An Exposure-Response Approach for Assessing Bottom Trawl Impacts on the Composition and Function of Epibenthic Communities in Northern Hecate Strait, B.C

Bottom trawling has often come under attack due to its perceived negative impacts on benthic habitats. Bottom trawl fisheries have been found to significantly reduce benthic habitat complexity by removing or degrading foundation species – organisms that provide biogenic habitat structure and play a large role in shaping community structure and contributing to the productivity of commercially important species. Therefore, the response of foundation species to trawling disturbance may be indicative of the impact of bottom trawl fisheries and the overall sustainability of a fishery. Research regarding impacts of bottom fishing has often been limited to a few square kilometers and to capturing results from relatively short time frames. In order to assess the impacts of bottom fishing at scales more representative of true fisheries, we have developed an exposure-response research protocol, which involves surveying areas that have been previously exposed to a wide range of fishing efforts, allowing us to investigate impacts on larger spatial scales, such as Northern Hecate Strait, B.C. To begin, we mapped fishing effort and substrate type for each 1 km² grid cell using fishing events recorded by fishery observers from 1994-2009, and Canadian Hydrographic Service data. Survey sites were randomly chosen using a five trawling effort levels, two substrate types and two depth classes for stratification. In August, 2010, we conducted transects at each survey site using a remote operated vehicle (ROV), and collected still photographs of the benthos.  We chose a  random subset of photographs for analysis, in which each organism was identified to the lowest taxonomic level, and the substrate type was classified. Changes to the community’s functional and biological traits, over the gradient of trawling effort, which may be indicative of the effects of trawling, will be assessed through biological traits analysis and will be compared to changes in species richness and diversity indices, which are not always sensitive to disturbance. Identifying the relationship between trawling effort and benthic community response through the exposure-response methodology will inform fisheries management of the impacts of trawl fisheries on a regional scale, which is representative of true fisheries. The analysis will also act as a preliminary step to developing eco-certification assessment methods for B.C. trawl fisheries which have expressed interest in becoming more sustainable.