Ecology and biodiversity of benthic marine molluscs before and after the 2012 trawling ban in Hong Kong

Abstract

Trawling is a widely employed fishing method worldwide, but it can destructively damage the benthic ecosystem, non-selectively collect animals of all sizes, and lead to overfishing. To promote sustainable fishery, the Government of the Hong Kong Special Administrative Region implemented a trawling ban within Hong Kong’s territorial waters on 31th December 2012. As this policy intervention can lower the impact to marine benthic habitats that were formerly brought by trawling activities, gradual recoveries of these habitats and their associated fisheries resources are to be expected.

This study, aimed to test the hypothesis that the trawl-ban would lead to a recovery of the benthic molluscan community in Hong Kong waters in terms of their species richness, species composition and food web structure. Systematic trawl surveys were conducted between 2012 (pre trawl-ban) and 2014 (post trawl-ban) while temporal comparison was also made by contrasting these survey results with those reported in the available literature dated back to the 1970s.

The composition of mollusc communities within Hong Kong waters shown significant changes over the decades. The mollusc community in the 1980s was once dominated by scavenger and pollution-tolerant species, but their populations notably declined in 2014. Their declines can be regarded as a sign of ecosystem recovery, despite the abundance of commercially important molluscan species remained low. This temporal trend implies that the Government’s environmental policies such as measures for water quality improvement and the trawl-ban may have led to some slight improvements in the benthic ecosystem.

For the surveys between2012and2014, there were obviously spatial, seasonal and temporal variations in diversity and biomass of molluscs. Generally, the diversity indices were low in 2012, increased throughthe2013 wet season tothe2014 wet season but declined again inthe2014 dry season. The results also suggested that marine pollution (e.g. eutrophication, red tides and hypoxia) possibly played a role in shaping the mollusc community in addition to the influence of the trawl-ban. Furthermore, massive blooms of species, such as the sea urchin Temnopleurus reevesi and starfish Archaster typicus, were encountered during the survey. These blooming of these species could outcompete the molluscs and other benthic species, leading to a change in community structure.

Stable isotope analysis was applied to study the trophic dynamics among the six mollusc communities in Hong Kong, between wet and dry seasons, between 2012(pre-trawl-ban)and 2014(post-trawl-ban). Although the results were highly seasonally dependent, there were some positive post trawl-ban changes in terms of food web structure of the mollusc community at Southern Lantau and Po Toi areas, indicating some signs of ecosystem recovery. However, the other sites within and nearby Tolo area did not show any improvement. Therefore, the recovery is likely remained to be a slow process.

A lack of clear improvement might due to continued impacts from other anthropogenic activities, including illegal trawling, eutrophication (i.e., harmful algal blooms) and suboptimal water quality. Without addressing these anthropogenic impacts, a full recovery of the benthic ecosystems and their associated fishery resources remains uncertain.