Fishermen's News - The Advocate for the Commercial Fisherman

Gear for Efficient and Sustainable Fishing


This "bag" or cod end of pollock is being loaded into the fish holds of the F/V Bering Defender. Photo courtesy of Simrad.

Fishing, specifically trawling, requires a vessel to get to the right part of the ocean, have a set of electronics to locate and analyze the wandering targets and then determine their depth and direction of travel. At that point, the key is to deploy and position the trawl at the right depth and latitude/longitude to intersect and capture the fish.

Now that the cod-end is back to the surface and the fish are being dumped or pumped into the hold, are they the right fish? Are they the right size? To claim "sustainable" the answers to both questions should be 'yes'. A yes answer in this case means that the proportion of desired targets to bycatch is not too high. If the proportion is incorrect, besides being unsustainable, will regulations introduce fines or shut down the fisheries? If the proportion is too high, is there technology available to reduce the ratio? The answer to the last question is 'yes'.

Efficiency in a discussion of fishing generally means the components in the economic equation are being analyzed. If there is to be an increase in efficiency, the costs need to be reduced. Usually time and energy consumption are the most costly components in an economic equation. With trawling these two components are obvious. Time is time and energy is fuel. Logic dictates that an increase in efficiency means a reduction in time spent looking for fish or dragging a misaligned trawl around. In the case of a fishing vessel, time is fuel.

Once the fish are located, the first thing to do is to get the trawl to the fish. This means knowing the depth of the fish. If they are on the bottom, the bottom depth is where they are and your echosounder tells you this. If pelagic, the echosounder also tells you the depth of the fish, and the goal is to stop the trawl at the depth of the fish. This requires a sensor capable of knowing and informing the fishermen on the bridge of the depth.

On the way to the fish depth, the trawl is opening. Except in the case of pair trawling, opening the trawl requires trawl doors. Trawl door design is highly technical for several reasons, but not least because dragging a large piece of steel through the water is responsible for a huge portion of the fuel consumed. Doors are designed to open the trawl with spreading force (think "lift", like airplane wings) and while doors are not individually designed for each vessel, there is a balance sought between the trawl, the doors and the vessel's horsepower.

Old school bottom trawl doors opened the trawl as much due to the drag on the bottom as spreading forces. Door opening measuring technology has only existed for about 30 years, and much manual and visual effort went into examining the net and trawl doors for shine and wear. In the 1980s, two avenues of technology were developing to help with this situation. In 1986, third-wire trawl sonars were introduced to show, in real time, the shape of the net, the location of the net relative to the bottom, relative to obstacles previously avoided and of course the fish. The second developed technology of the 1980s was the adoption of wireless systems, starting with door spread sensors to measure the door spread. Because most vessels didn't install both systems, two schools of thought developed: one; if the net shape is proper, then the doors are flying correctly and two; if the door spread is correct, then the net is open properly. Both techniques work when things are operating well, but both can also give false confidence when something breaks or is tangled. Together, troubles are obvious more quickly and the result is an increase in efficiency. But of course cost is very definitely a factor. Simrad was a pioneer in both technology streams and is the only manufacturer that has, at last, incorporated the two in an effort to keep costs low but also because the wireless communication at the trawl is much more reliable than communication from the trawl to the vessel.

Efficiency is driving new trawl and trawl door designs, new efficient designs require finesse and finesse fishing requires more measurements from the trawl and trawl doors. Spread, of course, but height off the bottom and roll are also very important when finesse is necessary. Depth is generally more important than height off the bottom in pelagic fishing, but if the fish are deemed to be following a contour and the doors are less than about 50 fathoms off the bottom, height off the bottom is a better parameter to observe. To clarify; Depth is a "pressure depth" from the surface and Height-Off-the-Bottom" is an acoustic measurement of the distance to the bottom.

Following the doors are sweeps, sometimes really long sweeps. Where the sweeps touch bottom and how well they tend the bottom is interesting to know, as this is the follow-up herding that the door initiated. Bottom Contact sensors can measure a point of contact or can be set up to indicate a specific distance from the bottom, from a few inches to about a fathom.

The trawl wings are the forward part of the trawl. Webbing is also critical to herding, as generally the meshes are quite large and the fish could easily swim out were they not being herded.

The trawl wings lead to the mouth of the trawl. It is a fair assumption that if fish clear the mouth they will be caught. When watching fish enter the mouth of the trawl on a trawl sonar image it is fair to expect catch indicators on the trawl to be triggered soon thereafter. However that is not always the case. There are a couple of possibilities, the fish are small or they are escaping. Maybe they are small and escaping and that is probably okay, but if they are the target species and target size and still escaping the efficiency meter is stuck on zero while the fuel gauge drops!

Technology can help solve this problem. The most likely trouble is poor trawl geometry. A trawl is designed to fly square to the direction of travel and to flow faster than the current. If the trawl is skewed the large meshes can be almost perpendicular to the direction of travel and the fish swim right out. There are several ideas about how to accurately measure trawl skew; flow at the headrope, trawl door to vessel distance and trawl door to center of headrope distance. The trawl door to center of headrope has proven accurate. Running tide and currents can negatively impact the other two, but if the distance from the center of the headrope to each door is equal, the trawl is flying square.

Back to the trawl mouth again: the trawl sonar (third-wire system) shows the shape, and on good systems the distances can be measured on the screen. Trawl footrope to bottom distance or footrope contact with the bottom is visible until the footrope merges with the bottom, if the trawl sonar is tuned properly and installed in a good spot on the headrope. However if the vessel is not equipped with a 3rd wire system there are several wireless sensors that are helpful. A height sensor will measure the distance to the footrope or bottom. Usually this sensor will not accurately measure both the distance to the footrope and the bottom. A TrawlEye sensor gives the user an echogram picture like an echosounder and this sensor will show the distances to both targets until the footrope merges into the bottom. However the TrawlEye only shows the image directly under the sensor, unlike the trawl sonar that shows the entire opening.

Measuring parameters on the trawl are always of interest to the fishermen. Does the cod end raise as it fills? Does it roll in a turn? How much? Would accurate depth sensors placed all over the net be valuable? Can short distances, like the distance from a flap to the trawl body be measured? Can I put 12 sensors on the net to measure the catch in 10-ton increments? Today's sensors can answer all of these questions.

All of these questions and many more are related to increasing the efficiency of the trawl. Efficiency of the fishery requires that the right fish be caught. This brings into the equation both size and species.

Identifying size and species requires different tools. At present, Simrad's FX80 real time video camera is the only tool that identifies species and relative size of the fish in real time.

This image shows the net opening and fish during a bottom trawling session. Photo courtesy of Simrad.

Researchers have used recording cameras that are deployed, retrieved and then viewed. If the camera is operated properly and is pointed in the right direction and the lighting system works, then useful video is obtained. Acoustic solutions with respect to fish size are available in hull-mounted configurations and are being developed for trawl applications, but species identification is still the elusive "Holy Grail" of fisheries acoustics.

Many individual fishermen, fishing companies and trawl designers are working on the efficiency of the fisheries in order to increase sustainability. This effort is mostly in the direction of excluders and sorting grids. These forward thinkers are using sensors in many ways not initially conceived of by the sensor manufacturers. Innovation is not uncommon among fishermen!

All of the recent advances in trawl design, door design and trawl electronics will result in increased fishing efficiency, and all of this effort and testing have the potential to increase the sustainability of all trawl fisheries in which they are employed.


Powered by ROAR Online Publication Software from Lions Light Corporation
© Copyright 2020

Rendered 09/22/2020 19:26