Research Document - 2013/075
The extent and nature of exposure to fishery induced remobilized sediment on the Hecate Strait and Queen Charlotte Sound glass sponge reef
By J. Boutillier, D. Masson, I. Fain, K. Conway, G. Lintern, M. O, S. Davies, P. Mahaux, N. Olsen, H. Nguyen, and K. Rutherford
Abstract
Fishing activities currently take place within Adaptive Management Zone (AMZ) of the proposed Hecate Strait and Queen Charlotte Sound Area of Interest (AOI). DFO Science has been asked to (1) assess the nature and extent of risks associated with remobilization of sediment from fishing gear, and (2) provide managers with options to mitigate these risks.
This paper summarises fishing activities that have occurred in the AMZ from 2007 – 2011, and outlines a framework to estimate the footprint of the remobilized sediment. This framework describes: the intensity of the various fishing activities as they relate to remobilization of sediment; the various sediment types within each reef complex; the factors that affect the resettlement rates of remobilized sediment; and the dispersion of the remobilized sediment as a result of ocean currents in the region under a variety of fishing activity scenarios.
For the purposes of this paper, the potential footprint of remobilized sediment is determined for theoretical interaction of bottom trawl gear with sediment that has an average grain size and composition of: 55% silt (3.9 to 63um), 30% clay (0 to 3.9um) and 15% sand (63um+).with a calculated D50 = 20um. Two models were used to calculate the area of impact: a model which calculated the resettlement rates for the average settlement size and a dispersion model which used the resettlement rates within a regional oceanographic current model to estimate the area that would be covered. Three variations of the dispersion models were calculated for the area. The first model calculated the maximum sediment transport around the AOI. The second model calculated the potential area of impact based on historic bottom-trawl fishing boundaries. The third model calculated the potential area of impact if fishing were restricted to those days with the lowest tide cycles.
The findings of the sediment dispersion models were then used to inform the risk of exposure of the sponge reef to remobilized sediment under 6 mitigation scenarios. In addition, the information needs from biological and ecological science perspective were outlined for a cost-benefit analysis that could be used to evaluate the social and economic consequences of the various mitigation measures.
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