Fishery resources of the Gulf of Tonkin, Vietnam. The state of indicator species monitored by bottom trawl surveys

Abstract

In the period 2001-2004 four bottom trawl surveys were carried out in the Gulf of Tonkin to investigate the aquatic stocks in the Vietnamese sector (67,370km2). The Gulf has tropical characteristics and great diversity of commercial species, but lately fishing pressure seems to have grown excessively. The swept-area cruises followed a stratified random design, with a fixed station grid imposed on four depth intervals between 0 and 100m, and internationally accepted survey protocols. Three commercially important indicator species which differ in life-history and habit were selected for appraisal using commonly accepted single-species metrics. The methodology for data treatment included spatial statistics (kriging) to describe seasonal trends in distribution, and non-parametric re-sampling (bootstrapping) of station data combined with maximum likelihood analysis of maturation curves to estimate both total and spawning standing biomass. Chinese squid (Loligo chinensis Gray 1849), a small pelagic resource with very high turnover rate, showed a marked decline in standing biomass, from 16,000 tones in 2001 to 3,000 tones in 2004 (CV 27-14%). Remarkably, spawning biomass remained at stable 1,000 tones, and large squid (13cm ML and above) dominate the population. The Greater lizardfish (Saurida tumbil (Bloch, 1795)), a very valuable, and slow growing, demersal species, showed stable levels of total biomass, at around 3,000 to 5,000 tones (CV 13-25%). However, spawning biomass showed a marked 22% decline to 732 tones in 2004, despite a decrease in the maturation size, which might be a compensatory mechanism. Simultaneously, most of the stock was comprised of fish in the 11- 24cm range, and this is a marked constriction from the original 5-35cm range. The Largehead hairtail (Trichiurus lepturus Linaeus, 1785), a large pelagic, has become the major commercial pelagic species in the South China Sea. In the Gulf the total biomass declined from 3,600 to 2,600 tones (CV 24- 29%), but a highly unreliable estimate (CV 50%) raises it to 15000 tones in 2004. The estimates of spawning biomass showed equal development, varying from 2,500 tones in 2001 to 1,500 tones in 2003, and raising to 4,000 tones in 2004. Largehead hairtail also shows a marked constriction of sizes in 2004, and the largest sizes are now about 1/3 of L∞. Although the squid showed a permanent coastal affinity (20-30m deep) and is exposed to the extremely large and un-controlled coastal fleet year round, it seems to have the best potential for recovery. The Largehead hairtail and the Greater lizardfish showed coastal affinity during the NE monsoon but moved to offshore grounds in the SW monsoon. Management of these transboundary populations is not sole responsibility of Vietnam and requires co-ordination with China in the South China Sea.