A review on fishing gear in China: Selectivity and application

Zhongqiu Wang, Hao Tang, Liuxiong Xu, Jian Zhang

a East China Sea Fisheries Research Institute, Chinese Academy of Fishery Academy, Shanghai, 200090, China

bCollege of Marine Sciences, Shanghai Ocean University, Shanghai, 201306, China

cNational Engineering Research Center for Oceanic Fisheries, Shanghai, 201306, China

dKey Laboratory of Oceanic Fisheries Exploration, Ministry of Agriculture and Rural Affairs, Shanghai, 201306, China

eThe Key Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Shanghai Ocean University, Ministry of Education, Shanghai, 201306, China

fCenter for Polar Research, Shanghai Ocean University, Shanghai, 201306, China

Keywords:

Trawl

Mesh size

Shrimp/ fish separator

Stow net

Gillnet

A B S T R A C T

In the 1980s, China’s offshore marine fishery resources declined, thus fishing gear selectivity gradually became a focus of scientific fishery research. Studies of fishing gear selectivity mainly focus on five kinds of fishing gear:trawls, shrimp beam trawls, stow nets, gillnets, and pots. Cover net method is the main method used in the comparative study of codend selectivity of both trawls and stow nets. These results showed that (1) Bottom trawls, targeting Larimichthys polyactis (the Little yellow croaker) and Trichiurus haumela (the Hairtail), should have a minimum mesh size of the codend no less than 60 mm in the Yellow and East China Seas. While the codend should use a square mesh with a minimum mesh size of 40 mm due to the variety of targeted species in the South China Sea. (2) Pair pelagic trawls, targeting pelagic species such as Engraulis japonicus (the Japanese anchovy), Pampus argenteus (the Silvery pomfret), Scomberomorus niphonius (the Spanish mackerel), as well as bottom species such as L. polyactis and T. haumela, should have a minimum mesh size of the codend no less than 56 mm in the Yellow Sea. (3) Shrimp beam trawls should use shrimp/ fish separator devices (e.g., separator panels, vertical separator panels, and sorting grids) to improve intra-species selectivity and reduce by-catch of non-target fish. (4) Canvas stow nets, targeting L. polyactis and T. haumela, should have a minimum mesh size of the codend no less than 55 mm. (5) Gillnets, the minimum mesh size should be determined for catching traditional economic species, but also the optimal mesh size for newly developed species. Such as, gillnets with mesh size of 35-38.71 mm targeting Harpodon nehereus (the Bombay duck), 55 mm targeting L. polyactis and Nemipterus virgatus (the Golden thread fin bream), 72 mm targeting Dentex tumifrons (the yellow sea breams) or Scomberomorus commersoni with body length of 280 mm, 100 mm targeting P. argenteus, 104 mm or 146 mm targeting 2-3 year or 6-7 year S. niphonius, 120 mm targeting S. commersoni with body length of 380 mm, and 144 mm targeting Pampus chinensis (the Chinese pomfret). (6) Pots, the steel-framed round pots targeting Portunus trituberculatus (the swimming crab) should have a 33 mm height of escape vent or an 8 cm diameter escape hole, and the steel-framed round pots or accordion-shaped trap targeting Charybdis japonica (the Asian paddle crabs) should have a 30 mm height of escape vent installed on the side of pots. These results of fishing gear selectivity from the scientific basis for the implementation of fishing gear access systems, especially for identifying the minimum mesh size of fishing gear. In the future, the Fisheries Research Institute of China should conduct long-term investigations of selectivity in trans areas. Developing fishing gear access systems that are in line with a sustainable development is of great significance for the conservation and restoration of offshore fishery resources in China.

1.Introduction

Since the 1990s, the output of China’s marine fisheries (i.e., coastal waters and distant waters) has consistently ranked first in the world; the 2018 production of 12.68 million tons accounted for 15% of the world’s marine fisheries (FAO, 2020, p. 13). In the same year, the output of China’s coastal water fishery was 10.44 million tons, accounting for 71.22% of China’s fisheries. Among China’s coastal water fisheries, the Bohai Sea, the Yellow Sea, the South China Sea, and the East China Sea accounted for about 7.57%, 22.84%, 29.64%, and 39.95%, respectively.By fishing gear, trawl net, gillnet, stow net, purse seine, lines, and other gears accounted for 46.79%, 21.83%, 11.69%, 8.92%, 3.53%, and 7.24%, respectively. By catch species, fishes, crustaceans (i.e., shrimp and crab), shell fishes, cephalopods, and other species were 68.57%,18.95% (12.54%, 6.41%), 4.12%, 5.46%, and 2.90%, respectively(China fishery statistical yearbook, 2019, Fig. 1).

Fig. 1.Pro file of China’s fisheries in 2018 (Data resources: China fishery statistical yearbook, 2019).

Chinese coastal fishing grounds are characterized by a mixture of species and a scattered distribution. The yields of 19 fish species exceed 1% of China’s coastal fishery output. Among these fish species, two species account for more than 5%, four species account for 4-5%, four species account for 3-4%, five species account for 2-3%, and four species account for 1-2% (China fishery statistical yearbook, 2019,Table 1). However, the dominating fishing species vary widely in shapes, which increases the dif ficulty associated with research on fishing gear selectivity in China. To conserve China’s offshore fishery resources,relevant regulations on fisheries resource reproductive protection(draft) put forward the concept of the minimum mesh size measure in 1956. In December 1963, the Sino-Japanese Fisheries Agreement stipulated that the minimum mesh size of trawl operating in waters designated by both countries was 54.5 mm. The “Notice on the implementation of the minimum mesh size system for marine fishing nets”, issued in 2003 by the Ministry of Agriculture, People’s Republic of China, came into effect on July 1, 2004 (Appendix 1). After, the Notice issued in 2003 was replaced by the new Notice which included more types of fishing gear, issued in 2013 and came into effect on June 1, 2014(Appendix 2).

Table 1Dominated fishing species with different body shapes in Chinese coastal fishing grounds.

Studying the selection performance of fishing gear is an important means to ensure the conservation and sustainable utilization of fishery resources. According to the Chinese National Knowledge Infrastructure(CNKI) and the Web of Science, Chinese scholars have published 109 research papers on fishing gear selectivity since the early 1980s. This research focused on trawling gear (55 papers, including 26 papers on shrimp trawls), followed by stow nets (23 papers), gillnets (19 papers),and pots (12 papers). Most research has been carried out in the past 20 years (Fig. 2). According to its geographical distribution, research is mainly concentrated in the northern part of the Bohai Sea, the Lvsifishing ground in the southern part of the Yellow Sea, the Zhoushan fishing ground in the East China Sea, and the northern part of the South China Sea (Fig. 3).

Fig. 2.Selectivity research papers on fishing gear of China’s offshore fisheries since the 1980s by Chinese scholar in CNKI and Web of Science.

Fig. 3.Selected offshore study areas in China since the 1980s (Note: the text in the figure shows the “ fishing gear name - number of publications (and the publication years in parentheses)”).

This paper systematically reviews the research progress of main fishing gear selectivity in offshore waters, such as trawls, gillnets, stow nets, and pots, as well as the application of relevant results by fishery management. The results indicate that (1) Bottom trawls, targetingL.polyactisandT. haumela, should have a minimum mesh size of the codend no less than 60 mm in the Yellow and East China Seas. Because of the variety of targeted species, the codend should use a square mesh with a minimum mesh size of 40 mm in the South China Sea. (2) Pair pelagic trawls should have a minimum mesh size of no less than 56 mm in the Yellow Sea. (3) Shrimp beam trawls should use shrimp/ fish separator devices (e.g., separator panels, vertical separator panels, and sorting grids) to improve intra-species selectivity and reduce by-catch of non-target fish. The minimum mesh size of shrimp trawl should be no less than 35 mm to reduce by-catch of juvenile shrimp and appropriate adjustments should be made based on the main target shrimp species.(4)Canvas stow nets should have a minimum mesh size of the codend of no less than 55 mm. (5) Double stake stow net should have a minimum mesh size of the codend of no less than 35 mm. (6) Single stake stow nets, using canvas devices instead of traditional beams as expansion devices, still need further study to identify the most sustainable structure and minimum mesh size of the codend. (7) For gillnets,the minimum mesh size should be determined in terms of the minimum mature body length of the caught fish species. The optimal mesh size of gillnets is determined in terms of the body length composition of target species. (8) Pots should have a 33 mm height escape vent or an 8-cmdiameter escape hole installed on the side of the crab pot, as this improves catch performance and selectivity. Increasing the mesh size of eel pots effectively enables the release of juvenile eels.

These studies provide a scientific basis for a fishing gear admittance system and the formulation of minimum mesh sizes for fishing gear in China. However, these selectivity studies are mostly short-term tests aimed at single species, often lack systematic long-term assessments and suffer from potential effects of random uncontrol factors in the trials.Additionally, the current lack of uniform standards and comparative studies on selectivity of test methods results in the application of various methods. For example, studies on the selectivity of trawls and stow nets mainly used the cover method, while studies on the selectivity of gillnets and pots mainly used the combined comparison method. In the future,with the further strengthening of China’s fishery management system and the implementation of a fishing gear access system in the coastal waters of China, research on fishing gear selectivity will be strengthened. The Fisheries Research Institute of China should conduct investigations of selectivity on long-term and trans areas. A fishing gear access system needs to be formulated that is in line with the requirements for sustainable development, scientific management, and a rational development of fishery resources.

2.Studies of fishing gear selectivity in China

2.1.Selectivity of trawl nets

Trawl nets (including shrimp beam trawl) is an important fishing gear for China’s coastal fishery, and its output consistently ranked first,accounting for almost 47% of China’s coastal fish catch in 2018. However, a large number of non-target species and juveniles of target species are caught by trawls because of their lack of selectivity, which seriously affects fishery resources. To reduce by-catch of trawl fisheries, protect fishery resources, and achieve sustainable development of fisheries, the simplest and most common way is to increase the mesh size, thus increasing the selectivity of trawls. Studies on the mesh size selectivity of the trawl codend showed that larger mesh sizes of trawl increase the escape probability of juvenile fish. The selectivity of the trawl is determined by the minimum mesh size of the codend. The conclusions of research on the mesh size selectivity of trawls in China are summarized in Table 2.

Table 2Mesh size selectivity of trawl codend in China.

2.1.1.Bottom trawl

The main target species of bottom trawls areT. haumelaandL. polyactisin the Yellow and East China Seas. The project collaboration group for bottom trawl codend mesh size research in the East China Sea and the Yellow Sea (led by the East China Sea and the Yellow Sea Institute of the Chinese Academy of Fishery Sciences under participation of Yantai, Shanghai, Zhoushan Fisheries Company, Zhejiang Fisheries Research Institute, Xiamen Fisheries College, and others institute)worked toward alleviating the problems of juvenile by-catch and diminishing fishery resources. They conducted a systemic experiment between 1980 and 1981, with a total of 192 comparative experiments and 643 hauls of fishing comparing experiments, employing 19 commercial fishing and fishery research vessels. The results suggested that the minimum mesh size of bottom trawl mainly targetingT. haumelain the northern part of the East China Sea should be 56 mm (Rui, 1984).Huang et al. (2005) and Song et al. (2015), studied the mesh size selectivity of the bottom trawl codend by the cover net method. The results showed that the minimum mesh size of bottom trawl codend should be 60 mm for traditional diamond mesh or 55 mm for square mesh. Huang et al. (2016) suggested that the minimum mesh size of bottom trawl main target forT. haumelashould be 65 mm. Comprehensive analysis of the current status of fishery resources showed that the minimum mesh size of bottom trawl should be no less than 60 mm to protect key economic species such asL. polyactisandT. haumelain the Yellow and East China Sea.

However, in the South China Sea, bottom trawl targets a variety of species because of the multi-species status of this particular fishing ground. Key economic species includeD. maruadsi,N. virgatus, andS. tumbil. In view of the diversity of species targeted by bottom trawl in the South China Sea, the traditional diamond mesh is not effective in releasing multiple species. To solve this problem, in the early 2000s,researchers from the South China Sea Institute of the Chinese Academy of Fishery Sciences used a cover net method to compare the effect of square and diamond meshes on the release of juveniles of a variety of species in the northern part of the South China Sea. Yang et al. (2002a,2002b) showed that a square mesh has better selection performance than a traditional diamond mesh, and can more effectively protect juvenile fish resources in the South China Sea. Furthermore, round fish can easier escape from square mesh nets than fat fish. Yang et al. (2021a,2021b, 2021c) studied the size selectivity and catch efficiency of six diamond-mesh codends with different mesh sizes, 25-54 mm, forPennahia argentata(the white croaker),Caranx(Atule)kalla(the banded scad), andMuraenesox cinereus(the conger pike) using the covered codend method in the South China Sea. The results of those studies showed that 1). None tested codend was proved efficient to release undersizedP. argentata, 2). The codend with 30 mm mesh size would be the best choice to targetC.(Atule)kalla, and 3). The codend with 40 mm mesh size only 9.61% ofM. cinereusbelow the Market Reference Size(MRS) was retained, the problem of capturing juvenile fish was signi ficantly mitigated. However, the loss of commercial-sized fish was considerable, as about 50% fish above the MRS would escape from the codend with 40 mm mesh size. Comprehensively considering the status of various fishery resources, the trawl codend should use square mesh with a minimum mesh size of 40 mm. For targetingD. maruadsiandS.tumbil, a 50-mm square mesh of bottom trawl should be used to protect these particular fishery resources in the South China Sea.

2.1.2.Pair pelagic trawl

Pair pelagic trawl is an important kind of fishing gear in both the East China Sea and the Yellow Sea. In pair pelagic trawl, the height of the trawl mouth is significantly larger than that of the traditional bottom trawl. Target species include pelagic fish such asS. niphonius,P.argenteus,Engraulis japonicus(the Japanese anchovy), as well as bottom fish such asT. haumelaandL. polyactis. However, with the decline of fishery resources, the mesh size of pair pelagic trawl for small fish has decreased. In response to this problem, You et al. (2017) and Chen et al.(2018) studied the mesh size selectivity of pair pelagic trawls using the cover net method. You et al. (2017) found that the minimum mesh size of pair pelagic trawl should be 60 mm to protect the small and medium-sized fish species, such asT. haumela,L. polyactis, andD.maruadsi. Chen et al. (2018) showed that the minimum mesh size should be greater than 80 mm to protect the existing fishery resources in the Yellow Sea. To balance ecological and economic bene fits, Chen et al.(2018) recommended using 56 mm as the reference standard for the minimum mesh size of the pair pelagic trawls in the Yellow Sea.

2.2.Selectivity of shrimp trawl

Shrimp trawls are characterized by various catch species and high discard ratios. The dominant species of shrimp beam trawl arePalaemon gravieri(the Chinese ditch prawn),Exopalaemon carinicauda(the ridgetail white prawn),Parapenaeopsis hardwickii(the Spear littoral shrimp),T.curvirostris,Metapenaeus ensis(the greasy-back shrimp),Metapenaeus joyneri(the Joyneris shrimp),Solenocera crassicornis(the coastal mud shrimp),Crangon af finis(the Japanese sand shrimp),Penaeus monodon(the giant tiger prawn),Metapenaeopsis barbata(the whiskered velvet shrimp), andMarsupenaeus japonicus(the kuruma prawn). The mesh size of the codend of shrimp trawls is generally small (20-30 mm), resulting in a large proportion of immature fish and shrimp in catches. It has been estimated that the average discard ratio exceeds 62.3%, which is much higher compared with other fisheries. To improve the status of the fisheries and protect juvenile shrimp and by-catch, the key is to separate shrimp from by-catch fishes and release juvenile shrimp while ensuring suf ficient shrimp production. To this end, a series of experiments on shrimp/ fish separators and mesh size selectivity of the codend were conducted in China since the 1990s (Table 3).

Table 3Mesh size selection of shrimp trawl codend in China.

2.2.1.Shrimp/ fish separator

Beam trawls are highly efficient shrimp catching devices, but the size of their codend is generally small; thus, many economic fish species are mixed in catches. To improve the quality of shrimp catches and mitigate damage of shrimp trawls to fish juveniles, Chinese fishery researchers applied the cover net method to study the shrimp/ fish separation effect of separator panels and rigid grids. Sun and Yao (1998) studied the separation effect of 100-mm mesh size shrimp/ fish separator panels in Shengsi Fishery, Zhejiang Province. The results showed that this mesh size significantly improved the separator ratio, which reached 55-68% for shrimp and 95% for fish. Luo et al. (2007) studies the separation effect of two different mesh sizes (i.e., 130 and 110 mm) and mesh shapes (i.e., square and diamond) installed on the front of the shrimp trawl codend in Dapeng Bay, South China Sea. The results showed that the mesh size of separator panels significantly affects the separator ratio,but no significant effect was found when testing different mesh shapes,while installation of trawl efficiency devices (also known as turtle exclusion devices (TED)) was shown to effectively improve separator ratios. Zhang et al. (2008a) conducted a series of studies on the separation effect of installation of separator panels (60, 75, and 90 mm) or rigid sorting grids (bar spaces of 15, 20, and 25 mm) in shrimp beam trawl in Lvsi Fishery, Jiangsu Province. Their results showed that the efficiency of the separator increases with increasing size of the separator panel mesh or the bar space of the rigid sorting grid. No significant difference was found in the probability of different shrimps contacting separator panels or rigid sorting grids. However, for fish, the contact probability forCynoglossus abbreviatus(the three-lined tongue sole)ranges between 0.2 and 0.4, while the probabilities forL. polyactisandCollichthys lucidus(the spiny-head croaker) are 1. The differences in the probability with which various fish species contact the rigid grid indicated that the fish behavior in the trawl codend varies with species. Yang et al. (2018b) conducted a study on the separation ratios of separator panels with 40 mm square mesh for shrimp trawls in an offshore fishing ground in the northern South China Sea. The results showed that a separator panel with 40 mm square mesh can separate shrimp from bycatch. Both mantissa and weight separation ratios of separator panels with 40 mm square mesh forM. ensisandM. joyneriexceeded 90%,which is useful for improving the selectivity and addressing the negative impacts of shrimp beam trawls. However, the 50% selection body length of the main species is still small, and the mesh size of separator panels needs to be enlarged to improve separation ratios.

2.2.2.Mesh size of the codend

For the East China Sea, Sun and Wang (1999) and Sun et al. (2002)suggested that the mesh size of the shrimp trawl codend should be enlarged to 35-40 mm, as this can lead to the release of a large number juvenile shrimps (e.g.,P. hardwickii,S. crassicornis, andT. curvirostris)with body lengths shorter than 70 mm; this also significantly improves the quality of commercial shrimp in shrimp trawl catches in Shengsi and Lvsifishing grounds. Later, Zhang & Sun (2008) suggested that forP. hardwickiiandP. gravieri, square and T90 meshes achieve better selectivity than the diamond mesh, and a 35-mm square mesh codend achieved selective fishing in Lvsifishing ground. Chen et al. (2011a,2011b) suggested that the minimum mesh sizes forP. gravieraandC. af finisduring the spring fishing season should be 25 and 26 mm,respectively, while forP. hardwickiiandP. monodonduring the autumn fishing season, minimum mesh size should be 45 and 50 mm, respectively, in the Yangtze estuary.

For the Yellow Sea, Tang et al. (2010a) suggested that the shrimp trawl forT. curvirostrisshould use 33 mm mesh size for protectingT. curvirostrisresources at the coast of Shandong Province. Xing et al.(2015) suggested that the shrimp beam trawl for theP. gravieriinner mesh size should be 33-38 mm and forE. carinicauda, it should be 24-30 mm in the coastal waters of Panjin, Liaodong Bay. Zhang et al.(2018) suggested that the currently applied 25 mm mesh size of shrimp trawl in the coastal Hebei area seriously damages fishery resources,recommending a mesh size of 35 mm for targetingP. gravieri.

For the South China Sea, Zhang et al. (2007) suggested that the 35 mm mesh size of shrimp trawl codend is fitting for releasing juveniles of the four shrimp species ofM. ensis,M. barbata,M. joyneri, andM. japonicus. Yang et al. (2017a, 2018a, 2018c, 2019, 2020, 2021d)suggested that square mesh can effectively release slender and elongated fishes of by-catch and a 35-mm square mesh combined with a 30-mm diamond mesh (S35+D30) achieved the best selectivity forM. ensisassociated with the first mature length (80 mm). Comprehensively considering the resources in the northern South China Sea, Yang et al.(2018a) suggested that the 35-mm square mesh combined with a 25-mm diamond mesh (S35+D25) was appropriate for both ensuring yield and protecting fishery resources.

2.3.Selectivity of stow net

Stow nets are a common fishing gear in China’s coastal fisheries.Their output ranks third and accounts for about 12% of China’s total coastal output, which is lower than the outputs of trawls and gillnets.Stow nets are an important fishing gear in both the East China Sea and the Yellow Sea but are comparatively uncommon in the South China Sea.The main fish species targeted by stow nets in the East China Sea are L.polyactis,T. japonicus,P. argenteus,Setipinna tenuifilis(the hair fin anchovy),Coilia ectenes(the estuarine tapertail anchovy), andHarpodon nehereus(the Bombay duck). In the Yellow Sea, main fish species areL. polyactis,T. curvirostris, andArgyrosomus argentatus(the white croaker). In the South China Sea, main fish species areSiganus oramin(the pearl-spotted spinefoot),M. joyneri,Oxyurichthys microlepis(the maned goby),A. argentatus,Coilia mystus(the long-tailed anchovy), andEnedrias fangi(the fang blenny). However, the mesh size of stow nets is generally small and selectivity research for catch species has not been conducted to date, which exerts a serious negative long-term impact on China’s coastal fisheries. To promote the sustainable development of China’s coastal fisheries, the selectivity of stow nets should be studied.Research on the selectivity of stow nets dates back to the 1990s, when a series of selectivity tests of stow nets was conducted in the Yellow Sea,the East China Sea, and the South China Sea. Research focused on the impacts of mesh size and stow type (canvas, single stake, and double stake, Fig. 4) on selectivity (Table 4).

Table 4Selectivity research of stow nets in China.

Fig. 4.Three stow net types.(Sources from left to right: Zhang et al., 2016a, Zhang, 2004, and Sun et al., 2011).

2.3.1.Canvas stow net

The target species of canvas stow nets areL. polyactisandT. haumela.To alleviate the problem of the high ratio of juvenile fish in canvas stow nets, Cheng et al. (2001) and Xu et al. (2019, 2021) used the cover net method and the parallel method to study the selectivity of canvas stow nets in the East China Sea. The results of Cheng et al. (2001), Zhang et al.(2017), and Xu et al. (2019, 2021) showed that suitable mesh sizes of canvas stow nets targetingL. polyactiswere 68.3 mm, 54.1 mm (inner size of mesh), and 73 mm, respectively, and for targetingT. haumelasuitable mesh sizes were 77 mm, 58.1 mm (inner size of mesh), and 61.1 mm, respectively. Considering the status of the target speciesL. polyactisandT. haumela, a minimum mesh size of 55 mm for canvas stow nets meets the purpose of protecting juveniles of the target speciesL. polyactisandT. haumela.

2.3.2.Single-stake stow net

A new kind of single-stake stow net in which a canvas device replaced the traditional beam as expansion device, has been developed in the Lvsi Fishing Ground. The rapid development of new kinds of single-stake stow nets imposed heavy pressure on fishery resources. To understand and improve the selection performance of this new kind of single-stake stow net, Zhang et al. (2011a) used the cover net method to study the selectivity of different mesh sizes and shapes in the Lvsifishing ground. The 50% selection length (L50) ofL. polyactisandS. tenuifilisincreases with increasing mesh size for both diamond mesh and square mesh. For the same mesh size, the selectivity forS. tenuifiliswas not significantly different between diamond mesh and square mesh, while forL. polyactis, the selectivity of square mesh was better than that of diamond mesh. Because of the selectivity difference of square mesh for various body types of fish, and the reluctance of fishermen to use of square mesh, it is dif ficult to promote the use of square mesh. To overcome the problems encountered in the promotion of square mesh, Zhang et al. (2011b) used the cover method to study the circumference of the codend effect on selectivity. The results showed that the selectivity of a 30% reduced circumference codend was higher than the selectivity of both diamond and square meshes targetingL. polyactisandS. tenuifilis.The regular pattern of the reduced circumference codend on selectivity still needs further study. To obtain age information related to selectivity,Zhang et al. (2016b) used the cover net method to assess the age selectivity forL. polyactisandS. tenuifilisof 35 mm and 45 mm meshes.The results showed that the ages ofL. polyactisandS. tenuifiliscorresponding to the 35 mm and 45 mm mesh were significantly lower than the recommended age of capture. This refects that the size of the currently used meshes is too small to protect juvenile fish. Based on the above research results, the codend structure and minimum mesh size of new kinds of single-stake stow nets still need further testing.

2.3.3.Double-stake stow net

Because of the small mesh size of double-stake stow nets, which makes them prone to catch many juveniles of economically important fish and shrimp species, the fishery resources of China’s coastal waters are seriously damaged. Sun et al. (2006) used the cover method to study the size selectivity of diamond and square mesh codends forS. tenuifilisin Chinese stow net fisheries, showing that the 36.5-mm mesh size codends could release 50% juvenileS. tenuifilis. However, such a mesh size of diamond mesh codends are not selective for other species. Tang et al. (2010b) used the cover method to study the mesh size selectivity in the Yellow Sea, showing that the selectivity of a mesh size 33 mm forT. curvirostriswas better than that of 30- or 40-mm mesh sizes. Yan et al.(2014) also used the cover method to study the mesh size selectivity in the South China Sea. Their results showed that the L50differs for different species despite the same mesh size, and a mesh size of double-stake stow nets should be 35 mm for the South China Sea. Li et al.(2015) used the parallel method to study mesh size and shape selectivity in the Yellow Sea, showing that a 30-mm square mesh codend can balance fishery output and the protection of fishery resources. Li, Zhang,et al. (2016) used the multivariate statistical software PRIMER to analyze species selectivity. Their results indicated that a 30-mm square mesh is recommended for the fishing ofL. polyactisandT. haumelain autumn, while a 20-mm square mesh is suitable for the fishing ofE. fangi.Yan et al. (2018) used the cover method to study mesh size selectivity in the South China Sea. They found no significant seasonal difference in the minimum mesh size forC. mystusbetween spring and summer, which were 42.95 and 43.87 mm, respectively. Huang et al. (2018) used the cover method to study the mesh size selectivity of the square mesh escape window in the Yellow Sea. The size of the escape mesh forL. polyactisand theT. curvirostrisstow nets was estimated at 47.7 and 36.3 mm, respectively. Xu et al. (2020) also used the cover method to study the mesh size selectivity of stow nets and recommended a 40.59 mm mesh for targetingL. polyactis.

2.4.Selectivity of gillnets

Gillnets are a kind of traditional fishing gear in China’s marine fisheries, the annual output of which ranks second, accounting for about 21.76% of the total of China’s coastal fishery output (slightly lower than trawl). Gillnets have high selectivity and only capture a certain length range of target species; their effective length range is closely related to mesh size. The pursuit of the highest efficiency in the fishery by adjusting the mesh size leads to the coincidence of the range of dominant lengths and the resource structure. Therefore, imposing a minimum mesh size is the most effective management measure for gillnet fishery.The selectivity of gillnets was studied since the 1990s, with specific foci on the selectivity of various mesh sizes, gillnet types (i.e., single, double,or triple), and target species. The minimum mesh size of gillnets and the selectivity of different types of gillnets with the same mesh size are summarized in Table 5.

Table 5Selectivity research of gillnets in China.

2.4.1.Mesh size of gillnets

Because of the one-sided pursuit of economic bene fits by fishermen in fishing, the mesh size of gillnets has continuously decreased. This resulted in a high proportion of juvenile fish by-catch by gillnets, which severely damaged both fishery resources and the environment. To regulate the management of gillnet fisheries, it is necessary to identify the minimum mesh sizes of gillnets for targeting different species.

In the East and Yellow China Sea, the main target species of gillnet fishery areP. argenteus,L. polyactis, andS. niphonius. Li et al. (2010,2016b), and Xu et al. (2018) used the parallel control method to identify the minimum mesh size of gillnets in the East and Yellow China Sea. Li et al. (2010) suggested that the minimum mesh size of a gillnet targetingP. argenteusshould be 100 mm in reference to the currentP. argenteuscapture fork length in East China Sea. The use of triple gillnets should be prohibited. You et al. (2014) suggested that the optimal mesh size of gillnets for 3-4-year-oldS. niphoniuswas 121.5 mm during spring season in the central and southern Yellow Sea. Wang et al. (2016) suggested that a 55-mm mesh size of single gillnet used in Qidong, Jiangsu province, South Yellow Sea, effectively increased the release probability of juvenileL. polyactis, which is bene ficial for their protection. Li, Xiong,et al. (2016) suggested that the minimum mesh size of a single gillnet should be 50 mm in the East China Sea, as this meets the conservation needs of the main concurrent fishing speciesChelidonichthys spinosus(the spiny red gurnard) and maximizes fishing efficiency. Zhu and Xu(2018) suggested that the minimum mesh size of single gillnets for the minimum catchable size (150 mm) ofDentex tumifrons(the yellow sea breams) should be 72 mm. Xu et al. (2018) suggested that red tile fish can be targeted by triple gillnets, with a mesh size of 70.91 mm, as this can effectively protect current fishery resources.

In the South China Sea, the main target species of gillnet fishery areN. virgatus,S. commersoni, andH. nehereus. Zhang et al. (2010b) and Yang et al. (2016b) conducted sea trails to study the mesh size selectivity of gillnets forN. virgatus. Zhang et al. (2010b) reported that with decreasing mesh size, the species and quantity of catches increased, but the average weight of catches decreased. In 2014, the minimum mesh size was set to 50 mm forN. virgatusgillnet fishery by a circular from the Ministry of Agriculture. Yang et al. (2016b) argued that a 50-mm mesh size is not enough to protectN. virgatusresources and thus recommended to increase the minimum mesh size to 55 mm in the South China Sea.Yang et al. (2013, 2015) analyzed the relationship between the body shape ofS. commersoniand the mesh size of gillnets via the relative body girth. Their results showed that the capture position ofS. commersoniwas at the edge and the beginning of the second dorsal fin; the relative girth ratio of the captured fish ranges from 0.96 to 1.77, and the optimal relative girth ratio is 1.09. Yang et al. (2016a, 2017b) also studied the mesh size selectivity of gillnets targetingS. commersoni, showing that the relative effectiveness of six kinds of mesh sizes forS. commersoniwas 138＞142＞160＞100＞80＞120 mm. As the allowable size ofS.commersoniwas set to 380 mm, Yang et al. (2017b) recommended a minimum mesh size of gillnets of 120 mm. Yan et al. (2013) and Yang et al. (2015) analyzed the biological characteristics ofH. nehereusand the fishing performance ofH. nehereusgillnet fishery, based on fishing and on-site sampling data. Yan et al. (2013) found that the length range of caughtH. nehereuswas 166-252 mm, with a dominant length of 210 mm.The optimum and minimum mesh size were identified as 36 mm and 33 mm, respectively. Yang et al. (2015) recommended a minimum mesh size forH. nehereusgillnets of 35 mm, while Yan et al. (2016)recommended a size of 38.71 mm by mathematical modeling. To achieve sustainable use ofH. nehereusresources, it is recommended to strictly control the implementation of the minimum mesh size and select the fishing area and season carefully.

2.4.2.Types of gillnets

To study the selectivity of different types of gillnets targeting the same fish species, Guo et al. (2016) and Li et al. (2019) assessed catches,catch composition, dominant species of catches, and proportion of juvenile fish and compared the performance of single, double, and triple gillnets. They showed that single gillnets have better selectivity than triple gillnets for spindle-shaped fish; triple gillnets have a strong entanglement function, which is advantageous for catchingO. oratoria,Branchiostegus japonicus(the red tile fish),Cleisthenes herzensteini(the point head plaice), andLophius litulon(the yellow goose fish). Additionally, no obvious difference was found in fishery resource damage.Therefore, rationally formulating the mesh size of triple gillnets can achieve the purpose of resource protection. Xu et al. (2018) proposed to adjust the mesh size of triple gillnets to 70.91 mm when targetingB.japonicusin the East China Sea. Guo et al. (2016) and Li et al. (2019)proposed to include triple gillnets in the list of permitted fishing gear in Bohai Sea, and limiting the minimum mesh size of triple gillnets forPenaeussp (prawn) andO. oratoriato 60 mm. At the same time, the fishing area and fishing season where triple gillnets are allowed were stipulated, and the quantity of fishing gear should be strictly controlled.In this way, not only the conservation of fishery resources bene fits, but the stability of the fishery in the Bohai Sea can also be maintained.

2.5.Selectivity of pots

2.5.1.Steel-framed round pots

Steel-framed round pots (hereafter ‘round pots’, Fig. 5) are well known for their high species selection and have become the main fishing gear for crabs, such as the most important crustacean catchesP.trituberculatuandCharybdis japonica(the Asian paddle crabs) in the East China Sea and the Yellow sea (Zhang et al., 2010a; Yu. et al., 2021).However, the round pots present a high ratio of juvenile crabs due to their small mesh size that commonly used 32 mm in the East China Sea forP. trituberculatuand 25 mm in the Yellow Sea of China forC. japonica.

Fig. 5.Steel-framed round pots with an escape gap (left) or escape vent (right).(Sources from left to right: Zhang et al., 2010a and Zhang et al., 2021).

In order to release juvenile crabs, Wu (1996) designed the escape hole to install at the side frame of round pots by considering the fishing mechanism and behavior of crabs. The results showed that the round pots with an 8 cm diameter escape hole can protectP. trituberculatusfishery in the East China Sea. Zhang et al. (2010a) designed the escape gap on the side frame of round pots to release juvenile crabs. The results showed that the height of the escape gap should be 33 mm if 125 g is applied as the allowable capture standard weight ofP. trituberculatus.Additionally, the escape vent exerts almost no effect on the survival rate of escaping crabs. Bai et al. (2015) compared the catch frequencies ofP.trituberculatusamong experimental round pots with rectangular escape vent of 25 mm or 30 mm height, control round pots, and control stow nets. The results showed that individuals captured in the stow net were smaller than those captured by control round pots. Furthermore,large crabs were more likely to find and enter round pots than small crabs. Thus, population assessments would be biased if only the encounter selectivity information was considered while information about catchability was omitted. Zhang et al. (2020) conducted a series of sea trials to assess the effect of increasing mesh size on fishing efficiency and analyze the mesh size selectivity of round pots forP. trituberculatusthrough a comparison experiment. The results showed that it may be impractical to effectively release undersized crabs by simply relying on increased mesh size in crab pot fisheries due to that the 50% selection carapace width ofP. trituberculatuswas considerably smaller than the minimum landing size set by the Zhejiang Province fishery regulations even with 59.2 mm mesh size of the experimental pot. Then, Zhang et al.(2021) tested round pots with one and two escape vents and compared the catchability and size selectivity of these round pots with control round pots without an escape vent. The results showed that both one and two escape vents of round pots caught a significantly smaller number of sublegal-sized crabs whose carapace width is less than 116 mm, and there is no significant difference in size selectivity between the two types of round pots forP. trituberculatus. Yu et al. (2021) conducted a series of sea trials to explore a method to improve the size selectivity forC.japonicaby mounting escape vents on the side panels of each round pot. The results recommended the round pots with an escape vent size of 70 mm ×30 mm for the sustainable development ofC. japonicain the Yellow Sea of China.

2.5.2.Whitespotted conger pot

Conger myriaster(the whitespotted conger) is an important fishing target for China’s fisheries. In addition to trawl and stow net, China’s coastal fishermen also use longline and eel pots for targeting the whitespotted conger. As mesh sizes are small and selectivity is extremely poor, eel pots impose enormous damage to juveniles ofC. myriaster.Tang et al. (2010c) used the cover net method in a water tank in the laboratory to study the mesh size selectivity of eel pots. The results showed that the L50and selection range ofC. myriasterincreased with increasing mesh size of eel pot, and further increasing the mesh size of the eel pot effectively releases juvenile eels.

2.5.3.Accordion-shaped trap

The accordion-shaped trap, a unique pot used only in China, is the most commonly used gear in Chinese artificial reef areas (Fig. 6). Its target species are bottom-dwelling fish and crabs (Zhang et al., 2016c).Zhang et al. (2013, 2016c) observed the ingress behavior ofC. japonicaentering the accordion-shaped trap in water tank and conducted a series of sea trials in the artificial reef area of Zhuwang, Laizhou Bay, China.Both results showed that a 4.3 cm ×3.0 cm escape vent was considered appropriate forC. japonicafishing. Tang, Zhang, et al. (2019) simulated cover-net method to study selectivity of different mesh sizes and hanging ratios forSebastes schlegeliis(the black rock fish) in a fume tank.The results showed that a 70 mm mesh size with a 0.6 hanging ratio of fishing gear was the suitable size for protectingS. schlegeliis. However,the currently used accordion-shaped traps forS. schlegeliiwith a mesh size of 2 cm present poor size selection and affect fishermen’s ability to effectively utilize theS. schlegeliiresource. Tang, Liu, et al. (2019)explored a method to improve the size selectivity of accordion-shaped traps by mounting escape vents at the ends of each trap (Fig. 6 right).The result showed that the 7 cm escape vent size can be considered a gear option for sustainable resource utilization which was found to retain most of the matureS. schlegelii, while releasing the majority of immatureS. schlegelii. Ren et al. (2021) investigated the catch performance of accordion-shaped trap with different mesh sizes (20, 30, 40,50, 60 mm) was by using the cover net method in Laizhou Bay of Bohai Sea. The results showed that there 8 species were dominant (e.g.,O.oratoria,C. japonica) of 37 species in catches and the proportion of main speciesO. oratorianot reaching the length of capture was the smallest in accordion-shaped trap with 50 mm of mesh size. Through comprehensive analysis, the accordion-shaped trap with the minimum mesh size of 50 mm meets combined with the 7 cm escape vents at the ends of trap will meet the requirements of main catch species production and release the juvenile of dominant species.

Fig. 6.Accordion-shaped trap with escape vent.(Sources from left to right: Zhang et al., 2016; Tang, Liu, et al., 2019).

3.Methodologies applied in the reviewed papers

3.1.Methodologies increase selection performance

The research involved the effect of change in mesh size, mesh shape and structure (e.g., diamond mesh, square mesh and T90 mesh), and mesh number in the circumference on the selective performance (Song et al., 2015; Yang et al., 2002a, 2002b; Bayse et al., 2016; Sala et al.,2016, 2017). The shrimp beam trawl has also related to shrimp- fish separator devices (e.g., separator panels, vertical separator panels, and sorting grids) to improve intra-species selectivity and reduce by-catch of non-target fish (Luo et al., 2007; Sun & Yao, 1998; Yang et al., 2018b;Zhang et al., 2008). Bycatch excluding grid is also used in the trawls or stow nets to release large marine creatures such as turtles, mammals,sharks, etc (Brewer et al., 2006; Brˇci´c et al., 2015; Hamilton & Baker,2019). The escape vent or escape hole were installed on the pots to release the juveniles of targeting speciesP. trituberculatus,C. japonica(Zhang et al., 2010a, 2016c, 2020, 2021; Bai et al., 2015; Tang, Liu,et al., 2019; Wu, 1996; Yu et al., 2021; Zhang & Zhang, 2013).

3.2.Methodologies for experimental

The covered net method and paired-gear method (e.g., twin trawl,trouser trawl, parallel haul and alternate haul) are the main experimental methodologies applied in towed fishing gear (Wileman et al.,1996). In the reviewed papers, cover net method were the main method used in the comparative study of codend mesh size selectivity both trawl nets and stow nets (Yang et al., 2021a, 2021b, 2021c; Chen et al., 2018;Huang et al., 2005; Rui, 1984), and the paired-gear method were often used in the double-codends or multi-codends trawl (Zhang & Sun, 2008;Zhang et al., 2010; Yang et al., 2019; 2020). While studies on the selectivity of gillnets and pots mainly used the paired-gear method due to the characteristics of these two types of fishing gear (Li et al., 2010,2016b; Xu et al., 2018). SUN et al., (2004) demonstrated that the 50% retention lengths, the selective ranges and the selective factors ofS.tenuifilisobtained from the parallel haul method are respectively 7.8 percent larger, 24.5 percent less and 7.4 percent larger than those estimated from the cover net method, but no significant differences among them were found. Madsen and Holst (2002) study showed similar results, also no significant difference between the two methods. Furthermore, Herrmann et al. (2016) showed that the sampling effort needed to achieve a specific uncertainty level for the selection parameters was always around 10 times higher for the paired-gear method than for the covered net method. Therefore, the covered net method is still one practical method for selective research on trawl nets and stow nets by adopted hooped cover, kites cover or other methods separating the codend and cover to reduce the impact of cover netting on fish escape(Madsen & Holst, 2002; Wileman et al., 1996).

4.Application of selectivity results in fishery management

4.1.Minimum mesh size of different gear types

Limiting the mesh size and formulating fishery regulations from the basis for protecting fishery resources. Selectivity results have been used for regulating fishing gear, e.g., by fishing gear admittance systems in China. The GB 1779-1989 and “Notice of Ministry of Agriculture 2003”stipulated a minimum inner mesh size of the trawl codend of 54 mm for the East China Sea and the Yellow Sea. The GB 1780-1989 and “Notice of Ministry of Agriculture 2003” stipulated a minimum inner mesh size of the trawl codend of 39 mm for the South China Sea. The GB 1780-2005 was promulgated in 2005 according to the research during 2002-2004 by Yang et al. (2002a) and Zhang et al. (2004) from the South China Sea Research Institute of Chinese Academy of Fishery Sciences. GB 1780-2005 stipulated a minimum inner mesh size of the square mesh codend of 40 mm (front and rear are diamond mesh, and the middle segment is square mesh) of a single layer after soaking shrinkage.

The “Notice of Ministry of Agriculture 2003” allowed only few types of fishing gear. In 2009, the Ministry of Agriculture launched a special investigation on marine fishing gear that was initially completed in the National Marine Fishing Gear Catalogue. The catalogue divides 85 kinds of marine fishing gear into the following three categories: permitted (30 kinds), prohibited (13 kinds), and transitional (42 kinds). Corresponding restrictions on minimum mesh size, fishing gear specifications, and carrying quantity were set. In 2013, the Ministry of Agriculture and Rural Affairs issued the “Notice concerning the banning of the use of 13 types of fishing gear, including two-boat single-piece multi-bag trawl”and the “Notice of the Ministry of Agriculture on Implementing the Minimum Mesh Size System for Marine Fishing Specimens and Transitional Fishing Gears”, effective from January 1 and June 1, 2014,respectively. These notes are based on a series of studies on fishing gear selectivity. The mesh sizes of 45 main marine fishing gear were limited in these notices.

4.2.Minimum mesh size for economically important fish species

To effectively protect juvenile fish resources and promote the restoration and sustainable use of marine fishery resources, the Ministry of Agriculture issued the “Circular of the Ministry of Agriculture on implementing the allowable size of capture for 15 economically important fishes and the management system for the ratio of juvenile fish” on February 11, 2018 (Appendix 3). The Ministry of Agriculture requires that China’s fishery researchers should study the relationship between the minimum mesh size and the selectivity parameter L50with various mesh structures in each fishing area of China according to the minimum catchable standard as specified in the notice. In 1968, the FAO recommended that the mesh size should be selected to retain half of all fish of critical size. This means that the allowable size of capture is equal to the L50. Seven important economic fish species have been studied, and the corresponding minimum mesh sizes are listed in the following:71.7-85.9 mm forT. japonicus, 66.9-71.1 mm (or 66.6 mm for square mesh) forL. polyactis, 63.8 mm (or 51.5 mm for square mesh) forD.maruadsi, 104.2 mm forP. argenteus, 129.8 mm forS. niphonius, 101.6 mm forS. japonicus, and 75.3 mm forTrachurus japonicus(the Japanese jack mackerel).

5.Summary and discussion

5.1.Fishing gear selectivity research institute

Since the 1980s, the Chinese Academy of Fishery Sciences (Yellow Sea, East China Sea, and South China Sea Fisheries Research Institute),Shanghai Ocean University, Ocean University of China, Dalian Ocean University, Marine Fisheries Research Institute of Zhejiang Province,and Zhejiang Ocean University have studied fishing gear selectivity. So far, 109 research papers have been published (see detail in Fig. 7).Among these papers, the majority were published by the South China Sea Research Institute of the Chinese Academy of Fishery Sciences (35)and the Shanghai Ocean University (28). In addition, Ocean University of China, Shanghai Ocean University, Dalian Ocean University, and Zhejiang Ocean University have trained 13 talents who study fishing gear selectivity in China (see detail in Fig. 8). These efforts lay a solid foundation for the development of China’s fishing gear selectivity research in the future.

Fig. 7.Research papers on fishing gear selectivity by the Fisheries Research Institute of China.

Fig. 8.Theses on fishing gear selectivity from four Ocean Universities of China.

5.2.Fishing gear selectivity in China

Research on the selectivity of Chinese fishing gear began by assessing bottom trawls, and then focused on shrimp trawls, stow nets, gillnets,pelagic trawls, pots, and other fishing gear. The conclusions of fishing gear selectivity research over the past 40 years are summarized in the following:

(1)The minimum codend mesh size of the bottom trawl targetingL.polyactisandT. haumelashould be no less than 60 mm in the East China Sea; the minimum mesh size of the pair pelagic trawl in the Yellow Sea should be no less than 56 mm; the bottom trawl in the South China Sea should use square mesh with a minimum mesh size of no less than 40 mm because of the variety of catch species (Yang et al., 2003).

(2)Shrimp beam trawl using shrimp/ fish separator devices (e.g.,separator panels, vertical separator panels, or sorting grids) can improve intra-species selectivity and reduce non-target fish bycatch; the minimum mesh size of shrimp trawl should be no less than 35 mm to reduce by-catch of juvenile shrimp and appropriate adjustments should be made based on the main targeted shrimp species.

(3)The minimum mesh size of canvas stow nets should be no less than 55 mm, which meets both the economic and conservation goals for main the target species ofL. polyactis,T. haumela, andP.argenteusin the Yellow and East China Sea; the minimum mesh size of double-stake stow nets should be no less than 35 mm; to identify the best codend structure and minimum mesh size of new kinds of single stake stow net using canvas devices instead of the traditional beam structure as expansion devices, further research is needed.

(4)Gillnets are a kind of highly selective fishing gear that only captures individuals with a certain length. The range of effective body length is closely related to the mesh size of gillnets. The minimum mesh size of gillnets should be determined in terms of the minimum mature body length of the targeted fish species, and the optimal mesh size of gillnets is determined in terms of the composition of body lengths of target species. The target species vary with the mesh size of gillnets, and the species that can be targeted with the same mesh size are relatively similar for various types of gillnets.

(5)Installing an escape vent at a height of 33 mm or an 8-cm-diameter escape hole on the side of crab pots can improve catch performance and selectivity; increasing the size of eel pots can effectively release juvenile eel.

This research provides a scientific basis for the development of a fishing gear admittance system and the formulation of minimum mesh sizes for fishing gear in China. However, the composition of the catch is complex and varied because of the variety of species in China’s coastal fishing grounds. Therefore, fishing gear selectivity research should comprehensively consider the resource status of each species. Currently,China’s fishing gear selectivity research mostly focuses on short-term tests of single fish species, while lacking systematic long-term tests,and failing to consider the potential impact of uncontrollable factors in the test. The test methods used to study selectivity lack uniform standards and different methods are commonly not compared. With the strengthening of China’s fishery management system and the implementation of a suitable fishing gear access system for the coastal waters of China, research on fishing gear selectivity will be strengthened. The Fisheries Research Institute of China should conduct systematic, longterm, andtrans-area investigations on fishing gear selectivity. The catch characteristics of different fishing gears during various fishing seasons in each sea of China should be fully recognized, and the protection of fishery resources and the interests of fishermen should be comprehensively considered. The formulation of a fishing gear access system that meets the goals of sustainable development, scientific management of resources, and the rational development of fishery resources holds great significance for the conservation and restoration of fishery resources in the coastal waters of China.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Grand No. 31902426), the Shanghai Sailing Program (19YF1419800), and the Special project for the exploitation and utilization of Antarctic biological resources of the Ministry of Agriculture and Rural Affairs.

Appendix

Appendix 1.Notice on the implementation of the minimum mesh size system for marine fishing nets by the Ministry of Agriculture, People’s Republic of China (issued in 2003)

Appendix 2.Notice on the implementation of the minimum mesh size system for marine fishing nets by the Ministry of Agriculture, People’s Republic of China (issued in 2013)

Appendix 3.Allowable sizes of capture for 15 key economic fish species in Chinese coastal waters