Effects of the trawl ban on demersal fish communities in the marine environment of Hong Kong, South China

Abstract

Fisheries resources of Hong Kong had been overexploited by the 1970s due to a drastic increase of mechanized fishing vessels, particularly bottom trawlers. The trawlers catch marine organisms non-selectively, and exert severe physical damage to benthic ecosystem. To rehabilitate the depleted fisheries resources, a year-round ban on trawling has been implemented in territorial waters of Hong Kong since 31 December 2012. This study aims to investigate whether the trawl-ban can facilitate recovery of local demersal fish communities through a field-based study.

Standardised surveys using shrimp trawlers were conducted with a valid scientific research permit. During 2003–2007 and 2012–2016, two demersal fish communities in eastern and western waters, respectively, were sampled. Two additional communities in southern waters were sampled during 2012–2016 only. Pre-trawl-ban status of demersal fish resources in eastern and western waters was assessed. The fish communities were in poor condition as low-valued, small-sized, short-lived, fast-growing fish species of low trophic level were dominant, while commercially-valuable, large-sized, high trophic species were rarely caught.

Data collected during wet and dry seasons of 2004, 2012 (pre-trawl-ban) and 2013–2015 (post-trawl-ban) were compared. At inner Tolo (EI) and outer Tolo (EO) of eastern waters, there were higher total abundance, proportion of predators and abundance of large-sized fishes in dry season after the trawl-ban. Significant changes in community composition were also detected at EI and EO during dry season, with more higher trophic fishes after the trawl-ban. At inner estuary (WI) of western waters, positive responses were observed with increases in abundance of large-sized individuals and mean maximum length in wet season. Moreover, data from 12 consecutive months of 2004, 2013/14 and 2015/16 were also compared. There were more fishes from low and high trophic levels, and increased species richness and large-sized fishes at EI after the trawl-ban. Increased total abundance and biomass, and dominance of predators in terms of absolute number and percentage were detected at WI.

Stable isotope analyses were employed to investigate the trawl-ban’s influence on trophic structure in dry season when there is less interference of terrestrial inputs of carbon and nitrogen associated with rainfall. The EI, EO and WI communities had greater trophic diversity in terms of δ15N range, δ13C range and total area of the δ13C-δ15N bi-plot space after the trawl-ban in 2015 when compared to 2012. Responses of eight selected fish species to the trawl-ban were, however, unclear.

After three years of the trawl-ban, initial signs of recovery were detected at EI, EO and WI only. Other anthropogenic impacts, including illegal trawling, expansion of non-trawling fishing efforts and reclamation may have been responsible for a lack of obvious short-term recovery in the other three communities, though apparent recovery probably requires a longer period to establish.

The trawl-ban is an important management intervention for rehabilitating the marine resources in local waters. This study not only revealed some initial signs of ecosystem recovery three years after the trawl-ban, but also provided essential baseline information for assisting fisheries management in Hong Kong and South China Sea.