Sorting Grids were used in the shrimp fleet (Pandalus Borealis) in the late 1980's. The purpose was to fish shrimp in areas where there was a lot of small fish, without getting this as bycatch.
Since then the use of grids has become increasingly widespread, and are now being tested in various fisheries.
Scanmar developed early on a highly sophisticated sensor, Trawl Speed/Symmetry sensor, which made very precise measurements of water velocity into and across the trawl opening. To obtain absolute accurate measurements the sensor that was developed had an inclinometer (angle measurer) which was used to correct the measurement if the sensor is not lying horizontally.
It was therefore easy to create a variation of the sensor, the Grid sensor, which measures the water flow through the sorting grid and the grid’s angle. For shrimp grids (Pandalus Borealis) is grid is most efficient around 45 degrees.
- Water flow through the sorting grid is the most important observation: It is not rare that the grid is being blocked by the skate, seals, sharks, rocks, etc. In some seasons there are large numbers of jellyfish that clog the mesh in the bags so that water doesn’t flow through. When this is the case it’s time to end the haul.
- With shrimp (and fish that with no swim bladder) the bag sinks as it fills. It is common to start the haul with an angle above 50 degrees and end it when the angle approaches 37-38 degrees.
- Changes in the angle give a good indication of the catch at any time, and the log curve also shows where the influx was the greatest.
Trawleye and Grid sensor used on a shrimp trawler. The shrimp influx is visible on the left side of the screen which displays the Trawl eye’s detection. It can also be seen as blue markings in the log image of the Grid sensor to the right. The grid angle has fallen (ascending curve) from about 550 to 440. Impact in the angle is corresponding to the influx of shrimp.