Two echograms, 120 kHz (top) and 38 kHz (bottom).

This page provides technical details intended for a reader interested in the specific parameters of our hydroacoustic work at ABL. Acoustics can be used to study most of the species found throughout the water column or can be focused on a specific species by using the right frequency. The common frequencies used for fisheries research include 18 kHz, 38 kHz, 70 kHz, 120 kHz, 200 kHz, and 400 kHz. We mainly collect data using the 38 and 120 kHz frequencies. Fish with air bladders produce strong backscatter signals and are well detected by the 38 kHz frequency. The 120 kHz works well for fish with no air bladder like eulachon.

Examples of transducers.

The dark blue layer in the top echogram (above) is absent in the bottom. This is a layer of weak scattering juvenile eulachon which is clearly seen at 120 kHz but not at 38 kHz. The color scale on the left shows the intensity of the signal with brown being most intense, having the strongest target strength. The red line at the bottom of both echograms is the ocean bottom. Acoustic signals are transmitted and received using frequency specific transducers which are mounted directly on a boat’s hull or towed in a towbody like the four pictures to the right. The top right picture is of the bottom of a fin with two red transducers with two frequencies, the smaller one is 120 kHz and the larger is 38 kHz. This towbody can be towed by a 21 footer and up like the 98 foot John N. Cobb in the top left photo with the towbody circled.

Acoustic data collected at Iutak Inlet, AK.

The graphs to the right are representations of the data collected from acoustic signals. These data are referred to as mean volume backscatter (Sv) or nautical area scatter coefficient (NASC). They also show a typical box type survey layout used to best cover an area. The area surveyed was lutak Inlet near Haines Alaska. The left graph above shows Lutak Inlet before any pre-spawning eulachon have showed up while the right graph shows eulachon throughout the inlet. Combining our acoustic survey and counts of sea lions our study showed a strong correlation between feeding lions and eulachon arrival with the eulachon peaking 2-3 days before sea lions.

Example of frequency specific results in 120 kHz and 38 kHz range.

A key point to using acoustics is to know your equipment’s limitations, specific frequency or frequencies for specific signals and studies. The graph to the right shows 120 kHz in blue and 38 kHz in red. It illustrates 38 kHz is best for strong backscatterers and 120 kHz is only OK but 120 kHz works very well for weak scatterers and 38 kHz does not work well at all.

Contact:
Dave Csepp
Auke Bay Laboratories
Alaska Fisheries Science Center, NOAA Fisheries

Ted Stevens Marine Research Institute
17109 Pt Lena Loop Rd
Juneau AK 99801
Dave.Csepp@noaa.gov