It is well known that large quantities of fish disappear under the bottom gear when trawling for demersal fish. This is the case in all commercial fishing, as well as when researchers conduct their bottom trawling trials. Researchers have documented this with video recordings and use of collection bags under the trawl. Some claim that as much as 50% may escape when the fish is pressed down on the seabed.
The Bottomcontact sensor has a solid construction.
New understanding of the problems
By having focus on this problem for many years, Scanmar has arrived at two important conclusions:
- Use of underwater ROV and video recording we can determine that practically all fish enters the trawl in the midsection. The side gear directs fish towards the midsection. It turns out that the side gear also lifts more easily and loses bottom contact much more often than the mid-gear. This is because the doors lift or that they jerk across uneven seabed.
Fish therefore usually disappear underneath the side gear, and this has to be the initial area of focus
- Although it is obvious that it is of vital significance that the mid gear keeps bottom contact, it is how hard it presses down on the seabed that is critical.
On uneven seabed, with strong current, it is important that the gear is rigged to presses hard on the seabed with as little movement within the gear as possible.
On soft seabed it can be rigged a lot lighter; the doors move more evenly and the gear will dig down, creating unnecessary towing resistance, and possibly fill the trawl with sand and mud.
Scanmar has therefore developed a new type of Bottom Contact sensor. Both in practical fishing, and for research purposes it has proven to satisfy all requirements for registering bottom contact. It is also capable of showing whether the distance between the trawl gear and seabed is due to uneven seabed, or because the seabed contact has been lost.
The sensors measure the angle in the same way as the Angle sensors on the trawl doors, and it is therefore easy to register how hard the gear presses on the seabed, or if the seabed contact is lost. When the seabed contact is lost, the angle will increase, while the ScanFactor, or stability, shows if the gear moves unevenly.
In the shooting phase the sensor will hang vertically, and therefore reports a large angle, and we see how the angle reduces when the sensor touches the seabed. It settles down in a stable position once the gear is on the seabed, and reports when towing can begin.
While towing, it can be observed that this angle changes as the door distance varies. This shows that the square is of importance for how hard the gear presses on the seabed. Therefore the Bottom Contact sensor is a good indicator of the correct towing speed in each individual haul, even if this is at the expense of the square, and whether the warp lengths should be adjusted for the best square possible, without this impacting the desired bottom pressure of the gear.
In practice the reason the bottom gear loses contact has been shown to most commonly be due to one or both of the trawl doors losing seabed contact. In which case you also receive the negative effect of the sweep lines not working as expected.
It appears from what is written above, and “everyone” knows instinctively, that there is a connection between the towing speed (water flow into the trawl), the trawl’s symmetry around the current, the doors’ angle and distance on the one side, and the gear’s bottom contact on the other.
When the Bottom Contact sensor shows an undesirable change you can therefore immediately determine what the reason is by monitoring the Door Distance, Door Angle, Trawl Speed and Symmetry.
And vice versa; fluctuations in these sensors will then lead to changes in the bottom contact.