It is important for fish farmers to track the number of fish in the seine, but doing so usually requires going in and actually netting some fish. According to a new study, echo detection technology could soon be used as a simpler and more accurate alternative. Catching some farmed fish on a regular basis is not only difficult and time-consuming, but also stressful for animals, and it doesn't always provide accurate fish population estimates. This is where echo detection comes in.
This sonar-like technique — which involves sending an acoustic pulse into the water and then detecting its echo on underwater objects — has been widely used in commercial fish finders. However, such devices are usually only used to show the location and approximate size of schools of fish. Assessing the actual number of fish can be difficult because animals at the top of the shoal tend to obscure the animals below, shielding them from sound pulses, so they cannot be clearly detected.
As part of the EU-funded informfish project, scientists from the Sintef Institute in Norway and the Hellenic Centre for Marine Research (HCMR) in Greece set out to tackle the problem.
Doing so requires starting with two seine nets — each with a known number of salmon — with an echo detector between them. The researchers then continued to take readings as they varied the distance between the two fences and the distance between each fence and the echo detector.
By comparing the various readings of the device with the number of known fish in the pen, the consistent relationship between the information provided by these readings and the actual number of fish can be determined. The echo detector can then be calibrated to provide proven reliable estimates of fish density within the purse seine.
"We are making exciting progress and I look forward to further development," said Dr. Walter Caharija of Sintef. "We're building a foundation from which we're learning how to use echo detectors to better estimate biomass within production seines."
The Norwegian company Kongsberg Maritime has expressed interest in commercializing the technology.