THE EFFECTS OF TRAWL GEAR ON SOFT BOTTOM HABITAT

Funded by NOAA - National Marine Fisheries Service

In January 2001, Boat Kathleen A. Mirarchi, Inc. and CR Environmental, Inc. were awarded a NOAA Cooperative Research Project to study fishing gear-induced habitat impacts to soft bottom habitats in Massachusetts Bay off Scituate, Massachusetts. To date, most of the research on  the effects of otter trawling on the seafloor has focused on long-term cumulative changes to gravel bottom or rocky substrate and few studies have looked at trawl effects on soft bottom habitat types.   Using local fishermen’s knowledge, two soft bottom fishing tows, a lightly fished area (Little Tow) and a heavily fished area (Mud Hole) were selected for the study.  At these sites in 120 to140 of water, the bottom and water column were characterized before and after six repetitive passes with a smooth bottom otter trawl to investigate the immediate impact of trawling. Areas were surveyed for benthic organisms, sediment surface characteristics, water column parameters, and fish and fish prey, epifauna and infauna.  An expansion of the project has been approved by NOAA to investigate chronic impacts on soft bottom habitat from weekly trawling.  Information from this on-going and potential future study will help improve the database used to manage fisheries in soft bottom habitat.  

Full Text of 2001 Acute Impact Study Here (Navigable HTML with Figures, Plates and Tables in PDF format)

Participating Fishermen and Vessels:  The project represented a cooperative research project effort involving 10 fishermen and 4 fishing vessels from the port of Scituate and 8 local scientists and technicians. The 72 ft F/V Andrea J performed the trawling, the 65 ft F/V Yankee Rose towed an experimental dredge, the 62 ft F/V Christopher Andrew performed side-scan and underwater video operations and the 42 ft F/V Lady Irene was outfitted with a side-mounted davit, hydraulic winch, and sieving table to perform the benthic sampling operations.    

The Survey Areas:  A reconnaissance survey was performed in June 2001, and wide area side-scan and single beam bathymetry performed to identify  (two) 1000m x 100m control lanes and two 1000m x 100m trawl lanes of similar bottom type.  At Little Tow and the Mud Hole, side-scan operations were performed with CR’s Edgetech digital side-scan system consisting of a Model 272 dual frequency towfish, top side processor with an  Analog Control Interface (ACI), and the Chesapeake Technology “SonarWiz” software. Side-scan mosaics were created using “SonarWeb” software, the georeferenced data exported to ArcView GIS, and the trawl and control lanes, benthic sampling stations, and video transects overlaid on the maps. 

The Experiment:  In July 2001, a major 10-day field effort was performed during which six bottom trawls were consecutively made over each of the selected lanes and the pre- and post-trawling surveys were performed.  High resolution side-scan data was collected to identify the areas of trawl activity and obtain detailed geologic features. At the trawl and control lanes (8) 1000 meter pre- and post trawl video transects were performed with a light weight aluminum video sled equipped with a Deep Sea Power and Light high resolution color camera, 250 watt lights and video overlay system. A total of 24 detailed 300 foot ROV transects perpendicular to the trawl and control lanes were also performed with a Benthos MiniROVER Mk II ROV equipped with a high resolution video camera, Benthos Mini Camera and strobe, and an ORE transponder.  Over 20 hours of underwater video footage and 600 still photos were obtained on this survey. 

Using a Ted Young grab sampler, 144 benthic samples were collected for benthic analyses and 48 samples for grain size analyses. In addition, CTD casts were performed before and after trawling.  In November 2001, a benthic sorting class was held in Scituate for a small group of local Scituate fishermen who will perform the preliminary sorting of the benthic samples. 

Preliminary examination of the benthic samples and video footage reveal deep furrows from the trawl doors and smoothing of the bottom from the sweep of the net at the soft bottom sites. Scavengers such as lobsters and crabs were observed feeding in the trawl tracks immediately after trawling. Amphipod tubes appeared to be stunted by the trawl activity but there were signs of tube rebuilding within hours of trawling. The preliminary fisheries data indicate declining catches of flounder after repetitive trawls but steady catches of dogfish that may indicate that dogfish are being attracted to the area. 

Benthic sample sorting, identifications, and analyses, fish stomach analyses and the analysis of the side-scan data, video tapes, and still photos have been performed.  The final data analysis and interpretation are nearly complete and a report will be furnished to NOAA in late 2002.

These preliminary findings were presented at the recent Marine Habitat Characterization and Mapping Workshop in Sebasco Harbor, Maine by CR Environmental in a short talk and poster session.

Participating Scientists and Oceanographers:

Sample Video Clips Collected During Video Sled Drifts in Trawl Areas

Double-click to Play or Right Click and "Save Target As" to Download MPEGs
Little Tow Clip 1    Little Tow Clip2    Mud Hole Clip 1    Mud Hole Clip 2

Photographs and Sonographs        (more photos here)