For 30 years Scanmar has delivered systems for monitoring the fishing gear used by fishing and research vessels.
Before Scanmar was established there were only a few simple trawl sounders in use, and the first Scanmar systems were also for the catch itself; TrawlSounder, Depth, Catch and Temperature sensors. The fishing gear was largely controlled by checking the wear and tear on the bottom gear and measuring the angle between the wires. Back then, however, one usually fished under less demanding conditions than are found today.
The most interesting thing that has happened, seen from our point of view, is that more and more people have put focus on their gear. When Scanmar introduced Distance sensors more than 25 years ago, we were greeted with incredible skepticism. Virtually no one could see a need for them. Today, most fishermen find them indispensible. The same was true of TrawlSpeed / Symmetry sensors.
The speed at which the trawl (and doors) passes through the water and the influence of underwater currents are what matter most for effective fishing:
- Low towing speed causes the fish, especially the largest, to escape and it reduces the door spread.
- High towing speed leads to overspreading of the doors, lost bottom contact, and a bucket effect in the trawl with the result that fish disappear through the panels.
- If the trawl doors and trawl opening are not adjusted for cross current the doors will be affected differently. This leads to different angles, instability, and fluctuations in the spread of the doors. Often bottom contact is lost on at least one door.
- A trawl that is skewed in relation to water inflow (asymmetrical) leads to incorrect geometry of the trawl and fish escaping directly.
Gradually, most fishermen have become aware of this, in no small way owing to their experience using Distance sensors, TrawlSpeed / Symmetry sensors and Angle sensors on the trawl and the trawl doors to allow them to understand the scope of the problems.
These experiences, records of trawl hauls, and our own experiences have been so interesting that we have put some extra emphasis on finding out why some fishermen perform better than others under the same conditions. Therefore, we have looked closely at the trawl and trawl doors by studying recordings and reports from various vessels and our own observations on board. The first thing that stands out is that things that should be consistent are not and that only small changes can have a major impact. Here are some examples:
- Three new, completely identical trawls were tested using the same gear. During towing, one sweep showed a 3.8 meter vertical opening, another sweep showed a 4.2 meter opening, and the third sweep showed a 4.8 meter opening. We do not know what caused it, but it has obvious significance for the towing resistance and catching ability.
- On another occasion we recorded that one trawl door was 7⁰ more inward than the other and became more unstable. When we checked, it turned out that the warp was attached 4 cm higher (from the shoe) than on the other.
Let’s look at the differences that have an impact on the trawl doors and trawl movement, and therefore on capture efficiency.
Trawl doors
- Size, weight and construction are perhaps what most people think of immediately. But there are many other things, such as the details of the design, foils, curvature, size of the upper and lower panel, mounting points for the warp and the back strap, etc. that have much greater significance than one might at first think.
- In practice, changes in depth of fishing, the relation between length of warps and depth, and the condition of the bottom lead to substantial changes in the doors angles, stability and ability to spread.
Trawl
- Trawl shape, mesh size, thickness of netting, knots, length of the intermediate section and so on are all of great significance for trawl efficiency in general, but also so that fish will quickly be moved back in the bag and not left in the trawl to reduce water flow and create a bucket effect.
- Trawl efficiency is affected by towing speed, bucket effects resulting from towing speed or filling of the bag, and twisting of the trawl and the cod end.
Trawl and Trawl Doors proportionality
- Whether it is new or old equipment, there are minimal chances for a trawl and trawl doors to match 100%. And if they were to do it, wanting to fish at different depths with different warp lengths, wind, currents and bottom conditions mean there will be differences in how the trawl and trawl doors match.
- As mentioned above, it is important that the water speed in the trawl opening is optimal (not so low as to allow fish to escape and not so high as to create a bucket effect in the front part of the trawl), and that it is 90⁰ on trawl opening. Therefore it is necessary to adjust the back straps and sometimes the mounting of the warps so that the trawl can be towed at the speed for which it is designed.
Some of the most common problems caused by the gear (and conditions):
Bottom trawling
- The doors lie down in the landing phase and it takes time before they rise
- Varying door distance
- Unstable doors
- The doors will lose bottom contact
- The gear loses bottom contact
- Incorrect vertical opening
- Improper towing speed
- Asymmetrical trawl geometry
- Cross currents
- Difference in warp length
- Bucket effect in the trawl
- Twist of the bag
Pelagic trawl
- The doors are at different depths
- Wrong towing speed
- Unbalance in the horizontal plane
- Bucket effect in trawl and bag
Every haul is affected by the vessel's movement as a result of wind and waves, while underwater currents and bottom conditions are of great significance for trawl efficiency.
What for many is perhaps most surprising is how big the impact is when caused by only small changes in those conditions.
Trawl doors:
- Changes in speed through the water as a result of underwater currents immediately affect the doors’ roll angle (vertical angle), and therefore also the spread rate (distance) and stability, leading sometimes to loss of bottom contact. These problems spread to the sweeps and trawl and result in an inefficient haul.
- Cross currents mean different forces act on the doors in relation to the angle of impact, providing a different roll angle, stability and spread rate for each of the doors.
The port door loses bottom contact and the trawl collapses. (1 and 2) A lot of pitch and roll makes the door unstable and loses bottom contact. (3) The trawl doors lie down and lose contact.
The Trawl:
- Impact on the trawl doors will have immediate impact on the trawl in that both the horizontal and vertical openings are changed while the gear easily loses ground contact.
- Change in towing speed (water speed in the trawl opening) causes fish to escape if it is too low and, because of bucket effects produced in the trawl, allows fish to escape through the panels if it is too high.
- A cross current (the angle of the water entering the trawl, which should be 90°) will cause the trawl to be towed skewed. In this case the mesh is closed on one side and stretched on the other, thus allowing the fish to go straight out.
- Because of the trawl’s shape, where the rear part is substantially smaller than the opening, a bucket effect is formed where the flow is large compared to the area it can exit. The bucket effect allows fish to escape if the mesh size allows it.
- All cod ends and most trawls twist as they fill. The amount of twist decreases as the bag becomes full, but before that point the twist reduces water flow and causes loss of catch.
- The conclusion is that in every haul the gear is influenced by factors that affect the vessel as well as underwater currents, changing ground conditions, etc. In order to achieve the most reliable results it is absolutely necessary to continuously monitor the elements that are affected and immediately make adjustments so that the haul is once again optimal.
Sensors increasingly being used and recommended by Scanmar:
- Angle, Distance and Depth sensors on the doors
- TrawlEye and TrawlSpeed / Symmetry sensor in the trawl opening
- Tunnelflow sensor in the tunnel, and Twist sensor on the trawl and cod end