Comparing size selectivity and exploitation pattern of diamond-mesh codends for Southern velvet shrimp (Metapenaeopsis palmensis) in shrimp trawl fishery of the South China Sea

Abstract

In this study, size selectivity and exploitation pattern of six diamond-mesh codends with different mesh sizes, ranging from 25 to 54 mm, for Southern velvet shrimp (Metapenaeopsis palmensis) were tested and compared in a shrimp trawl fishery of the South China Sea (SCS). We used a codend with a mesh size of 25 mm (D25) as a starting point, which is the minimum mesh size (MMS) currently regulated in the studied area. Four different fishing population scenarios were applied to quantify and compare how mesh sizes of codends used would impact the size selectivity and exploitation pattern for the target shrimp species. The results demonstrated that the D25 codend was not proper for protecting juvenile shrimp at the studied area. By applying this legal codend, L50 (50% retention length) of the target shrimp species was below its minimum conservation reference size (MCRS, 7.0 cm total length), the retention probability of shrimp with a length of MCRS was above 95% CI [91–99] and more than 43% of undersized shrimp was retained. To mitigate the bycatch issue of undersized shrimp, increasing the mesh size in the diamond mesh codend is a simple and effective option. However, the loss of catch efficiency for marketable shrimp is a major concern while increasing the mesh size. A good compromise between releasing undersized shrimp and maintaining the legal individuals is manifested by using the codend with 35 mm mesh size (D35). Our study will be beneficial for the management of shrimp trawl fisheries in the SCS.

Effect of codend mesh sizes on the size selectivity and exploitation pattern of cocktail shrimp (Trachypenaeus curvirostris) in shrimp trawl fishery of the South China Sea

“…L50 and selection range (SR) can be referred to as parameters for size selectivity. Usually, larger mesh sizes in the codend bring about larger L50 values so that more undersized individuals of the target species can be released (He, 2007;Melli et al, 2020;Yang et al, 2021a;Yang et al, 2021b). However, the probable increase of SR values, if exceeding a certain extent, would induce the loss of marketable individuals.…”

Simple and effective: T90 codends improve size selectivity and catch efficiency compared with diamond-mesh codends for mantis shrimp (Oratosquilla oratoria) in demersal trawl fishery of the South China Sea

“…Following the recommendation of Wileman et al (1996), the cover net was 1.5 larger and longer than the tested codends, and its mesh opening was about 12 mm (Table 1). In order to remove the potential covered effect, a total of 12 flexible kites were mounted to the covers (He, 2007;Grimaldo et al, 2009;Yang et al, 2021). Additionally, underwater video recordings systems, composed of GoPro HERO4 and a framework, were applied to check how these kites work during the experiments.…”

“…The water depth in the fishing grounds varied between 18 and 39 m. All fishing procedures were identical to the ones of commercial fishing. Prior to the experimental fishing in 2020, there was a similar codend selectivity experiment, in which mantis shrimp served as a bycatch species, carried out by the same vessel on the same grounds (for detailed information, please refer to Yang et al, 2021). The experimental data of this earlier sea trial can be used to supplement the present study by generating another fishing population scenario of mantis shrimp.…”

“…This MMS regulation is simple, but its effectiveness is questionable. Recently, Yang et al (2021) conducted a selectivity experiment using six diamond-mesh codends, with mesh sizes ranging from 25 to 54 mm, in the SCS. The target species was Southern velvet shrimp (Metapenaeopsis palmensis), whose body size is often smaller than that of mantis shrimp.…”

“…The target species was Southern velvet shrimp (Metapenaeopsis palmensis), whose body size is often smaller than that of mantis shrimp. Their results demonstrated that the legal codend with a 25-mm mesh size was not proper for protecting juvenile shrimp and that a mesh size increased to 35 mm selectivity would be better (Yang et al, 2021). Will increasing the mesh sizes of diamond-mesh codend improve size selectivity for mantis shrimp in the SCS and substituting the T0 codend with T90 result in a positive outcome?…”