the test pots during the sea trials, categorized by carapace length class. Notably, in the case of existing crab pots, the highest catch frequency was observed for the 60–64 mm carapace length group, which includes the boundary value (64 mm) distinguishing undersized crabs from legal-sizeds crabs. Conversely, the two types of pots equipped with escape vents exhibited a significantly lower catch frequency for sizes ≤64 mm.

The sea trial yielded a total of 1806 swimming crabs, with 882, 442, and 482 captured in the control, basic, and door types, respectively (

Table 4. Number of crabs caught per pot type and sea trial.

Trial (Date)	Gear No.	Undersized Swimming Crab Number (Count)			Legal-Sized Swimming Crab Number (Count)			Total Swimming Crab Number (Count)
		Control	Basic Type	Door Type	Control	Basic Type	Door Type	Control	Basic Type	Door Type
1st (13 October 2021)	1	52	6	7	23	25	21	75	31	28
1st (13 October 2021)	2	30	1	2	19	20	24	49	21	26
1st (13 October 2021)	3	24	1	6	18	18	22	42	19	28
1st (13 October 2021)	4	5	0	0	1	1	0	6	1	0
2nd (14 October 2021)	1	31	3	7	27	32	30	58	35	37
2nd (14 October 2021)	2	37	2	8	45	36	29	82	38	37
2nd (14 October 2021)	3	26	2	9	11	6	16	37	8	25
2nd (14 October 2021)	4	13	3	2	17	11	22	30	14	24
3rd (28 October 2021)	1	41	12	9	30	36	27	71	48	36
3rd (28 October 2021)	2	58	1	7	32	39	33	90	40	40
3rd (28 October 2021)	3	24	5	7	38	33	36	62	38	43
3rd (28 October 2021)	4	54	12	5	32	31	44	86	43	49
4th (29 October 2021)	1	36	8	8	27	25	32	63	33	40
4th (29 October 2021)	2	25	12	13	21	20	15	46	32	28
4th (29 October 2021)	3	13	1	3	10	12	8	23	13	11
4th (29 October 2021)	4	32	5	10	30	23	20	62	28	30
Total		501	74	103	381	368	379	882	442	482

). Among these were 678 undersized crabs, with 501, 74, and 103 captured in the control, basic, and door types, respectively. Additionally, 1128 legal-sized crabs were captured, with 381, 368, and 379 in the control, basic, and door types, respectively.

The total weight of swimming crabs caught in the sea trial was 307.3 kg, with weights of 131.5, 84.1, and 91.7 kg recorded for the control, basic, and door types, respectively (

Table 5. Weight of crabs caught per pot type and sea trial.

Trial (Date)	Gear No.	Undersized Swimming Crab Weight (g)			Legal-Sized Swimming Crab Weight (g)			Total Swimming Crab Weight (g)
		Control	Basic Type	Door Type	Control	Basic Type	Door Type	Control	Basic Type	Door Type
1st (13 October 2021)	1	5508	820	833	4787	5496	4361	10,295	6316	5194
1st (13 October 2021)	2	3290	135	296	3649	4050	5088	6938	4185	5384
1st (13 October 2021)	3	2392	145	794	3826	3797	4766	6218	3942	5560
1st (13 October 2021)	4	579	0	0	159	151	0	737	151	0
2nd (14 October 2021)	1	3504	407	776	6012	6215	6491	9516	6622	7267
2nd (14 October 2021)	2	4259	216	1101	9092	7632	6340	13,351	7848	7441
2nd (14 October 2021)	3	2768	257	912	2329	1334	3177	5097	1590	4089
2nd (14 October 2021)	4	1471	393	261	3360	2299	4837	4831	2692	5098
3rd (28 October 2021)	1	4590	1650	1204	5345	6965	5289	9935	8615	6493
3rd (28 October 2021)	2	6439	50	991	5860	7725	6700	12,299	7775	7691
3rd (28 October 2021)	3	2799	685	1030	7608	6574	7387	10,407	7258	8417
3rd (28 October 2021)	4	6348	1649	704	6078	6009	8447	12,426	7658	9151
4th (29 October 2021)	1	4086	1137	1136	5423	5112	6636	9509	6249	7772
4th (29 October 2021)	2	2691	1571	1852	3900	3782	2704	6591	5353	4556
4th (29 October 2021)	3	1591	142	387	2240	2538	1904	3831	2680	2291
4th (29 October 2021)	4	3699	694	1240	5785	4427	4100	9483	5120	5340
Total		56,014	9951	13,517	75,451	74,104	78,226	131,463	84,053	91,742

). The total weight of undersized crabs was 79.5 kg, with weights of 56.0, 10.0, and 13.5 kg recorded for the control, basic, and door types, respectively. Furthermore, the total weight of legal-sized crabs was 227.8 kg, with weights of 75.5, 74.1, and 78.2 kg recorded for the control, basic, and door types, respectively. Consequently, the number and weight of swimming crabs caught in conventional pots without escape vents were found to be greater than those in pots equipped with escape vents. Similarly, the number and weight of Undersized swimming crabs caught were higher in pots without escape vents compared to those with escape vents. However, little differences were observed in the number or weight of legal-sized swimming crabs caught across different pot types.

The average bycatch rates of undersized crabs per pot for the control, basic, and door types were 57.2%, 15.0%, and 22.3%, respectively, based on catch number, and 51.0%, 13.4%, and 19.9%, respectively, based on weight (Figure 12). Hence, the average bycatch rate of undersized swimming crabs per pot was 34.9–42.2% lower in catch number and 31.1–37.6% lower in weight in pots equipped with an escape vent than in those without vents.

When comparing the two types of pots with escape vents, the bycatch rate of undersized crabs in pots with doors was 7.3% higher in catch number and 6.5% higher in weight than in those without doors.

Regarding the average catch of legal-sized swimming crabs per pot, the control, basic, and door type captured 1.40, 1.72, and 1.62 individuals with weights of 276, 346, and 334 g, respectively (Figure 13). This suggests that pots with escape vents caught 0.22–0.32 more individuals and 58–70 g more weight per pot than those without escape vents. Notably, there was no significant difference in the legal-sized swimming crab catches between the two types of pots with escape vents.

There were no significant differences in the variance of undersized crab bycatch rates among fishing gear sets within each sea trial date or between sea trial dates (

Table 6. Results of homogeneity of variance tests for fishing gear sets and sea trial dates.

Test the variance between fishing gear sets in each sea trial date:
Trial date	Fligner–Killeen Test
13 October 2021	X2 = 3.2067, df = 3, p-value = 0.3608
14 October 2021	X2 = 5.7389, df = 3, p-value = 0.125
28 October 2021	X2 = 1.9661, df = 3, p-value = 0.5795
29 October 2021	X2 = 2.4152, df = 3, p-value = 0.4908
Test the variance between sea trial dates:
	X2 = 3.1037, df = 3, p-value = 0.3759

). Therefore, when comparing the bycatch rates of undersized crabs across different pot types, the random effects of fishing gear set and date variation were not considered in the statistical analysis. The statistical test results (

Table 7. Statistical test results on the bycatch rate of Undersized crabs from sea trials.

		Catch Number	Catch Weight
Normality (control)	Shapiro–Wilk test	W = 0.87744, p = 5.515 × 10−14	W = 0.86508, p = 9.848 × 10−15
Normality (basic)		W = 0.58583, p < 2.2 × 10−16	W = 0.55735, p < 2.2 × 10−16
Normality (door)		W = 0.65881, p < 2.2 × 10−16	W = 0.62244, p < 2.2 × 10−16
Homogeneity of variance	Levene’s test	df = 2, F = 19.404, p = 6.206 × 10−09	df = 2, F = 30.995, p = 1.227 × 10−13
Omnibus test	Kruskal–Wallis	Χ2 = 187.31, df = 2, p < 2.2 × 10−16	Χ2 = 180.17, df = 2, p < 2.2 × 10−16
Post hoc (control, basic)	Dunn’s test	Χ2 = 187.31, Z = 12.454396,	Χ2 = 180.17, Z = 12.182958,
		p = 1.985243 × 10−35	p = 5.746914 × 10−34
Post hoc (control, door)	Dunn’s test	Χ2 = 187.31, Z = 10.675365,	Χ2 = 180.17, Z = 10.517396,
		p = 1.991021 × 10−26	p = 1.077467 × 10−25
Post hoc (basic, door)	Dunn’s test	Χ2 = 187.31, Z = −1.967149,	Χ2 = 180.17, Z = −1.853923,
		p = 0.07374901	p = 0.09562537

) showed that the bycatch rate of undersized crabs per pot was significantly higher in the conventional pot (control) without an escape vent than in the two pots (door type and basic type) with escape vents in terms of the number of individuals (p < 0.01) and weight (p < 0.01). There was no significant difference in the bycatch rate of undersized crabs between the pot with the door less escape vent and the pot with a door (p > 0.05). The catch of legal-sized crabs per pot was significantly lower in the conventional pot without an escape vent than in the two pots with escape vents (

Table 8. Statistical test results on the legal-sized crabs from sea trials.

		Catch Number	Catch Weight
Normality (control)	Shapiro–Wilk test	W = 0.86539, p = 1.028 × 10−14	W = 0.88735, p-value = 2.397 × 10−13
Normality (basic)		W = 0.86089, p = 4.744 × 10−13	W = 0.93881, p-value = 7.955 × 10−08
Normality (door)		W = 0.88711, p = 3.282 × 10−12	W = 0.94787, p-value = 1.938 × 10−07
Homogeneity of variance	Levene’s test	df = 2, F = 2.3921, p = 0.09217	df = 2, F = 3.8825, p = 0.02103
Omnibus test	Kruskal–Wallis	Χ2 = 16.569, df = 2, p = 0.000252	Χ2 = 18.447, df = 2, p = 9.867 × 10−05
Post hoc (control, basic)	Dunn’s test	Χ2 = 16.56867, Z = −3.891723,	Χ2 = 18.447, Z = −3.9191261,
		p = 0.0001493025	p = 0.0001333059
Post hoc (control, door)	Dunn’s test	Χ2 = 16.56867, Z = −2.817766,	Χ2 = 18.447, Z = −3.3339208,
		p = 0.0072538478	p = 0.0012844656
Post hoc (basic, door)	Dunn’s test	Χ2 = 16.56867, Z = 1.102624,	Χ2 = 18.447, Z = 0.6429264,
		p = 0.4052855089	p = 0.7804077492

), regarding the number and weight of individuals (p < 0.01). There was no significant difference in the catch of legal-sized crabs between the two pots with escape vents regarding the number of individuals (p = 0.40528) or weight (p = 0.78040).

4. Discussion

Owing to the funnel-shaped entrance of swimming crab pots, crabs of any size can pass through the entrance and enter the pot; however, once inside, they become trapped. If the mesh size of the pot is smaller than the size of the target individuals, no means of escape exists for the crabs trapped within the pot. Considering this characteristic, the escape vent is the only means for undersized crabs to enter the pot and escape.

Several studies have demonstrated the effectiveness of escape vents [2,3,16,17,18], which have helped direct recent research on optimizing escape vent performance. The performance of an escape vent is influenced by several factors, including its location, shape, number, and size. According to experimental observations of swimming crab behavior, the optimal location for the escape vent in cylindrical pots for undersized swimming crabs is the lower part of the side [16]. This is based on the behavioral tendencies of swimming crabs in pots. Crabs often move sideways with their backs against the side walls. If the crabs encounter an escape vent of an appropriate size to facilitate escape, they can maneuver their bodies and exit through the vent [3,16]. Therefore, in this study, the escape vents were positioned on the lower side, aligning with these research findings.

The shape of the escape vent was also anticipated to impact the escape rate of swimming crabs [3,6,17]. Oval escape vents, even if they have identical width and height dimensions to the rectangular escape vents, do not perform as effectively as the rectangular escape vents [17]. This is because when a swimming crab attempts to recognize and escape through a vent, a rectangular shape allows for uniform dimensions resulting in no obstruction or snagging of the crab during the turning process. By contrast, oval escape vents taper toward the sides, creating narrower passages, which can hinder swimming crabs from easily maneuvering their bodies. The performance of circular and rectangular escape vents varies across studies. Yu et al. (2024) [6] reported no statistically significant difference in the escape performance of undersized crabs between these two vent types, whereas Boutson et al. (2008) [17] found that circular escape vents exhibited superior performance compared to rectangular ones, despite both studies employing vents of identical dimensions. These discrepancies may be attributed to multiple factors, including experimental conditions (e.g., sea trials versus indoor experiments, soaking time, and the number of pots or trials conducted). Collectively, these studies suggest that the performance of circular escape vents is either equivalent to or exceeds that of rectangular vents. Should the findings of the former study hold—indicating no performance disparity between the two types—rectangular vents may be more likely to be adopted in fishery applications due to their smaller surface area, which could reduce manufacturing costs compared to circular vents.

Therefore, the location and rectangular shape of the escape vent proposed in this study were deemed suitable for undersized swimming crabs.

Given the diversity of swimming crab species worldwide, it is imperative to ascertain the appropriate escape vent size for each species. While Zhang et al. (2023) [3] delved into escape vents suitable for juvenile P. trituberculatus among various swimming crab species, their study primarily centered on escape performance concerning the shape of the escape vent rather than its size. Consequently, it did not provide details regarding the escape vent size that could optimize the escape performance of undersized crabs. By contrast, our study derived and verified the optimal size for enhancing the escape of P. trituberculatus. According to Zhang et al. (2023) [3], the rectangular escape vent exhibited superior performance in reducing the bycatch of undersized crabs, showcasing a minimal bycatch rate of 3% in comparison to the conventional pot devoid of an escape vent (the bycatch rate of conventional pots was set to 100%). However, the catch rate of legal-sized crabs using the rectangular escape vent was only 81% of that observed in the conventional pot. In contrast, the bycatch rate of undersized crabs captured in the basic type of pot proposed in this study was 15.0% of that of the conventional pot. Meanwhile, the catch rate of legal-sized crabs was notably higher at 121.1%, approximately 40.1% above the result of Zhang et al. (2023) [3].

Notably, as the escape vent size increases, so does the likelihood of undersized crabs escaping; however, if it surpasses a certain threshold, the legal-sized crab catch diminishes. Thus, the optimal escape vent should strike a balance, facilitating reduced undersized crab bycatch without significantly affecting the legal-sized crab catch, which is vital for commercial purposes. Consequently, the outcomes of this study hold significance as they represent the first attempt to derive an escape vent size suitable for safeguarding undersized crabs while preserving the legal-sized P. trituberculatus catch.

In the initial planning stages of this study, it was anticipated that the catch of legal-sized crabs in pots with escape vents would remain comparable to that in conventional pots. However, the results revealed that the legal-sized crab catch in pots with escape vents surpassed that in conventional pots in terms of the number of individuals and weight. This unexpected outcome might be attributed to the phenomenon where the escape vents enable undersized crabs to exit the pot, thereby creating additional space within the pot. This is consistent with the findings of Miller (1990) [32] and Broadhurst et al. (2017) [33], who reported that the conspecific antagonistic behavior among swimming crabs for food and space facilitates the escape of undersized individuals. Moreover, they highlighted the significance of escape vents in reducing congestion inside the pot, consequently enhancing the catch rate of legal-sized crabs.

We also evaluated the efficacy of both basic and door types. The door pot escape vent featured doors attached to the top and bottom of the escape vent frame. While the bottom door remained open, the top door was closed. Consequently, the escape vent’s height was halved compared to that of the basic escape vent. The door is designed to prevent the entry of undersized crabs and other fish species through the escape vents, with the door operable only by applying force from the inside of the pot. Hence, it was anticipated that the use of doors would result in a lower bycatch rate of undersized crabs. However, no significant differences were observed in the bycatch rate of juvenile crabs or the catch of legal-sized crabs between the two pots with escape vents. This suggests that the ingress of individuals from outside the pot through the escape vents does not influence the bycatch rate of juvenile crabs or the catch of legal-sized crabs.

Escape vents can be utilized not only for the egress of undersized crabs but also for mitigating ghost fishing. Globally, the loss of pots during fishing operations is a frequent occurrence, and ghost fishing resulting from abandoned pots poses a significant threat to the depletion of crab populations [34]. The strategic integration of escape vents with biodegradable materials may be considered to reduce ghost fishing impacts. For instance, fabricating escape vents from biodegradable materials or using biodegradable threads to secure escape vents to pots could ensure that, in the event of pot loss, the vents or the securing threads degrade, thereby creating a larger opening for trapped crabs and other species to escape. This mechanism could consequently reduce the incidence of ghost fishing. This approach is intended to be explored in future research.

Bycatch includes organisms unintentionally caught during fishing activities besides the target species, and it has various impacts on the ecosystem [35]. Immediate effects include the reduction of populations of specific species and the loss of biodiversity, while long-term consequences may involve changes to the structure and function of the ecosystem. In particular, bycatch can influence predator–prey interactions, potentially compromising ecosystem stability [35]. The implementation of escape vents may effectively reduce the bycatch of undersized crabs; however, it can alter predator–prey interactions within the pot. When certain species escape through these vents, the balance between predators and prey inside the pot may be disrupted. For instance, if undersized crabs or small prey species exit through the escape vents, predators remaining in the pot may experience a decline in survival rate or population due to food scarcity. Conversely, if the escape vents are sufficiently large to allow predators (e.g., mollusks or crabs) to escape, the survival rate or population of these predators may increase, while prey species could benefit from reduced predation pressure, leading to an elevated survival rate [36].

Undersized crabs escaping through the vents may attract the attention of predators such as fish or marine mammals, which could subsequently associate the pot with a food source [37]. This may heighten predation pressure on both the escaping crabs and those remaining inside the pot, particularly if predators learn to target pots equipped with escape vents. Such effects could trigger cascading impacts on local predator–prey dynamics, potentially modifying the behavior or distribution of predator species within fishing grounds. The protective effect of escape vents on undersized crabs may increase crab populations, potentially lowering the survival rate of organisms in food competition with them. Furthermore, if crab populations become disproportionately large relative to their prey, food shortages may ensue, which could, in turn, reduce crab populations [36].

From an industrial perspective, a key concern is that the use of escape vents may decrease the catch of fishermen targeting small crabs. This could impose an economic burden on fishery workers, with additional considerations including the costs associated with purchasing and installing escape vents. Consequently, research and policy development are required to optimize the efficacy of escape vents while mitigating industrial concerns. Given that escape vents are likely to be constructed from plastic, their production and disposal alongside pots could contribute to micro plastic generation, posing long-term risks to ecosystems and human health. Therefore, the development of escape vents made from biodegradable materials should be considered in parallel with their widespread adoption.

Active research on escape vents for undersized crabs in pots has demonstrated their effectiveness, leading several countries to recognize their importance in protecting crab resources. For example, policies in countries such as the United States require crab pot fisheries to include escape hatches [38,39]. However, despite the high commercial value and popularity of swimming crabs in Korea, there is currently no requirement for the attachment of escape vents to protect undersized swimming crab resources. This highlights the urgent need for policy interest and understanding in this area.

While the effectiveness of escape vents in safeguarding undersized crabs has been established, future research should aim to maximize efficiency. One approach is to investigate how the escape rate varies with the number of vents. Furthermore, studies examining the impact of immersion depth, light intensity, and visibility of escape vents on the escape rate are anticipated to contribute significantly to enhanced efficiency.

5. Conclusions

In this study, we conducted aquarium experiments and sea trials to design and evaluate the effectiveness of escape vents, allowing undersized swimming crabs (P. trituberculatus) to escape from pots and reducing the bycatch of undersized swimming crabs in the West Sea of Korea. The key findings are as follows:

• The optimal escape vent height and width for swimming crabs with carapace lengths of 64 mm or less are approximately 34.1 mm and 69.1 mm, respectively.
• The average bycatch rates of undersized crabs are 57.2%, 15.0%, and 22.3% in terms of the number of individuals in the control, basic, and door types, respectively. In terms of weight, the corresponding rates are 51.0%, 13.4%, and 19.9%. Additionally, the average legal-sized crab catch is 1.4, 1.72, and 1.62 individuals and 276, 346, and 334 g, respectively.
• Comparing pots with escape vents to those without, the former has an average 38.6% lower bycatch rate of undersized crabs in terms of the number of individuals and 34.4% lower in terms of weight. Furthermore, the legal-sized crab catch is significantly higher in pots with escape vents.
• No significant differences were observed in the bycatch rate of juvenile crabs or the catch of legal-sized crabs between pots with escape vents featuring doors and those without doors.

In conclusion, this study provides compelling evidence that escape vents significantly reduce undersized swimming crab bycatch in pot fisheries, offering a viable mechanism to enhance population sustainability. The findings lay critical groundwork for informing future regulatory frameworks, potentially driving the adoption of escape vent standards to balance ecological and economic outcomes. Implementing this solution in crab pot fisheries will contribute to the conservation of swimming crab resources and the development of sustainable fisheries practices.