Length-Based Assessment of Fish Stocks in a Data-Poor, Jointly Exploited (China and Vietnam) Fishing Ground, Northern South China Sea

Abstract

The Beibu Gulf is one of the most important fishing grounds in the South China Sea (SCS), and the fisheries resources in this area are exploited by both China and Vietnam. In recent decades, some indications of overfishing have appeared, including declining catch rates, frequently changing catch composition, and shrinking body sizes in main commercial fish species. Due to limited data availability, only a small subset of exploited fish stocks in this area has been assessed. Here, we applied two length-based methods, electronic length frequency analysis (ELEFAN) and length-based Bayesian biomass estimation (LBB), to stock assessment of nine exploited fish species in the Beibu Gulf. There were total 53, 652 length records of 30 target stocks used in this study during the survey period from 1960 to 2015. The results showed that the two length-based methods presented different ability in estimating exploitation rate (E), and the estimated E ranged from 0.34 to 0.87 using ELEFAN method while ranged from 0.26 to 0.86 using LBB method. The prior information from ELEFAN method was effective for LBB method, as there were significant differences in 66.7% of the 30 target stocks in estimated Linf, and 93.3% in estimated B/BMSY, using LBB method with and without prior information. The estimated Lc/Lc_opt and B/BMSY of LBB method suggest a pressing situation for the fisheries in the Beibu Gulf, as 86.7% of the 30 target stocks had been suffering from growth overfishing (Lc/Lc_opt < 1), and 83.3% had been overexploited or fully exploited (B/BMSY ≤ 1.2). In addition, we suggest using both ELEFAN and LBB methods to fit length-frequency data of data-poor fish stocks because they are complementary in estimating management reference points. We also emphasize collaboration mechanism should be established by China and Vietnam for the sustainability and recovery of fishery resources in the Beibu Gulf.

This is what we know: Assessing the stock status of the data-poor skipjack tuna (Katsuwonus pelamis) fishery in the South China Sea

“…The SCS occupies a central position in the Indo-West Pacific region, and is the third largest marginal sea in the world (Li et al, 2019). Fish diversity and fisheries resources within it are extremely rich (Zhang et al, 2021a), there are limited data available for many of its fisheries (Zhang et al, 2017). Shallow water (< 200 m) fisheries resources in the northern SCS had been overfished in the 1990s (Zhang et al, 2017;Zhang et al, 2021a).…”

“…We regard a stock to be healthy if B/ B MSY > 1; slightly overexploited where 0.8< B/B MSY ≤ 1; overfished where 0.8 ≤ B/B MSY ≤ 0.5; heavily overfished where 0.2< B/B MSY ≤ 0.5, and collapsed when B/B MSY ≤ 0.2 (Palomares et al, 2018). Stocks are also considered to suffer from 'growth overfishing' (when fish are harvested at an average size that is smaller than the size that would produce the maximum yield per recruit) if the estimated L c /L c_opt < 1 (Zhang et al, 2021a), where L c_opt represents the length at first capture that maximizes the catch and biomass. More details of LBB…”

“…Nevertheless, only one fifth of the global landings comes from assessed species, and less than one eighth of the world's fisheries are sufficiently well managed or have detailed stock assessments for management purposes (Costello et al, 2012;FAO, 2022;Kindong et al, 2022). The severity of this problem is increasingly recognized, and various types of data-poor stock assessment methods have been put forward over the past 20 years as part of a solution (Zhang et al, 2021a;Liao et al, 2022). Among these methods, length-based assessment models are widely used because length-frequency data are rather cost-effective and easy to collect (Quinn and Deriso, 1999;Hordyk et al, 2015;Hordyk et al, 2016;Mildenberger et al, 2017;Froese et al, 2018;Chong et al, 2020).…”

High migratory propensity constitutes a single stock of an exploited cutlassfish species in the Northwest Pacific: A microsatellite approach

“…A downward trend of the total catch of cutlassfish in the NW Pacific area has been reported [ 131 – 134 ]. Some studies suggested that persisting fishing pressure had changed their growth and reproduction pattern, as seen in their precocious maturity and miniaturization [ 5 , 8 , 135 ], and this led to fitness reduction [ 136 ].…”

Evaluating Catch-Only Methods to Inform Fisheries Management in the East China Sea

“…Most data-limited methods which were employed to infer the status of fish populations in China Seas are catch-based or length-based (e.g., Zhang et al, 2018;Zhai et al, 2020;Han et al, 2021;Liao et al, 2021;Pauly et al, 2021;Zhang et al, 2021). However, either catch-based or length-based methods often produce contrasting estimates of stock status when using different models (Anderson et al, 2017;Pons et al, 2020;van Gemert et al, 2022), which is hard to objectively inform fisheries management.…”