Resource Assessment & Conservation Engineering  (cont.)

Trawl Impact Studies in Eastern Bering Sea

The Trawlex-01 cruise was conducted 15 June-15 July 2001 in the eastern Bering Sea to experimentally investigate possible adverse effects of bottom trawls on a soft-bottom community in the eastern Bering Sea and to evaluate a state-of-the-art side scan sonar and swath bathymetry system for exploration of benthic habitats.  Whereas earlier work focused on chronic effects of trawling (Trawlex-96 and Trawlex-97), the present study is a more process-oriented look at short-term effects and recovery.  The 155-ft trawler FV Ocean Explorer was chartered, and all scientific systems were successfully implemented including an ultra-short baseline (USBL) tracking system, two complete side scan sonar systems with tow winches, a trawl mensuration system, and a survey-grade integrated navigation system with DGPS, two gyroscopic compasses, and a vertical reference unit.  All systems were tested and calibrated during gear trials in Puget Sound, Washington conducted 30 May-1 June.

During the Alaska cruise, biological, physical and chemical characteristics of the seabed were randomly sampled in six experimental-control corridor pairs (see Figure 1).  Individual corridors were 20.9 km long and 100 m wide, representing the long-term average tow for commercial bottom trawls in the study area.  Biological sampling consisted of 15-minute research trawls for epifauna (n=72 total) and 0.1 m² van Veen grab samples for infauna (n=144 total at 2 per epifauna site).  At each infauna sampling site, a second grab sample (n=144 total) was collected for characterizing carbon and nitrogen levels in surficial sediments, as well as grain size properties.  Sampling effort in experimental and control corridors was equally divided before and after fishing in the experimental units with a commercial bottom trawl (NETS 91/140 Aleutian cod combination).  Each of the experimental and control corridors was also surveyed twice using a Klein 5410 side scan sonar system.

Preliminary observations indicate a very diverse epifaunal community (approximately 90 distinct taxa) on very-fine olive-gray sand at 60 m depth.  The seafloor appears to be brushed smooth in the preliminary side scan imagery, probably due to sizable storm waves and strong tidal currents that regularly disturb the area.  Occasional video deployments on the trawls indicated somewhat greater complexity, with at least some areas of the seafloor resembling the surface of a soccer ball with marbled coloration.  Significant numbers of derelict king crab pots were encountered, and there is evidence of extensive feeding by walrus.  Two conspicuous, as yet unidentified, targets were also encountered.  A more detailed characterization of the area will be possible once laboratory processing and analysis of the navigation, sonar, epifauna, infauna, and sediment data are completed.

The new NOAA Ocean Exploration program supported use of a Klein 5410 interferometric side scan sonar system.  This fully digital, multibeam system produces coregistered backscatter and swath bathymetry with four side scan beams and one interferometric beam each on the port and starboard sides of the towfish.  At this time, there are only three prototype Klein 5410 systems in existence (France, Japan, United States).  Side scan backscatter images contain quantitaive information about the sediment type and general roughness of the seabed, while swath bathymetry enables direct measurements of small vertical features on the seabed.  Both types of information are important when investigating relationships between geological features, benthic biota and fishing gear disturbance.  In addition to data collection for an analysis of change due to trawling, additional objectives of the deployment were evaluations of advanced remote-sensing technology for future broad-scale seafloor mapping expeditions and the feasibility of using ships of opportunity for this purpose.  Approximately 950 line-km of seabed were successfully sampled with the system and protocols were developed for implementing state of the art side scan sonar and navigation technology on a chartered commercial fishing vessel.  Additional information about this aspect of the project is posted on the NOAA Ocean Exploration web site:
http://oceanexplorer.noaa.gov/projects/alaskatools01/alaskatools01.html.

The Trawlex project had considerable technical support from its multidisciplinary partners.  The U.S. Navy’s Naval Undersea Warfare Center (Keyport, Washington) provided side scan sonar and navigation services at sea.  The University of New Hampshire-NOAA Center for Coastal Ocean Mapping/Joint Hydrographic Center (Durham, New Hampshire) assisted with electronic systems integration and calibration, as well as side scan sonar and swath bathymetry data processing.  The University of Alaska Fairbanks, Institute for Marine Studies (Fairbanks, Alaska) has responsibility for infauna sample processing and surficial sediment analyses.  Special arrangements with Klein Associates, Inc. (Salem, New Hampshire) made the Klein 5410 system available for use.

Finally, the Ocean Explorer charter with all equipment was transferred to the U.S. Coast Guard at the end of the Trawlex cruise to investigate the unexplained sinking of the FV Arctic Rose in the Bering Sea on 2 April 2001, which claimed the lives of 15 persons.  After a brief survey using the Klein sonar system, the Coast Guard located the sunken vessel in 450 ft of water.  A remotely operated vehicle was deployed and the identity of the vessel was confirmed.  Presence of the fully-configured and staffed Ocean Explorer in the Bering Sea greatly facilitated the Coast Guard’s official investigation of the tragedy.

 
2002 Cruise Objectives
The trawl effects study will continue in summer 2002 with recovery assessments in all six experimental-control corridor pairs.  The full biological and geophysical sampling regime will be used to characterize changes that have occurred after a 1-year recovery period.  Using a Before-After-Control-Impact “BACI” experimental design, baseline information on natural variability in control corridors will be statistically factored out of the recovery responses observed in the experimentally-trawled areas.  A 15-day cruise aboard the same charter vessel (plus 5 day weather contingency) is scheduled for June-July 2002.  Equipment mobilization should be “plug-and-play” based on careful demobilization after the 2001 cruise.  The experimental design will accommodate one additional series of (destructive) epifauna sampling and multiple years of grab sampling after 2002.

A possible second objective for 2002 field operations would be to use the Klein sonar system for high resolution reconnaissance mapping of the Bristol Bay seabed.  These surveys are intended to detect boundaries between distinct texture-bedform classes of seabed, rather than synoptic mapping which is impractical for large areas.  As a 7-10 day add-on to the bottom trawl study, the Klein 5410 sonar would be towed on long transects that extend from coarse substrates near shore, across the reworked sand “plains” found in the Trawlex study area, through the sand wave habitats observed in 1997 and terminate close to another shore.  Management of fishery-habitat interactions would benefit from the reconnaissance survey effort because, currently, only simple case studies of fishing gear effects are being conducted in the United States and elsewhere in the world.  An inherent shortcoming of this approach is the inability to extrapolate these findings to meaningful geographic scales.  Until it is possible to identify regions with similar sensitivities to fishing gear and then place replicate experiments in each, application of research findings will be limited to the areas actually studied.  Systematic investigations will require knowledge of boundaries (and associated uncertainty) for seabed types that support distinct biological communities.  The Bristol Bay work in 2002 would be the basis for developing protocols to achieve this goal.  Standardized measures of fish and invertebrate abundance from annual trawl surveys covering the entire Bering Sea shelf, including Bristol Bay, are available for boundary validation.  An important element of early work will be consideration of the optimum resolution for data acquisition, since spatial resolution and areal coverage (hence efficiency and costs) are inversely related.  Since the scale of biological response to environmental variability is unknown, it is imperative that data acquired during the Bristol Bay development phase is of the highest possible resolution, thus enabling correlative analyses over a broad range of spatial scales by a process of data decimation. From an exploration perspective, these surveys would yield high quality information about a poorly described area.

By Bob McConnaughey.

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AFSC Quarterly
Research Reports
Oct-Dec 2001