2016 SoE Marine Chapter - Case Study - Footprint of trawling

Brief description

The Marine chapter of the 2016 State of the Environment (SoE) report incorporates multiple expert templates developed from streams of marine data. This metadata record describes the Case Study "Footprint of trawling". The full Case Study, including figures and tables (where provided), is attached to this record. Where available, the Data Stream(s) used to generate this Case Study are accessible through the "On-line Resources" section of this record. ---------------------------------------- DESCRIPTION OF THE FOCUS OF THE CASE STUDY SOE 2011 stated that virtually all of Australia’s marine areas shallower than 1,000 m depth are, or have been, fished [by some means]. The most extensive direct human pressure on the seabed in Australia is demersal trawling for fishes, prawns and scallops. It is also commonly perceived that trawl gear has substantial direct impacts on seabed habitats, with most concern surrounding impacts on delicate long-lived structure-forming biota that may be easily damaged and slow to recover if trawling or other bottom-contact fishing occurs on such habitats. This habitat loss is considered to be one of the primary threats to marine ecosystems (SOE 2011). Despite this, to date there has been no national-scale quantitative analysis of the cumulative spatial extent of demersal trawl fishing [or “footprint”: the area (km²) of seabed trawled at least once in a specified period of time] that may be used to assess the potential for impacts of trawling on seabed habitats. Formal habitat assessments have been completed for only a small (5 of 22) number of Australian fisheries although these assessments include the largest fisheries and account for >70% of the total footprint. Some other fisheries have implemented qualitative consideration of habitat risks under Ecological Sustainable Development (ESD) objectives, and the Regional Marine Planning process attempted a national qualitative assessment of cumulative risks. Critically, however, the majority of regions lack suitable data on seabed habitats types and their distributions — a major gap that is an impediment to formal assessment of their status. PRESSURES/ISSUES OF IMPORTANCE Australia’s marine estate is very large and a national quantitative analysis is required to estimate the footprint of trawling and to identify regions that are most exposed and have most need to focus limited research resources towards understanding the presence, distribution and status of sensitive habitats. In recent years-to-decades the total annual effort in most Australia trawl fisheries has been declining, in some cases substantially (Figure 1). As a consequence, the footprints of these fisheries are also contracting as has been documented in several cases (GBR: Pears et al. 2012; TS: Pitcher 2013; NPF: W. Rochester pers. comm.; SET: Pitcher et al. 2015), although footprint contraction is not directly proportional to effort reduction (e.g. GBR: 2005 vs 2009 effort reduction 44% cf footprint decrease 19%; TS: 2005 vs 2011 effort reduction 72% cf footprint decrease 54%; SET: 5 yrs before vs after 2007 effort reduction 36% cf footprint decrease 23%). Although trawl effort and footprints have contracted, it nevertheless remains critical to determine where potential risks are greatest and where new surveys are required to document presence and distributions of sensitive seabed habitats. DATA STREAM(S) USED IN CASE STUDY The distribution and intensity of trawl effort in each fishery was mapped by interpolating and gridding position data of trawling events recorded in confidential fishing vessel logbooks, or electronic Vessel Monitoring System (VMS) data, for a 3–5 year period between 2007 and 2012 depending on the fishery. Each trawl event included the associated hours of trawling effort. Depending on the fishery, a ‘trawl event’ may comprise either VMS position polls at ~2 hourly intervals; start and end positions of each individual trawl; positions recorded manually during trawling operations (e.g. 3× per night); a cell location for the days trawling for each vessel (either 6 or 10 minute cells). The gridding was done at 0.01° resolution (~1.1 km² depending on latitude), because the distribution of effort is typically random at about this scale. Effort in hours was converted to trawl swept area per grid area ratios using information on gear width and tow speeds specific to each fishery, then mapped.

Lineage

Statement: QUALITY OF DATA USED IN THE CASE STUDY VMS data has the highest spatial and temporal resolution (<<100 m and ~2 hours); nevertheless, there is uncertainty determining which VMS polls are associated with trawling. Several data rules are employed to estimate which VMS polls represent trawling, such as vessel speed bands. Start and end positions of individual trawls potentially may more clearly identify actual trawling events; however, positions are recorded and entered manually and are subject to error, and trawls may not be straight lines between start and end positions which becomes more critical for longer trawls. Less frequent manual recording of positions during trawling operations has the same uncertainties but additionally may not sufficiently represent all areas trawled by a vessel in a given day and potentially may lead to underestimation of the overall fishery footprint. Recording of the days trawling by 6 or 10 minute cell location is too coarse (>100 km² and >300 km² respectively) for accurate footprint estimation and is most likely to cause over-estimation of fishery footprints even if more than one cell is fished on some days. High resolution effort data is an important requirement for accurate footprint quantification and habitat assessment for all fisheries.

Notes

Purpose
To describe the Case Study "Footprint of trawling" for use in the Marine chapter of the 2016 State of the Environment report.