Effects of the trawling ban on demersal crustacean resources (orders: decapoda and stomatopoda) in the marine environment of Hong Kong

Abstract

Trawling is considered to be one of the most destructive fishing methods to marine ecosystems worldwide as it homogenises benthic habitats, impacts the structure and functioning of benthic communities, and depletes fishery resources. To facilitate recovery of benthic ecosystems and associated fishery resources, the Government of the Hong Kong Special Administrative Region has implemented a ban on trawling activities in its territorial waters since 31 December 2012. This study aimed to investigate whether the status of demersal crustacean resources improved after the trawl-ban. Standardized field surveys were conducted in eastern and western waters during 2003–2007 and 2012–2016 respectively, and in southern waters during 2012–2016. The statuses of commercially important crustaceans in eastern and western waters pre-trawl-ban were assessed. Biomass, species richness and proportion of carnivorous crustacean species in western waters were significantly higher than that of eastern waters. The crustacean assemblages were impacted considerably by multiple stressors such as eutrophication and overfishing. Diversity, abundance and biomass of commercially important crustaceans from pre-trawl-ban (2004) and post-trawl-ban (2013–2014 and 2015–2016) periods were compared. Increased diversity (J’ and EXP H’) in Tolo Channel of eastern waters (EI) and higher abundance and biomass of crabs in Outer Tolo of eastern waters (EO) were detected post-trawl-ban. In the outer estuary of western waters (WO), a significant decrease in the level of disturbance was observed post-trawl-ban, with increased biodiversity in crustacean assemblages and higher abundance and biomass of crabs. No positive changes were detected in southern waters. The results of stable isotope analyses on crustacean samples collected in the dry season of 2012 and 2015 revealed that the trawl-ban led to improvement of trophodynamics in some sites. The community in EI and the inner estuary of western waters (WI) showed longer food-chain-lengths (FCL), higher trophic diversity (TA) and mean trophic levels of carnivorous crustaceans (MTLoC) in 2015 when compared with that of 2012. Longer FCL and greater TA were recorded in the crustacean community of EO in 2015. WO community showed an increase in FCL and MTLoC in 2015. However, the communities in the southern waters remained unimproved in the stable isotope profile and displayed decreases in the key trophic parameters. Population dynamics of four dominant predatory mantis shrimp species in western waters among the three periods (i.e., 2004, 2013–2014 and 2015–2016) were examined. The results confirmed the detection of recovery signs in all four mantis shrimp species, with significant increases in mean weight and mean carapace length three years after the trawl-ban. Overall, initial signs of recovery of benthic crustacean communities were observed in eastern and western waters, especially in WO, only three years after the trawl-ban. Although a full recovery of the stressed coastal environment requires a longer period, it can be hindered by the ongoing marine construction works (e.g. Third Runway System), increased fishing pressure from non-trawling means, and illegal trawling activities. This study established a baseline for future comparison for the investigation of any long-term effect and effectiveness of the trawl-ban as a management intervention.