Do You Know Where Your Fish Are?
Customer: Cornell University
Rising costs of ship time and increasing budgetary constraints limit current fisheries acoustic survey practices, while the resulting limited spatial coverage and relative infrequency of such surveys compromise their value to fisheries managers. Liquid Robotics’ Wave Glider allows scientists to collect data in a wider range of weather conditions and using fewer man-hours than typical shipboard acoustic surveys while operating at a greatly reduced cost.
“This is a game changer. The Wave Glider gives us the means for continuous acoustic monitoring of fish and micronekton populations, at a fraction of the operational costs of shipboard surveys.”
- Chuck Greene, Cornell University
How We Helped
As part of an NSF-sponsored research project, Cornell University, BioSonics, Inc. and Liquid Robotics collaborated to design and manufacture a custom tow-body containing a specially modified dual-frequency BioSonics echosounder with 70-kHz and 200-kHz transducers.
The echosounder provided high-resolution, full water-column echograms. While these were used to monitor for the mesopelagic boundary layer, they could also be analyzed for many types of information: bathymetry, bottom type (e.g. sand vs. rock vs. mud), quantitative biomass estimates of fish and other organisms, presence of boundary layers in the water column, etc. While the raw data was downloaded when the vehicle was recovered, low-resolution summaries of the data were retrieved via satellite connection while the vehicle was deployed.
The Wave Glider with its compliant cable was able to effectively tow the echo sounder tow body and maintain it in a horizontal orientation suitable for the collection of high-quality acoustic data. The system was shown to be an effective method for collecting high-quality acoustic data for studying patterns in the distribution and migration of marine organisms. It also holds great promise for applications oriented towards continuous assessment of commercial fish stocks and the monitoring of marine ecosystem health.
Testing was performed in Kealakekua Bay off the west coast of Hawaii Island during March 2012 in an operational test area of 1 nm x 100 m. The Wave Glider was instructed to continuously run the operational box, with the transducers alternating pings in both active and passive mode. The speed through water of the Wave Glider pulling the tow body during this test was approximately 1.2 knots in a moderate sea state.
Echogram of the 70 kHz transducer in a moderate sea state
Echogram of the 200 kHz transducer in a moderate sea state
The ocean bottom can be seen at a depth of 90 to 100 meters as the upper bright red line in the echograms. (The lower bright red line at a depth of 180 to 200 meters is caused by sound backscattering from the bottom, off the ocean surface, and off the bottom again, so it is at roughly twice the true depth.) Significant biological backscatter is visible all along the ocean bottom at a depth of 70 to 90 meters, as well as a likely aggregation of fish at approximately 60 meters.