So You Want to Know: What is an acoustic survey?

First published in the MLA Newsletter, October, 2012.

An acoustic survey is a method of gathering information on the abundance of a fish species using echo sounders. Sound is a form of energy which travels farther and faster underwater than in the air. So using the distance a sound travels and the record it makes when it bounces off of something underwater can tell a researcher a good deal about the creatures passing by underwater.

In an acoustic survey, a transducer emits a pulse of sound. The transducer can be either fixed or mounted on a moving vessel. The sound moves through the water until it bumps into something, such as a fish, and part of its energy is reflected back to the transducer. Another device, called an echo sounder, amplifies the returned energy and sends it to a recorder where it registers as a mark.

The sound emitted by a transducer will register on the echo sounder differently depending on whether it hits something solid, water-filled or air-filled. The swim bladders of herring make a distinctive mark because herring have relatively large swim bladders which are filled with air, making the fish stand out against the water in which they swim. Researchers must edit out the echoes received from non-targeted fish in order to gain a clearer picture of the size and density of the herring school.

Acoustic surveys are generally done on spawning fish because during the spawning season the fish predictably aggregate in larger schools. Groundfish stocks, however, have long been subject to acoustic surveys to estimate stock abundance. Today, numerous groundfish stocks are being managed at least partly on the basis of acoustic data collected through acoustic surveys. Atlantic cod, haddock, pollock and redfish are surveyed using acoustic devices in Norway; Canada uses the method to keep track of cod and Pacific hake.

Herring acoustic survey, Casco Bay, September 17, 2012. Image courtesy of Curt Brown.

The herring acoustic survey taking place in Maine could influence the stock quota set by the New England Fisheries Management Council (NEFMC). When NEFMC cut the herring quota in 2009, the National Marine Fisheries Service (NMFS) had not conducted a survey to estimate the herring population. The NEFMC’s next evaluation of herring will take place in 2014.

Lead scientist Graham Sherwood from GMRI has already seen many schools of herring in the data the ten participants have sent back. The data is recorded on a computer aboard each vessel then downloaded to a thumb drive that is sent to GMRI for analysis. Data from each set transect will be received at GMRI until the end of November.

Since herring is such a valuable part of lobstering, this survey, the first of its kind, will provide useful data to managers in the herring and lobstering industries. “We’re really excited to see what’s out there,” said Sarah Paquette, MLA’s industry communications assistant. “And it’s great to hear from the guys involved each week about their trips and what the acoustics picked up along their transects. Everyone has been great about this project.”