New Paper: Using acoustics to learn about harbour porpoise behaviour in tidal environments

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New Paper: Using acoustics to learn about harbour porpoise behaviour in tidal environments

Last week our colleagues at SMRU and PAMGuard published a paper on an approach they developed to successfully find and track harbour porpoise in tidally active areas. Their paper entitled Passive acoustic methods for fine-scale tracking of harbour porpoise in tidal rapids was published in the Journal of the Acoustical Society of America.

Their study developed a way to record the diving behavior of harbour porpoise in tidally active environments. These areas are often important feeding areas for these small marine mammals. Porpoise use the predictability of tidal flows in these areas to find food. This predictability is also attractive as a source of clean renewable power.

The mixture of marine mammals and the physical infrastructure required to harness the power generated by tidal flows presents a conundrum for regulators. How will this technology impact the animals that live in, and visit these areas (including fish and birds)? The prospect of animals colliding with tidal generators is a big concern and is the driving force behind much current research (including much of what we do at SMRU Consulting) into the environmental risks associated with tidal energy development. However, we still do not fully understand the risks of collision.

To get a handle on understanding how tidal energy developments may impact marine mammals it is important to understand how these animals use their environment. This presents challenge when marine mammals spend most their lives beneath the surface.

A commonly applied method for understanding how marine mammals use their underwater environment has been to attach special GPS tags to animals. But, tagging animals is hard and costly. Therefore, other more cost-effective means are sought after to study marine mammal behaviour.

Photograph by Bill Curtsinger

Photograph by Bill Curtsinger

Studying harbour porpoise comes with its own set of challenges. Porpoise can be difficult to spot at the surface because of their understated surfacing behaviours, but porpoise are chatty. They vocalize a lot and this allows us to eavesdrop on them to learn more about what they are doing and how they use their environment.

Harbour porpoise produce highly directional, high frequency clicks. The authors of this new study used these characteristics to develop a passive acoustic monitoring (PAM) method to find (localize) and track harbour porpoise underwater.

Hydrophone (underwater microphones) arrays have commonly been used to track vocalizing cetaceans (whales, dolphins, porpoise) underwater. All that is needed are a series of connected hydrophones. Then by using the time that it takes a whale call, or a dolphin click to arrive at each hydrophone researcher can calculate where in the water column that animal is.

Macaulay and his colleagues adapted what is called in the acoustics world as a large aperture vertical linear hydrophone array so that they track harbour porpoise in 3D. To translate this, they essentially adapted a system where hydrophones were widely spaced on a vertical line and added in a combination of sensors that allowed them to calculate exactly where in the water column each hydrophone was (i.e. in 3D). This meant that when localizing porpoise clicks they could also determine where in the water column the click was coming from.

Figure 6 in Macaulay et al. 2017: Example of dive fragments of harbour porpoises in the Sound of Sleat, Scotland. (a) shows an example of geo referenced tracks and bathymetry. (b) shows an example of track fragments in depth and time. Note size differences in fragments in both graphs. Tracks can be tens of seconds long, forming a significant portion of an animal’s dive, or can be just a few seconds long, providing a “snapshot” location of an animal.

The study used PAMGuard’s click detector module to identify harbour porpoise clicks and combined their localization and tracking methods with a new PAMGuard module, developed specifically to calculate animal positions.

Macaulay and his co-authors successfully demonstrated that their system could not only find harbour porpoise in the water column but that they could do this in tidally active areas with strong currents. They were also able to show that the methods they developed to automate data analysis were effective.

Both the hydrophone array and the data analysis methods developed in this study are shown to be successful and a promising new cost-effective survey method for uncovering the secrets in the fine-scale behaviour of harbour porpoise in tidally active areas -a tool that will be particularly valuable to the developing tidal energy industry and regulators trying to understand risk.

 

 

 

 

 

 

To see the full details of this study check out the following link:

Macaulay, J, J Gordon, D. Gillespie, C. Malinka, and S. Northridge. 2017. Passive acoustic methods for fine-scale tracking of harbour porpoises in tidal rapids. Journal of the Acoust. Soc. Am. 141:1120-1132

 

SMRU Consulting has worked on a number of projects related to understanding harbour porpoise presence, distribution and habitat use in tidal environments. Some of these include:

Past and present monitoring of harbour porpoise in the Bay of Fundy using C-PODs.

SMRU Consulting is also actively involved in the ongoing maintenance of PAMGuard and the efforts to develop new modules, upgrade the software and provide user support.

 

 

About the Author:

Frances is a Research Scientist at SMRU Consulting North America. Check out her bio under the "About Us" tab.

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