Oral Presentation Australian Society for Fish Biology Conference 2017

Reconstructing the footprint of an Australian penaeid-trawl fishery to support ecosystem-based management (#30)

Craig J Noell 1 , Simon Clark 2 , Steve Shanks 3
  1. SARDI Aquatic Sciences, Henley Beach, SA, Australia
  2. Spencer Gulf & West Coast Prawn Fishermen's Association, Port Lincoln
  3. PIRSA Fisheries and Aquaculture, Adelaide, SA

Using GIS techniques, the trawl footprint of the eco-certified Spencer Gulf prawn fishery was estimated with the view of developing a cost-effective surrogate measure of the fishery’s impact on the benthic ecosystem. Based on trawl midpoints recorded for ~40% of all shots from 2003–2016, and trawl distance, width and simulated direction, individual trawl paths (line segments) and swept areas (polygons) were reconstructed. Owing to a consistent spatial trawl pattern and relatively stable annual total effort during this 14-year period, random subsets of midpoints representing ~5.5 years with 100% coverage were obtained to describe and predict the change in trawl footprint. Two alternative estimates of trawl footprint were calculated: (i) a swept-area estimate, which is the amalgamated area of trawl polygons, and (ii) a trawl-intensity estimate, which is the total area of pixels at sufficiently high resolution and above a predetermined minimum trawl intensity (cutoff). The amalgamation of polygons resulted in a total swept-area (at ~5.5 years) of 4,282 km2, with less new ground trawled each year. By comparison, the trawl-intensity estimate was 4,224 km2 at a resolution of 30 × 30 m and cutoff of 0.5 h km-2 yr-1. Exponential decay models fitted to swept-area and trawl-intensity footprint estimates with each additional year yielded projections of 4,468 and 4,253 km2, respectively, which equate to <20% of the total area of Spencer Gulf. The pending introduction of electronic logbooks in the fishery theoretically should eliminate some positional errors that are, otherwise, undetectable. Until then, the trawl-intensity estimate is preferred since, not only can it be updated within a few hours (cf. days for the swept-area estimate), errors can partly be offset by adjustment to the intensity cutoff. We suggest that regular trawl-footprint estimates, supplemented with monitoring bycatch species of interest, should be considered in an ecosystem-based management context for this fishery.