A Team Listening to the Saguenay Fjord and its Belugas
Since mid-August, a team of researchers from the Maurice Lamontagne Institute, the Rimouski Institute of Ocean Sciences and the Saguenay–St. Lawrence Marine Park has been listening to the Saguenay Fjord and its belugas using a set of drifting hydrophones – aquatic microphones – that capture sounds heard underwater.
About 105 km long, the Saguenay Fjord is one of the habitats favoured by the St. Lawrence beluga that are commonly seen from its mouth at Tadoussac, as far as Baie Sainte-Marguerite, about thirty kilometres upstream. The Fjord is well known for its particular marine environment, characterized by brackish water near the surface and a thick layer of cold, salty water below. Its depth reaches more than 250 m in some places. However, it is much less known for its underwater sound characteristics.
The beluga is a marine mammal that makes extensive use of sounds to exchange information with other members of its pod, and moves underwater and hunts its prey using its advanced ultrasonic echolocation system. It therefore needs a healthy sound environment to perform these vital functions.
What are the sound qualities of the beluga habitat along the Saguenay Fjord?
This is the question posed by the team of researchers led by Yvan Simard, Director of the Fisheries and Oceans Canada Research Chair in underwater acoustics applied to ecosystem research and marine mammals at ISMER-UQAR.
In search of answers, the team conducted two campaigns to measure the underwater sound environment along the Fjord last August and October. It used specialized acoustic systems to record a wide range of sounds, including the various whistles and ultrasonic echolocation clicks of the belugas. These autonomous systems were suspended from buoys deposited in the Saguenay. They then drifted with the current, collecting acoustic recordings at several depths in the two characteristic water bodies of the Fjord. Their position, provided by GPS, was transmitted in real time to the research vessel via the SPOT satellite network. Thus, using an application on a smartphone connected to the Internet, the team was able to follow the trajectory of the drifting hydrophone networks remotely.
A unique environment for the propagation of underwater sounds
The geomorphological and sedimentary characteristics of this narrow, steep-walled glacial valley make the Fjord a unique environment for the propagation of underwater sounds. In water, sound travels at a speed of about 1.5 km/second, or 5,400 km/hour. A sound emitted in the center of the Fjord is immediately reflected in several directions by the rocky walls. Its echoes can be heard for several seconds. This is called reverberation and can be heard in very reflective places, as with music or singing in a cathedral.
The Fjord is not a straight line, but rather a broken line with several curves. In the detours between the straight segments, sound transmission is greatly affected by the degree of curvature. Only certain sounds can travel to the neighbouring segment.
In addition, glacial deposits separate the Fjord into three basins with differing areas, depths and sediments accumulated in the bottom. All of these features generate changing sound conditions along the Fjord.
In the coming months, the team will analyze the abundant collection of acoustic recordings and oceanographic surveys from the measurement campaigns to provide a general picture of sound levels and sound propagation conditions in these bodies of water. The recordings collected at the various sites along the Fjord include measurements of the natural sound environment, without sources of noise generated by humans, as well as measurements of small boats or merchant ships in the presence of belugas. This project will allow the team to lift part of the veil hiding the underwater acoustic landscapes of the Fjord.