New Norwegian research believes it can better estimate the tonnage of a school of fish
Marine research has developed a method that can better estimate the size of a school of fish, primarily for herring and mackerel.
The method, published in a new scientific article , uses raw data from the sonar plus physics and advanced math. A simple figure is published for how many tonnes the herring or mackerel hour is, plus a value for how safe the estimate is, writes Havforskningen
The industry is free to incorporate the new algorithm into its sonar. (see case here)
“The precision so far seems to be plus or minus 20 percent. But this presupposes that the skipper is patient and takes the time to inspect the shoal from several sides. Only then will this method get enough information about the fish’s behaviour, which is absolutely crucial for accuracy,” explains marine scientist Héctor Peña who has been involved in the experiments.
This is because the echo ability is lower in a fish swimming directly towards you than in a fish measured from the side. To compare with what we see with the eyes, you can think of the silhouette of the fish seen from the front versus in the side profile. And when it comes to echoes, the difference is much, much bigger.
The sonar must therefore be allowed to check the shoal in 360 degrees before it makes its calculations, says Peña.
The researchers have initially developed the calculation method for herring and mackerel, but the principle is transferable to other species.
To arrive at it, they have used modern Simrad sonar with 64 beams which they calibrated to scientific standard. The more sonar rays, the more information you get about the shoal – a bit like the resolution of a digital image.
When all the beams are calibrated to a known size, usually a metal reference ball that the scientists fire down, they together provide information that is reliable enough to calculate both the volume and density of the school of fish.
Checked against whole shoals
– To assess the accuracy of the measurements, we helped measure and capture a total of 76 herring and mackerel hours with chartered purse seine boats.