Shrimp beam trawl is one of the most important fishing gears in the South China Sea (Yang et al., 2002a). It has been estimated that there were 600 shrimp beam trawlers (125 092.1 kW) operating in the coastal waters of Guangdong province, and harvesting a great amount of by-catch product (Yang et al., 2015). The target species of shrimp beam trawl in the South China Sea are Greasyback shrimp (Metapenaeus ensis) and Shiba shrimp (Metapenaeus joyneri), but their production substantially vary according to the fishing seasons and fishing grounds. At some seasons, especially in the spring, they were often replaced by banded scad (Caranx (Atule) kalla) to be the main target species. The banded scad, used to be a by-catch species of bottom trawls in the South China Sea, has gradually become the commercially valuable target species of trawls and gillnets, as other traditional fish stocks have already deteriorated.
Trawling fisheries, especially shrimp trawl fisheries in tropical and subtropical areas, are notorious for their by-catch and discards problem (Hall et al., 2000; Kelleher, 2005). The situation is the same to shrimp beam trawl fishery in the South China Sea, because relatively small meshes are needed to retain the shrimp species and they usually operate in multispecies fishing grounds. Diamond mesh codends with 20 or 18 mm mesh size are often used in commercial fishing of shrimp beam trawl. The selectivity offi shing gear plays an important role in the exploitation of fish stocks and fisheries management (Mahjoub et al., 2011). It is necessary to investigate the selectivity of shrimp beam trawl to its main species, due to its poor selective properties. There are some studies on the selectivity of diamond mesh codend and square mesh codend of bottom trawl fishery in the South China Sea (Yang et al., 2002b, 2002c, 2003). According to their studies, the diamond mesh codend is good at releasing flat fish, while the square mesh codend has better selective properties for round fish. Some researcher suggests that a combined diamond and square mesh codend might be ideal for trawl fisheries in the South China Sea (Yang et al., 2003). In China, shrimp beam trawl belongs to the same legislative category of fishing gears as bottom trawl. But the fishing gears and methods of shrimp beam trawl are completely different from those of bottom trawl. Because the bottom trawl fishing vessels usually drag one trawl-net using otter boards, and the dimension of its net is much larger than that of shrimp beam trawl. While fishing vessels of shrimp beam trawl use two sticks (or beams) to operate their nets, they can simultaneity drag as much as 20 nets at one fishing operation. At present, most studies of selectivity focused on the bottom trawl, few works about the selectivity of shrimp beam trawl has been done in China (Yang et al., 2002b, 2003; Huang et al., 2016).
At present, there is no regulation about the minimum landing size (MLS) of banded scad in the South China Sea. According to Chen and Jiang (1990), the first maturity length for female banded scad is 67 mm total length, for male banded scad that length is 60 mm. In this study, the average maturity length of female and male banded scad, 63.5 mm total length, is temporarily used as its MLS in the present study.
In 2013, a minimum mesh size of 25-mm length of bar for the diamond mesh codend of the shrimp beam trawl was promulgated by the Chinese government. But this minimum mesh size has not been validated, and selectivity of diamond mesh codend of shrimp beam trawl to main target species is still unknown. A lot of work have been done to compared the selectivity of square mesh codend with that of diamond mesh codend (Broadhurst et a., 2006; Deval et al., 2009; Sala et al., 2015), most of these work proved that the selectivity of square mesh codend is better than diamond mesh codend. But in multispecies trawl fisheries, a combined netting codend of diamond mesh and square mesh was suggested, as to some species, such as flatfish, the selectivity of diamond mesh codend is better than that of square mesh codend (Yang et al., 2003; Frandsen et al., 2010, 2011). At present, there is no study about the selective properties of combined square mesh and diamond mesh codend reported in the South China Sea. The aim of our study is to test two traditional diamond mesh codends, codend with mesh size of 25-mm and codend with mesh size of 30-mm, and compare their selectivity with those of four combined square mesh and diamond mesh codends.2 MATERIAL AND METHOD 2.1 Fishing experiment
A series of fishing experiments were conducted in the northern South China Sea in August 2014, August 2015, and August 2016, respectively. The main fishing area was 21°33′–21°48′N, 112°07′–112°20′E, in which the water depth was mainly about 5–12 m, and sand and mud in the bottom. Two traditional shrimp beam trawl vessels were used in fishing experiments, named "Yueyangdong 12507" (length overall: 16 m, engine power: 79 kW) and "Yueyangdong 12081" (length overall: 21 m, engine power: 98 kW).
A commercial shrimp beam trawl-net was used in the experiments. The total stretched length of the net was nearly 7.15 m, with a net mouth of 375 meshes (with a mesh size of 28-mm). Two sticks, both with a length of 2.2 m, were equipped around the mouth of the net, the upper one was made of bamboo, and the other was made of steel. During fishing operation, two large beams (nearly 20 m in length) were stretched at both sides of the vessel, and then 12 trawl-nets were shot and equipped in order. In our experiments, all tested codends were placed at the same position and fished with others commercial nets, respectively.
Six codends with different mesh sizes and configuration were designed. The first and second codend represent the traditional diamond mesh codends, with mesh size 25 (D25) and 30 mm (D30), respectively. The other four codends were novel designs, made of combined square and diamond mesh codends, in which the square mesh codends were placed in front part connected with the anterior extension section of the net. The combined netting codends were defined as the S35+D18, S25+D25, S30+D25 and S35+D25 codend, respectively, according to their mesh sizes of the square mesh and diamond mesh (Fig. 1 and Table 1). All these codends were made from polyethene (PE) single twine, and had the same stretched length (approx. 1.5 m) and circumference (approx. 0.5 m, with a hanging ratio of 0.35 for diamond mesh codends). The experiments were conducted with the covered codend method. The cover was made of single twine PE netting with a nominal mesh size 15 mm. The specification of tested codends and cover were listed in Table 1 and Fig. 1. Under the test schedule, all designed codends were planed to have at least 10 replicated hauls test. However, as some unpredicted situation occurred, for instance, the trawl nets were blocked by great amount of jellyfish, some codends could not reach that level.
For each haul, all catches from both codend and cover were handled separately, and classified into species levels. Each of species were weighed and counted, and the total length was measured. Due to large number of individuals, random sub-sampling was conducted at some hauls. All data were collected onboard.2.2 Data analysis
Selective curves were fitted by the logistic equation as given by Fryer (1991) and Wileman et al. (1996). In the logistic equation, the retention probability of a fish with l length entering the codend can be expressed as follow:
where r(l) is the probability that a fish of size class l would be retained, v1 and v2 are selective parameters to be estimated. By the parameters v1 and v2, the 50% retention length (L50) and selection range (SR) can be obtained as: