Fishermen's News - The Advocate for the Commercial Fisherman

Trawling in the Age of Technology


A wireless trawl eye allows the sensor to "see" small targets including shrimp with the acoustic range to distinguish fish swimming with or feeding on these shrimp. Photo courtesy of Simrad.

Trawling is an old and somewhat indiscriminate way to catch fish. As techniques advance, the ability to devastate fish populations exists and is obvious. Regulations ensue. For example: increased horsepower allows vessels to tow larger nets. The goal being increased trawl door spread and the ability to sift through more water volume; therefore in some places the result is to regulate the amount of horsepower. Technological advances in trawl door design allow for increased door spread with reduced horsepower. In other regions total time to fish is regulated, resulting in higher horsepower vessels in order to speed to the fishing grounds to meet the time regulations. The point is that the desire to regulate and control the situation is met with advancements in other aspects of the overall trawling system. The goals are noble. The fisheries need to be regulated, even the most contentious, conservation minded fisherman can benefit from the science that usually results from the regulatory process. But it certainly is interesting to watch the varied ways that politicians try to grapple with the issue.

One of the goals of regulating fisheries and trawling in particular should be to encourage advancement in techniques and technology. Not in an "apply for an exemption" fashion but to make sure that the regulations are written broadly enough to stimulate ideas. Look up 50 CFR 223.207 - Approved TEDs, for example. It is specific enough that it deters innovation or advancement in design. Contrast this approach with the hard cap on salmon in the Pollock trawl fishery. "Here is the 'hard cap' number, industry. Don't exceed it." Everyone with an idea is free to try implementation. This is by far the better approach. Both industry and science are hard at work to resolve this issue. At the same time these innovations and ideas are being reviewed and evaluated for future hard caps and for other trawl by-catch issues. The amount of knowledge being gained through these exercises is tremendous. Knowledge for knowledge's sake is fun, but knowledge for a purpose, sustainable fisheries, in this case has tangible value.

There are three main areas where technology is advancing in trawling; Trawls themselves, where materials and designs are changing. The tasks the trawls are asked to perform are not just to capture the fish but to capture some and release or exclude others based on size or species. For trawl doors, the original task was to simply create enough spread to open the net. This has expanded to include opening the net while staying off the bottom and doing it with less drag and fuel consumption. In some places specialized doors and techniques are used to fish among or along pipes lying on the bottom without doing the pipelines any damage. Finally, with trawl electronics a significant amount of effort is going into the technology because this is how the trawl and trawl door effort are measured in the actual fishing environment.

Catch Monitoring is the generalized phrase that is used to encompass the overall electronic measuring effort for both trawling and seining. There are two portions to the electronics effort: wireless and third wire.

Wireless devices communicate acoustically with the vessel, each other and at times with a third wire receiver. Wireless sensors, "eggs" in industry parlance, have generally been used to measure a parameter; door spread for example but also depth, height, temperature or filling rate of the cod end. Advances in the technology allow measurement of trawl door pitch and roll as well as water current direction relative to the trawl.

As always, when a fisherman is presented with a tool designed, developed and manufactured for a particular task, they find another way to use the tool or a different parameter to measure. This type of collaboration between the fishermen and the manufacturer is incredibly valuable to the industry and exponentially advances the technology. As a very basic example depth sensors designed to give the depth and sinking rate of a seine were quickly adapted to trawl doors and subsequently used all over the trawl to give the fisherman an idea of how the entire trawl is behaving. Does the cod end fly higher than the headrope? Is the mid-section of the trawl ballooning? The depth of any point on the trawl can be measured and an accurate comparison can be made to another point on the trawl. Separately or in conjunction with the depth the height from the sensor point to the bottom can be measured. This was originally conceived as a way to measure the headrope to bottom distance. This is used to confirm the design criteria of the trawl and to "see" how the door spread and trawling speed affect the opening height. Door spread is also a parameter of supreme importance. Door spread morphed quickly into wing spread and even into measuring if and how far an excluder flap was opening.

Trawl Eyes, an echosounder on the headrope, have been around for some decades providing the fisherman with confirmation that the trawl is open and fish are entering the net mouth. Advances in this technology have followed advances in echosounders and the new Trawl Eye sensor from Simrad now includes CHIRP. This allows the sensor to "see" small targets including shrimp with the acoustic range to distinguish fish swimming with or feeding on these shrimp. Trawl Eye sensors are now routinely used in the trawl tunnel either with another trawl eye or third wire trawl sonar located at the headrope.

One of the advantages of wireless sensors is the ability to determine the geographic location of the sensor. This technology has been available from Simrad since the very late 1980s and is in use in certain segments of the market usually based upon the environment. For example if the area being fished is flat and free of obstacles, position is not overly critical. However if the area varies greatly in depth, has many obstacles or strong currents then the trawl position is important. Trawl position can only be determined if both the range and the bearing to the trawl can be accurately measured. This requires fairly sophisticated electronics and there rumors of other companies entering the trawl position market.

Third Wire catch monitoring systems have also been around for some decades and have generally been used by larger vessels. Third wire systems are usually more reliable in terms of constant communication due to the difficulties in the wireless communication link. However the cost of a winch and wire and the maintenance keep this technology out of the hands of some fisheries. There are two main advances in the technology related to third wire systems. More and faster data is one and real time video is the other.

Faster update rates with trawl sonars are possible due to the advances in multibeam technology. Multibeam technology allows an entire trawl opening image to be displayed every 3 seconds or so and is compared to a scanning sonar which updates the trawl opening image every 30-45 seconds. The amount of data coming from a multibeam sonar compared to a scanning sonar is extreme and this has necessitated advances in the communication link. Typical third wire cables have coax copper or even steel conductors and this limits (particularly the steel) the amount of data transfer that is possible. Future advancements will require going to a higher quality cable or paying particular attention to the location of cable runs within the vessel.

The biggest advancement in third wire trawling is Real Time Video! Simrad pioneered this technology for the ability to really see the performance of salmon excluders in pollock trawls. Since the advent of these real time cameras the use has blossomed to include krill trawlers for gear observation, hake trawlers for species identification and now even shrimp trawlers to determine catch rates and the location of the concentration of the shrimp!

Multibeam technology allows an entire trawl opening image to be displayed every 3 seconds. Photo courtesy of Simrad.

Trawling technology will continue to advance and developments such as opening the trawl at depth to safely release unwanted fish at depth or opening and closing "flaps" on trawl doors to allow for the trawl to be actively steered are in the design phase. Different materials to decrease drag and increase fuel efficiency are also underway. The desire on the part of fishermen to target the right size and right species at the right time is very strong. The desire to increase the quality of the product, which increases the value to the consumer, is also prevalent. All of these factors make the future of trawling very healthy, profitable and environmentally sound.

Michael Hillers has been with Simrad Fisheries since 1984 and specializes in maintaining the link between the fishermen and the engineering and development teams at the various Kongsberg factories that design and manufacture the fishing acoustics.


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