A Novel System for the Deployment and Retrieval of Trawl Gear from the Bow of a Small Research Vessel

Trawls are an effective and adaptable gear capable of sampling a variety of habitats and fish communities. A long-established fisheries method in oceanic, estuarine, and large lake environments (Hayes et al. 1996), trawling has also become a viable sampling option for researchers in freshwater rivers (Herzog et al. 2005), reservoirs (Michaletz et al. 1995), and smaller lakes (Allen et al. 1999). Researchers use small-mesh benthic trawls extensively in the monitoring and assessment of Scaphirhynchus sturgeon populations on the Missouri and Mississippi rivers (Welker and Drobish 2010; Ridenour et al. 2011; Sechler et al. 2012; Gosch et al. 2015), and commonly use surface and midwater trawls as an effective gear for collecting larval fishes, age-0 Asian carp (Hypophthalmichthys species), and small-bodied pelagic fishes (Michaletz et al. 1995; Isermann et al. 2002; E. Pherigo, U.S. Fish and Wildlife Service, Columbia, MO, personal communication). Trawls have become an important gear in long-term sampling programs for multiple agencies (Gutreuter et al. 1995).

Freshwater trawl designs and applications have continued to evolve but advances in the deployment and retrieval systems for these trawls have stagnated. On many inland freshwaters, crews deploy trawls from both the bow and the stern of research vessels. Crews often retrieve bow trawls by hand, requiring at least a three-person crew, including the boat operator (Herzog et al. 2005; Welker and Drobish 2010). Some larger bow-trawl boats and most stern-trawl boats are outfitted with hydraulic winch systems for net retrieval (Welker and Drobish 2010; W. Bouska, DLH Corporation, personal observation).

Retrieving a trawl by hand is labor intensive compared to powered systems, and extensive trawling can lead to crew fatigue and increased chances of injury. Hydraulic retrieval systems reduce manual labor, but on vessels powered by outboards motors, hydraulic systems typically require a separate generator for operation, resulting in a loud work environment and increased operating costs (e.g., engine fuel, oil, hydraulic fluid, maintenance time). Hydraulic winch systems may also require a significant investment of deck space to accommodate the spools, control mechanisms, hydraulic fluid reservoir, and power source, limiting their application to larger research vessels (> 5.5 m). Our objective was to develop an electric trawl retrieval (ETR) system for bow-trawling that would reduce effort and noise levels while maximizing useable space. Furthermore, the ETR system needed to be interchangeable amongst research vessels ≤ 5.5 m in length and adaptable enough to handle a variety of trawl types and sizes.

The ETR system we developed utilizes two 12V, high-speed winches (commonly referred to as a “pot pullers” or “line haulers”) coupled with aluminum davits. Many different line haulers are commercially available, but for our purposes we wanted a product that was powered by a 12V motor and capable of producing high torque and line retrieval speeds under load up to 1 m/s. The line haulers also needed to be light enough to mount on small (≤ 5.5 m) watercraft while maintaining stability. We selected Electra-Dyne Co., Inc., ED7500-10 XXHD line haulers for our system, which weighed 29 kg each and drew approximately 50 A per motor under normal net retrieval conditions. If a snag dramatically increased the load on the line hauler, the draw could spike over 100 A. As per the recommendation of the line hauler manufacturer, we protected the circuit with 100-A household-style breakers. We powered the haulers with two dual-purpose (deep cycle/starting) 1,100 cold-cranking A, 200-Ah lead-acid batteries connected in parallel. We then connected this battery bank to the isolated charging circuit of the boat motor, allowing the battery bank to be constantly charged. It should be noted that not all boat motors have an isolated charging circuit, and it is not recommended to charge the battery bank off of the normal charging system as this could easily overdraw and burn out the stator. If an isolated charging circuit is not present, we recommend using an onboard charger that can be plugged into a 120V outlet.

The aluminum davits stand 1.83 m tall and each consists of a vertical 5-cm outside-diameter solid aluminum rod with a 135° angled coupling affixed to the top. From the coupling extends a 76.2-cm hollow aluminum tube that protrudes out slightly over the bow. We bolted a 453-kg capacity block to the end of the davit, and mounted the line hauler below the coupling (Figure 1). We attached the davits to the boat by two specially fabricated receivers consisting of 5-cm inside-diameter by 61-cm vertical-height aluminum pipe, welded to a frame of 6.25-cm square tubing bolted to the front deck (Figure 1). The receiver design is specific to a particular research vessel because it ties into horizontal spars and strengthened points on the deck. However, research vessel operators can modify the basic design in terms of length or number and placement of mounting brackets, to work on numerous vessels. Once we had mounted the receivers to the deck, we angled the davits outward approximately 16° and set them in place with a hitch pin.

We designed a line loading device to transition the trawl lines from the deployment and trawling phase into the hauler pulleys for retrieval. Each loading device consists of an L-shaped lever made of 2.5-cm-diameter aluminum rod, 30 × 6.5 cm. We bolted a V-shaped roller (Electrodyne part 20001) to the long arm of the lever so that in the downward position, the roller rests above the hauler pulley. We mounted the loading levers to the line haulers with brackets made from 0.6-cm aluminum flat stock with their fulcrums 11.4 cm above the hauler pulleys, and angled toward the stern approximately 18° off vertical (Figure 2). The line haulers were controlled by two low-amperage switches. We placed the control switches next to the throttle, allowing the boat driver to control retrieval while driving. However, switch placement is flexible and could be near the davits for operation by the deckhand if desired. Both switches could be activated simultaneously with one hand, making line retrieval relatively uniform.

To use the ETR system, crews attach the trawl doors to the trawl lines (we used 1.3-cm-diameter polytron solid-core line) with quick links. Other line types may work in the line hauler although we had trouble with hollow-core lines compressing and not properly seating in the hauler pulley. Crews should premeasure and mark trawl lines to correspond to the length of line required for trawl type and sampling depth. Once the crew has chosen the trawl line length, they route each line from the trawl doors through the block pulleys on the end of the davits, around the guide pulley that is positioned slightly in front of and beneath the hauler pulley, and then up and around the large V-grooved pulley of the line hauler (Figure 3). They then give the line a firm tug to set it into the groove and secure the ends of the lines onto a cleat below each davit. They attach the trawl to the doors and place it on the front deck; the boat driver then activates the two switches by the throttle causing the line haulers to raise the doors up to the block pulleys and coil the trawl lines on the floor below. The crew then routes the lines from where they exit the line hauler pulley, up and around the rollers of the loading levers (Figure 3). At this time the hauler pulleys are still gripping the lines, holding the trawl doors up to the block pulleys, and the trawl is ready to be deployed.

After a trawl location is selected, the driver reverses the craft along the sampling path, and the deckhand deploys the net off the bow into the water by hand. The drag from the net will pull the trawl doors from a vertical hang to a diagonal orientation. When this occurs, the deckhand grips the slack end of each line where it exits the loading lever and pulls the lines toward the rear of the boat. This action removes the line from the V-shaped groove of the hauler pulley, and transfers the tension on the lines from the hauler pulleys to the loading levers and the crew member's hands. As the driver reverses, the deckhand keeps hand tension on the lines as they feed out on the loading lever rollers. When all the slack is let out of the lines, the tension is transferred to the cleats and the loading levers. Once the trawl is fully deployed and fishing, the deckhand raises the loading levers from their downward position to about 45°, causing the lines to fall off the loading lever rollers and into the V-shaped groove of the line hauler pulleys. The line haulers are now loaded and ready to retrieve the trawl (Figure 3; Video S1).

When the trawl is completed, the two line haulers are activated by the driver as they keep the craft stationary or in slow reverse. As the line is brought in, the line haulers coil the line neatly on the deck. The high retrieval speed of the line haulers (up to 1 m/s) keeps the doors spread until they are pulled from the water up to the block pulleys. If the switch operator does not stop the line haulers before the doors hit the block pulleys, the circuit breaker will trip, halting the retrieval. The deckhand then pulls the trawl onto the deck, ending the trawl sample (Video S1). If subsequent trawls are required at the same site, the deckhand must simply route the slack end of the line around the loading lever roller and prepare it for deployment. If travel is required to reach the next trawl site, the deckhand can safely secure the boards against the block pulleys by cleating the lines, or lower the boards and lay them on top of the trawl before getting underway.

In the case of a snagged trawl, the boat operator drives slowly toward the snag as the line haulers are activated to bring as much trawl line into the boat as possible. When all the slack is into the boat, the lines are cleated and the boat maneuvers forward into the snag to try and free the trawl. Safety considerations specific to the ETR system include keeping hands and loose clothing away from the line hauler pulley and other pinch points during deployment or retrieval. Also make sure the space where the trawl line coils on the deck is clear of obstructions, and that during deployment the deckhand is mindful to wear gloves to avoid rope burn and to keep hands and feet out of any coiled line as it is being fed into the water. Good communication between the boat operator and the deckhand is critical for safety

Our primary goal in developing the ETR system was to make trawl net deployment and retrieval from small research vessels more efficient, easier, and safer. Since development, the ETR system has been used extensively and successfully in support of the Missouri River pallid sturgeon (Scaphirhynchus albus) habitat assessment and monitoring program. Research vessel crews successfully deployed and retrieved more than 1,000 benthic trawls in the Lower Missouri River, Missouri (net river miles 33–180), during 2014 and 2015, in depths ranging from 0.5 to 6.0 m, and with trawl line lengths ranging from 25 to 60 m. We sampled with two different small mesh benthic trawl configurations (4 mm mesh; widths 2.4 and 4.9 m; lengths 1.8 and 7.6 m; Table 1), and deployment and retrieval could be safely accomplished by one deckhand and one boat operator (Video S1). We also tested larger experimental surface trawls with the ETR system and although deployment and retrieval were successful, the greater size of these trawls (widths 7.92 and 8.25 m; lengths 11.5 and 11.58 m; Table 1) was cumbersome for one deckhand to lift into the boat alone after retrieval. Trawls of this size are likely the threshold for operating the ETR system with a two-person crew.

Under typical sampling conditions we performed a maximum of 40 trawls/d. The battery bank we used to power the ETR system was continually charged by the boat motor and seemed able to power the system indefinitely under this type of duty load without any additional maintenance charging. The first iteration of the ETR system was powered by a battery bank independent from the boat motor and required recharging between field excursions. However, the independent battery bank was still able to provide sufficient power for our sampling needs, making the ETR a viable option for watercraft powered by motors not equipped with isolated charging circuits. Different sampling protocols that require more consecutive trawls or result in additional line hauler operation time may experience greater power draws and require a larger battery bank than described. Crews could add additional batteries or batteries with higher ampere hour ratings to extend the time needed between charges.

The total weight of the ETR system (two line haulers, two davits, two davit receivers, and battery bank) was approximately 140 kg. We used the system interchangeably on two different research vessels, both 5.5 m in length, without compromising stability or maximum weight capacity recommendations. While it is likely that the ETR system could be safely used on boats under 5.5 m in length, be sure to consider the weight capacity recommendations of your research vessel.

Given the exemplary performance of the ETR system on the Lower Missouri River, a large, swift, turbid river full of snags and obstructions, we feel the ETR system would also be suitable for small-craft bow trawling applications in almost any other aquatic habitat. Our suggested design was developed specifically for the extensive benthic bow trawling required by the research program and was not an exhaustive test of the capabilities or limitations of the ETR system. The sampling protocol for which the ETR system was developed included only the two benthic trawls that we tested, and limited sampling depths to a maximum of 6.0 m. We speculate that the ETR system would continue to be effective in greater sampling depths with increased trawl line lengths, and with additional sizes and types of trawls. The utility of the ETR system for different research applications may only be limited by creativity and ingenuity.

Please note: The Journal of Fish and Wildlife Management is not responsible for the content or functionality of any supplemental material. Queries should be directed to the corresponding authors for the article.

Reference S1. Gutreuter S, Burkhardt R, Lubinski K. 1995. Long term resource monitoring program procedures: fish monitoring. National Biological Service, Environmental Management Technical Center, Onalaska, Wisconsin, July 1995. LTRMP 95-P002-1.

Found at DOI: http://dx.doi.org/10.3996/072015-JFWM-059.S1; also available at http://www.umesc.usgs.gov/documents/reports/1995/95p00201.pdf (765 KB PDF).

Reference S2. Welker TL, Drobish MR. 2010. Missouri River standard operating procedures for fish sampling and data collection. Volume 1.5. U.S. Army Corps of Engineers, Omaha District, Yankton, South Dakota.

Found at DOI: http://dx.doi.org/10.3996/072015-JFWM-059.S2; also available at http://www.fwspubs.org/doi/suppl/10.3996/022012-JFWM-013/suppl_file/10.3996_022012-jfwm-013.s7.pdf (2.64 MB PDF).

Video S1. Video of small-mesh trawl deployment and retrieval using the electric trawl retrieval system developed primarily for pallid sturgeon (Scaphirhynchus albus) habitat assessment and monitoring in the Lower Missouri River, Missouri, 2014 and 2015.

Found at DOI: http://dx.doi.org/10.3996/072015-JFWM-059.S3 (102 MB MPG)

We would like to thank Ruslan Grigoriev, Jeff Finley, Jeremiah Smith, Jared Knerr, and other staff members at the Columbia Fish and Wildlife Conservation Office for help in building and testing the ETR system, and he Associate Editor and two anonymous reviewers for their constructive comments that improved earlier drafts of this manuscript.

Schematics were created using Google SketchUp.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.