Optical Trawl Surveys of Groundfish: A Video Trawl Survey of Western Gulf of Maine Atlantic Cod (Gadus morhua)

This research provides a minimally invasive method for collecting trawl survey data to quantify abundance, distribution and biomass. This system can be adapted to any otter or midwater trawl net. It can be adapted for scientific or increased fishing information.

To begin, arrange the survey net and pressure-sensitive winch on the vessel's deck. Attach the survey trawl doors to the vessel's winch cables. Then secure the winch to the deck using a temporary weld.

Next, wire the winch power cable into the vessel's breaker box to establish a stable electrical supply. Attach a cable block to a mount on the vessel's A-frame or similar support structure and route the coaxial cable through the block toward the stern, positioning it adjacent to the net drum. Attach the junction box to the rigid polyethylene cylinder.

Then mount the cameras and lights onto the cylinder. Connect the cameras and lights to the junction box using cables fitted with waterproof connectors. First, secure the computer and monitors to a stable workstation in the wheelhouse.

Then connect the GPS devices to the computer to enable real-time spatial data acquisition. Connect the net mensuration receiver to the computer and connect cameras and lights to the power in the vessel's wheelhouse. After verifying proper equipment function, launch the mapping software to display the vessel location.

Check if the time and date stamps are correctly displayed in the video-recording software. Enter the tow number to be overlaid on video. Launch the net mensuration sensor software and begin recording the wing spread and doorspread data.

Using the vessel's hydraulic winches, deploy the net and doors slowly and start recording the video. Turn on the lights after the cylinder enters the water. Once the vessel's hydraulic winches are locked into position, proceed to record the location and speed data.

Stop recording location and speed data once the vessel's hydraulic winches are engaged for haul back, indicating the end of the tow. Switch off the lights once the net and doors are hauled back and the lights reach the surface. Similarly, capture video and data at all stations.

Prior to completing a closed cod-end tow to collect biological information, secure the cod-end of the net using the cod-end clip, document the trawl catch aboard the vessel following standardized procedures. Offload all data from the wheelhouse computer to external hard drives. Upload all videos to the annotation software.

Trained technicians shall review and annotate all target species and unidentified fish. During the 10 days of sampling, 47 usable survey tows were completed, including four closed cod-end and 43 open cod-end tows during daylight hours. The length-frequency distribution shows that Atlantic cod captured during the survey ranged from 41 to 81 centimeters.

The length-weight relationship demonstrates the expected increase in weight with fish length, confirming consistent biological measurements from the cod sampled during the survey. You can estimate abundance of any species collected in trawl nets. It also provides species diversity and increases your range of size selectivity, particularly for small fish and invertebrates.

Operating delicate optical equipment in harsh environments is challenging and requires some electrical repair skills and access to spare parts. Automating video analysis using AI and integrating stereoscopic measurements are the next steps in development. I see this system being applied to many different studies, but in essence, it increases the amount of water use sample for the same or less cost than traditional scientific surveys.