Identifying indicators for early-stage ecosystem recovery following a ban on trawl fishing

Abstract

Hong Kong’s government imposed an indefinite territory-wide ban on trawl fishing in December 2012 in an attempt to facilitate ecosystem and fisheries recovery. Understanding if and to what extent recovery is proceeding is a challenge that is further complicated by spatial diversity of habitats from estuarine to oceanic, and strong monsoon seasonality. As such, Hong Kong has high marine biodiversity that suggests spatially variable recovery responses, and strong migratory patterns that infer recovery will not simply be a response to localised reductions in fishing mortality. We focus on examining benthic fish communities using year-long monthly trawl survey data at twelve sites. Detecting early-stage recovery within benthic habitats will require identifying suitable ecological indicators, and here we focus on two levels of organisation: i) at a community level by examining food web structure (using stable isotope approaches) and ii) at the species level by identifying resident species (based on phenological patterns) and trawl-sensitive species (based on a maximum size index). Isotopic proxy measures of food web structure show strong seasonal and spatial differences in trophic structure that infers a need to account for site and season specific recovery, and allows identification of appropriate food web indicators to examine recovery. At a species level, quantifying proportion of community composed of resident and trawl-sensitive species infers what proportion of species will likely benefit from a localised trawl ban and thus what recovery might look like, while providing an appropriate baseline indicator of community composition against which to assess future recovery. Given the scarcity of trawl bans globally, and the context of high diversity and migratory patterns, we discuss our approaches to identifying effective ecological recovery indicators and the key insights this study provides into the likely effectiveness of a localised resource management policy within a larger region of high exploitation.