Hydrophone arrays, FTW!

Figure 1. Photo of Amy and Emily deploying the array off the back deck of the R/V Ocean Starr.
Figure 1. Photo of Amy and Emily pulling in the array at the end of the day.

We do a lot of things out here on the CalCurCEAS cruise – we play cribbage, we eat cookies, we ride the stationary bicycle – but mostly we do these two things, A LOT:

1) Look for whales and dolphins.
2) Listen for whales and dolphins.

Part 1, the “Look” part, is within the realm of the visual team, who stand bravely in wind and cold and rain (and sometimes gorgeous, balmy sunshine) and scan the surface of the ocean for animals that might pop up to say hi and or take a quick breath of air.

Part 2, the “Listen” part, exists because, well, it’s pretty difficult to see what animals are doing once they dip down beneath the waves. And we all know that’s where the fun stuff happens (synchronized swimming practice? underwater tea parties? you never know!).

Since I’m on the acoustics team, I thought I’d give a little overview of how our operation works.  We eavesdrop on the animals using a pair of hydrophones, which are basically underwater microphones. If we just used one hydrophone, it would be okay, we could hear the animals. But adding a second allows us to not only hear them, but figure out where they are.

Figure 1.  Top: Array geometry, showing how we can tell which direction a sound is coming from. Bottom: Signals measured on each of the hydrophones.
Figure 2. Top: Array geometry, showing how we can tell which direction a sound is coming from. (The poor creature pictured is my sad attempt at a striped dolphin, if you were wondering) Bottom: Signals measured on each of the hydrophones.

The pair of hydrophones are built into the end of a cable that is spooled onto a winch on the back deck. Every morning we head out there, don our life jackets, hard hats, and boots, and reel out the cable until the hydrophones are 300 meters behind us. The hydrophones are spaced one meter apart, and once the vessel is up to speed, they are roughly at the same depth.

The upper part of Figure 2 (blue) shows a cartoon schematic of the hydrophone setup. Here’s what it all means:

H_1 and H_2 – hydrophones #1 and #2

Known quantities:

d_h – distance between H_1 and H_2. For our array this distance is 1 meter

c_w – measured sound speed in water (approximately 1500 m/s, but depends on temperature and salinity)

Measured/derived quantities:

\Delta t – time delay between when the signal arrives at H_1 and H_2

d' – distance associated with the time delay \Delta t, derived using c_w

Unknown quantity:

\theta – angle between the incoming ray path and the array baseline. This is what we’re solving for!

OK, that seems complicated. But feel free to ignore the math, of course. The basic idea is that depending on where the sound comes from, it can arrive at the hydrophones at different times. For example, in the image above, it hits hydrophone 1 first, and after some amount of time, it hits hydrophone 2. The signals from each of the hydrophones are sent upstairs to the acoustics lab (see bottom part of Figure 2). The call shows up at slightly different times on each of the hydrophone channels, and we can measure that time delay \Delta t very precisely.

Using the time delay \delta t and the measured sound speed c_w, we can obtain distance d' using:

d' = c_w * \Delta t

So now we’ve got a right triangle where we know the hypotenuse and one other side, and you know what that means – trigonometry time!! Everyone’s favorite time! We finally have what we need to solve for the angle \theta.

\theta = acos( \frac{d'}{d_h})

We now know what angle the dolphin is at relative to the array. Huzzah! But wait. There are just a couple of little details that you need to remember (see Figure 3). First: you don’t know how far away the dolphin is. Second: there’s this pesky thing called “left-right ambiguity” *.


From the perspective of the array, there’s no difference between an animal calling from an angle \theta to the left and an animal calling from an angle \theta to the right.

Figure 3. Left-right and range ambiguity.
Figure 3. Left-right and range ambiguity.

These are fundamental limitations of the method, but we can (sort of) overcome them. As the vessel moves along, and we estimate angles at different locations, we end up with a location where most of the bearing lines intersect. If the vessel is traveling in a straight line, we can get a good idea of range – how far the animal is from the trackline. We just won’t know which side of the trackline it’s on. But if the vessel makes a turn, the new bearings estimated after the turn will resolve which side of the line it’s on!

Figure 4. Bearing estimates (red lines) taken at different locations along the track line. Probable location is where most of the lines intersect.

At this point you might be wondering, Michelle, what assumptions are you making when you do these calculations? So here they are:


  • The array is horizontal
  • The animals are calling at the surface
  • The animals are staying in approximately the same location for the duration of the measurements

So there you have it. That’s how we locate animals underwater with a towed, 2-element hydrophone array.


* Yin, one of the amazing visual observers on the cruise, thinks “Left-right ambiguity” would be a great name for a band, and I agree.

** assumptions are made to be broken

Whales in a Noisy Ocean


In the ocean, sound rules.  Unlike on land, where animals (just like us humans!) get a lot of information from light, animals in the ocean have evolved to take advantage of sound, which is much more effective than light under water.  Light gets absorbed quickly, but some sounds, like the low-frequency calls of blue whales, can travel hundreds of kilometers under water, under the right conditions.

Whales have developed really fascinating ways of using moans and clicks and songs to help them get the things they need in life – friends, mates, food… what more could a whale want?  Some whales find food by making sounds and then listening for echoes that bounce off of their prey.  Using those echoes, they can figure out where their food is (mmm, delicious fish!).  Whales can also use sound to communicate with each other.  A mother and calf might need to keep track of each other as they swim.  Or a male might show off his sweet singing voice to attract a female.


Unfortunately, noise in the ocean is on the rise, and it’s making life tougher for whales. Some of the sources of human-caused noise include ship traffic, seismic exploration for oil & gas and sonar testing. It’s worth noting, though, that there is noise in the ocean that is not caused by humans. Earthquakes, volcanoes, breaking waves, rain and even lightning are some of the things that add to the background noise. The thing is, whales have spent thousands of years evolving to deal with these natural noise sources – but in the last few decades they have suddenly had to get used to the growing din caused by human activities. And, as Chris Clark from the Bioacoustics Research Program at Cornell University explains, “it’s not just one ship. It’s ten thousand ships”. It’s the same as if only one person litters on the side of the road, it might not be such a big deal. The problem arises when everyone does it, all at the same time.


Scientists are still not sure exactly how noise affects whales – different whales might be more or less sensitive to particular sounds, and might respond in different ways. For example, right whales in the Bay of Fundy showed lower levels of stress hormones when ship traffic stopped briefly following the 9-11 terrorist attacks [1]. Killer whales off the coast of Washington state and British Columbia have increased the volume of their calls so that they can be heard above vessel traffic [2]. And in the most severe cases, some beaked whale strandings have been linked to mid-frequency naval sonar operations [3].

Excessive noise in the ocean causes a sort of masking effect – meaning that the noise is loud enough that the whales can’t hear what they normally would need to hear in their environment, whether it’s echoes from fish, or a signal from another whale. Not being able to find a mate or find food or find each other is a serious problem, especially for species that are already endangered.

But there is still hope. Now that we’re starting to realize how harmful noise can be, we’re finally in a position to actually do something about it. Small steps can make a big difference. Even slowing ships down can substantially reduce the overall noise. In recent years, the US Navy has funded a lot of basic research that has taught us a huge amount about how marine mammals hear and use and produce sounds. We’ve still got a long way to go, but now that we’re aware that there is a problem, we can work on ways to fix it.

This post was done with help from Chris Clark and Kevin White from the Bioacoustics Research Program at Cornell University.  The PDF version can be downloaded here.


[1] http://news.sciencemag.org/sciencenow/2012/02/shhh-ocean-noises-stress-out-wha.html
[2] Holt, Marla M., et al. “Speaking up: Killer whales (Orcinus orca) increase their call amplitude in response to vessel noise.” The Journal of the Acoustical Society of America 125.1 (2008): EL27-EL32.
[3] D’Amico, Angela, et al. Beaked whale strandings and naval exercises. Space and Naval Warfare Systems Center, San Diego CA, 2009.

What does a fin whale call sound like?

If you play back this audio file, you’ll be listening to a couple of fin whales and an earthquake, and according to my dog, that’s some exciting stuff. No, seriously, I played it on my laptop the other day, and Trooper got all agitated, and started growling and barking in what I can only assume was confusion. (“where’s the fin whale?? it’s got to be around here somewhere!”)

The colorful figure at the top of the post is called a spectrogram.  Time marches across to the right.  Frequency increases upward.  And the colors basically indicate loudness – brighter colors are louder.  This particular chunk of data was recorded in the dead of winter just off the coast of British Columbia, Canada, under more than a kilometer of water. Even though the instrument is designed to measure earthquakes, it also picks up the very low, booming calls of fin and blue whales.  The spectrogram shows two slightly different calls alternating – one slightly higher pitched and one slightly lower.  We believe this is probably two fin whales passing near the seismometer.

You might notice that the audio clip is about 30 seconds long, but the spectrogram shows five minutes of data – that’s because the calls are down around 20 Hz, which is at the very lower end of the human hearing range.  (If you have tip top hearing, you are probably sensitive to sounds between 20 Hz and 20 kHz.)  I sped up the audio by a factor of 10, so that we can actually hear it – bloop… bloop… bloop…

At about 10:03am in the spectrogram, and 20 seconds into the audio recording, you can see/hear an earthquake in the background.  My seismologist colleagues tell me that this isn’t the distinctive crack of a primary or secondary phase arrival from an earthquake, but possibly the rumbling caused by a tertiary, or “T phase”, arrival.

So why did Trooper have a meltdown?  I guess you would too, if you thought you were suddenly surrounded by a couple of super high-pitched fin whales and an earthquake.

Eavesdropping on dolphins

Please welcome my next science+comics interview victim, Alexis!

Name: Alexis Rudd
Job: PhD student in Zoology at the University of Hawaii
Research: Alexis uses passive acoustic monitoring to study whales and dolphins off Hawaii

News Flash!! Whales and dolphins, a.k.a. cetaceans, hang out near the Hawaiian Islands! Okay, not a news flash at all, everyone knows that. But you may be surprised to learn that we don’t actually know much about them – what they’re doing, where they’re going, and why – especially once you get out to the deeper rougher waters further from shore. So why is it important to know about their hangout spots and behaviors? It’s because that kind of information can help us design and implement effective management and conservation strategies. Yes, cetaceans hang out near Hawaii, and we want to keep it that way!

Here’s the thing. Whales and dolphins (and loads of other animals) like to spend time where food is available. In the ocean, the big driver behind food abundance is primary productivity – phytoplankton, or plants that live in the sunlit upper layers of the ocean. And just like on land, different places in the ocean are more productive than others: some places are lush and green with plants and all the life they support, and other places are sort of like deserts – yes, animals live there, but it’s on a whole different scale. Here’s an image of productivity (actually, it’s an indicator of productivity – chlorophyll concentration, which satellites pick up by its color).


This picture shows green where there is a lot of productivity, and blue where there’s not much. So the deepest blues show where the ocean “deserts” are. The green places tend to be where nutrients are available – things like nitrogen and phosphate (yup, same stuff that’s in fertilizers for your garden), which might run off the land, or be brought up to the surface in upwelling zones.

But – not to worry! – it’s not exactly a dead zone around Hawaii. It just means that cetaceans might need to look for an oasis sometimes. One theory is that cold, nutrient-rich water gets pulled to the surface by cyclonic (counter-clockwise) eddies, triggering a cascade of activity up the food web – a very enticing fish/squid/zooplankton buffet indeed.

To address these kinds of theories, Alexis needs to figure out whether cetaceans are indeed seeking out certain environmental and oceanographic conditions. Often, studying cetaceans means getting on a boat and looking for them visually. This works reasonably well in calm waters, but it gets pretty tough to pick out a dolphin or a whale when the water is choppy. The prevailing winds are westerlies, and they come sweeping across the Pacific from California. Calm zones form on the leeward side of the islands, but the wind speeds up as it squeezes through the gaps, creating regions of high wind and rough seas.


Instead of looking for cetaceans visually, Alexis listens for the sounds they make. And here’s where the super creative part of her research comes in: she tags along on a tugboat that brings supplies to the different islands. Being the persistent scientist that she is, Alexis went out every two weeks for a year and a half, installing her underwater recording package (hydrophone) on the barge behind the tugboat. This setup was ideal in a lot of ways: first of all, people need their supplies all year round, so this vessel does regular trips. Second, having the hydrophone on a barge – separated from the loud tug engines – makes it easier to pick out cetacean sounds. Here’s what the basic setup looks like:


As the tugboat moves from island to island, it provides a great acoustic “view” of a very large area – the calm leeward side of the islands and also the windier regions between the islands. So Alexis is able to get a great “big-picture” idea of what’s going on.

She’s now collected hundreds of hours (!!) of recordings, and is going through the process of checking and documenting all of the identifiable cetacean calls. She links each call up to the location where the barge was at that time so that she can gather up all the data at the end and figure out whether there are correlations between the animals’ presence and environmental or oceanographic conditions.

Alexis has an excellent and very informative blog, and I encourage you to head on over and check it out: http://bioacoustics.blogspot.com/

She also did a guest blog post over at Scientific American Blogs, where she talks about the experience of partnering with commercial shipping to do her research: Towing my weight: partnering with commercial shipping for whale and dolphin research.

By the way, this is how an ocean themed interview should be conducted:


… with pirate hats all around.

Southern Resident killer whales with Juliana Houghton

It’s another science + comics interview!

Name: Juliana Houghton
Job: 2nd year Masters student in the School of Aquatic and Fishery Sciences at the University of Washington
Research: Southern Resident Killer Whales

Southern Resident killer whales live in the Eastern North Pacific Ocean. They are popular with residents and tourists of the Pacific Northwest and coastal British Columbia and are of particular interest because they are extremely endangered. There are only 88 individuals left! There are so few of them that each individual has been named: they have their “scientific” names (like L72 or J26) and also their informal names (like Mike or Cookie or Granny).

Sound is important
The Southern Resident killer whales (or “SRKW” population) are fish eaters, and they use “echolocation” to find their prey. You might have heard of bats using echolocation when they hunt in the dark, and in a way, the SRKWs hunt in the dark as well. As they move deeper into the water, visibility becomes restricted, and sound is a more reliable way to interact with their environment.


Watching whales
The Pacific Northwest is special because it is one of the few places in the world where you can go out and see these top predators in their environment, feeding and socializing with each other. One way to see them is to venture out on a whale watching tour. These tours are great for tourism and the local economy, and they also serve to increase public awareness. The downside is that the presence of whale watching boats in the vicinity of SRKWs might be disruptive: if they need to use sound to find food, then raising the background volume can interfere with that already difficult task.

Juliana’s master’s work is looking at the noise levels that SRKWs hear during busy whale-watching times. She does this by combining two unique datasets collected with researchers from NOAA Northwest Fisheries Science Center, Cascadia Research Initiative, UC Davis, and Killer Whale Tales.

The Experiment
The data for this project are acquired aboard small rigid-hull inflatable boats (RHIB) at times when whale-watching vessels are out. Using a long pole on the bow platform of the RHIB, an instrument package is carefully placed on a whale’s back using a suction cup. After a few hours of recording the ambient noise as the whale swims and dives, the suction cup is released and floats to the surface where the researchers can collect it.


While the hydrophone (or underwater microphone) records noise levels, the team on the RHIB scans the surface of the water for the tagged whale. Any time that they spot the whale, they use an instrument called a theodolite to measure the distance and horizontal angle of the whale relative to the RHIB, while also recording a description of the whale’s activity at that time (feeding, resting, communicating with other whales, etc). A second theodolite measures the locations of any other vessels within a radius of one kilometer, simultaneously recording characteristics such as vessel size, speed, and orientation relative to the whale.


Putting it all together
Juliana combines the noise level data with the vessel data to look at whether the noise levels that the whales hear is affected by the number of vessels, distance to those vessels and other factors. For example, when the tagged whale is sighted, you get a sort of snapshot of the vessels in the area. At the same time, you are listening in on the noise levels detected at the whale’s location (via the hydrophone data). This kind of information is extremely valuable because it can help guide the types of regulations that go into place for vessels in order to reduce the potential negative effects on the whales’ foraging and communication abilities.

Juliana would like to acknowledge her collaborators listed above, especially Marla Holt and Deborah Giles for their help with the acoustic and vessel location datasets.

Q: How loud are fin whale calls?

A:  Really loud!

Now that I’ve finally descended from my little soapbox (see this post), I guess it’s time to take my own advice.  I was inspired to go for it yesterday when I came across this blog post by Jessica Carilli: Why Geochemistry is Awesome.

Just last week(ish) my very first paper was published.  (Huzzah!)  If you feel so inclined, you can check it out for yourself here.  However, even I’ll admit that it’s a bit technical for someone who really just wants to get a big picture overview.  Here goes nothing!

“Source levels of fin whale 20 Hz pulses measured in the Northeast Pacific Ocean”

If you follow my blog, you probably already know that I’m pretty into fin whales.  They’re amazing and huge and, unfortunately, endangered.  And if we want to monitor their recovery and to avoid further risks from things like ship noise, ship strikes, and fishing activity, we need to have an idea of where they are and how they move around.

http://www.beringclimate.noaa.gov/essays_moore.html (photo by Lori Mazzuca)
http://www.beringclimate.noaa.gov/essays_moore.html (photo by Lori Mazzuca)

Problem is, despite being the second largest animal in the entire world, they are tough to keep track of.  They spend a lot of time far from the coast and a lot of time under water.  There are a few different techniques people use to study fin whales (and other whales, for that matter).  These include  visual surveys (from a boat or plane or land), radio and satellite tagging, and (of course) acoustics.  Each of these methods has its pros and cons. Passive acoustic monitoring is good because you can deploy instruments that can monitor for long time periods (months and even years), and you don’t need to worry about conditions at a certain time of year, or whether the animals are visible at the surface.

Let’s imagine for a moment that you have a hydrophone (which is an underwater microphone, essentially) and you want to listen to some fin whales.


There you are, hydrophone off the side of the boat, and you get lucky – you record a fin whale call!  Woo hoo!  Just hearing it is great.  But something else that is extremely useful is to be able to say how far away that whale was when it made the call (R in the picture).  That’s where some basic acoustics comes in.  If you measure how loud the call is at the hydrophone, and you know how loud the call was when it was generated, and you know something about the acoustic properties of the water, you can figure out how far away it was.  You can think of it like this: if you’re in a big room, and you shut your eyes, and someone else in the room starts talking, you can tell roughly how far away they are just by how loud their voice sounds in your ears.

Here’s the math, dead simple, I promise:

ML = SL - TL     (EQ 1)

In that equation, ML is “measured level”, SL is “source level”, and TL is “transmission loss”.  If you re-arrange the equation solving for TL, you get:

TL = SL - ML        (EQ 2)

What’s the deal with transmission loss?  Well, it accounts for the acoustic energy that is lost between the source (the whale) and the receiver (the hydrophone).  And it’s dependent on the range – the further apart the source and receiver, the greater the transmission loss.  Here’s the equation for transmission loss:

TL = 20 \log_{10}(R).         (EQ 3)

What all this boils down to is that if you know the source level (SL) and measure the receive level (ML), you can calculate transmission loss (TL), which you can then use to calculate range.  Awesome!  Except for one thing… source levels of fin whale calls are not really well known.  There have been very few published papers reporting fin whale source levels (e.g. [1],[2],[3]).  These results are useful, but because of the difficulties inherent in estimating source levels, relatively few calls were used in the final estimates (at most, 83).


This is where a BIG FAT DATASET really comes in handy.  And that’s just what we have:  a three year ocean bottom seismometer (OBS) deployment off the coast of Vancouver Island.  That’s 8 seismometers, collecting data for three years, at 128 samples every second.  And we see TONS of fin whale calls, especially during the winter months.

Fin whales have a very distinctive call.  It’s very low frequency, very loud, and only lasts for about a second.  And typically, a fin whale will make this call about one time every 25 seconds (although it does vary by location).  A couple of things make this call particularly handy for me.  First of all, they are so low frequency that they get picked up on OBSs.  Second, they are very similar from one call to the next, making an automatic detection algorithm relatively straightforward.  That means I can write code to tell my computer what the call looks like, and it will run through the data and find instances where that call shows up.  With hundreds of thousands of calls on 8 instruments, searching for the calls manually would take a hundred graduate students like a hundred years.  Okay, that’s an exaggeration, but it would take a LONG time.

How do we use this data to get at source levels?  The thing that makes it possible is that, in the first year of this three year dataset, we actually know where the whale is at the time of the call.  This is because my office mate, Dax, did his masters work on tracking whales near the OBS network.  Since we know where each call was generated, and we know where the seismometers are, we can calculate the range between source and receiver.


If you look back up to Equation 3, you can see that if we know the range, R, we can calculate the transmission loss.  And with TL and ML, we can estimate source level!  See, that wasn’t so bad…

A Slight Complication

If you know a bit about acoustics or seismology, you might have seen this coming:  OBSs don’t measure acoustic pressure level (ML) directly.  They actually measure ground velocity.  The amplitude of the ground velocity is definitely related to ML, but it’s dependent on what the seafloor is made of at that location, and angle at which the incoming sound hits the seafloor.  I would say that this is the most technical and complicated part of my paper, and since it is not critical to understanding the results, and also because it would take a long time, I will leave this for a separate blog post at some future date.  (if you’re especially curious, I encourage you to check it out in the actual paper).

The moment you’ve all been waiting for…

Or, um, you know.  Maybe not…  Anyway – the results!  A total of 1241 calls on 32 whale tracks were used to estimate source levels.  The mean source level was estimated to be 189.9 +/-5.8 dB re 1uPa @ 1m (see below for an explanation of this notation).  This is within the range of previous estimates, although slightly in the loud side.  The most recently published results were in 2008, where fin whale call source levels were measured in the Southern Ocean [1].  They found a mean source level of 189 +/- 4 dB, based on a total of 83 calls.

As part of the analysis, we looked at the variation of source levels over the duration of a dive and also between tracks.  We were surprised that we didn’t see any obvious trend over a dive – we expected that maybe as the whales ran out of breath, their calls would get progressively quieter, but we didn’t see any evidence of that.

Where the slop comes from

Part of reporting scientific results includes keeping track of where the uncertainties in the results come from.  The biggest contributors in this analysis were:  1) uncertainties in the location of the whales at the time of the call and 2) interference between the direct path acoustic arrival, and the “echo” that bounced off the sea surface.  Other potential sources of error include: estimated seafloor properties used to convert ground velocity to acoustic pressure level, sound speed profile, differences in the coupling between the seismic instruments and the seafloor.

The end… for now

And there you have it – that’s the gist of my paper.  I would love to dig into more of the analysis of the amplitude variations along tracks, and between individuals.  Maybe an even bigger “big fat dataset” would allow me to tease out additional clues…


** In my explanation above, I reported results as 189 dB re 1uPa @ 1m.  If you don’t study acoustics, that will probably look pretty mysterious.  Here’s what it means:  dB is decibels, which is a measure of loudness.  Decibels are measured as a logarithmic ratio of pressures:

dB = 20log_{10}\left(\frac{P_{meas}}{P_{ref}}\right)

Pmeas is the pressure you’re measuring, and Pref is a reference pressure. The reference pressure in water is 1uPa (micro Pascal) at a distance of 1 meter from the sound source.


[1]  Širović, Ana, John A. Hildebrand, and Sean M. Wiggins. “Blue and fin whale call source levels and propagation range in the Southern Ocean.” The Journal of the Acoustical Society of America 122 (2007): 1208.

[2] Charif, Russell A., et al. “Estimated source levels of fin whale (Balaenoptera physalus) vocalizations: Adjustments for surface interference.” Marine Mammal Science 18.1 (2002): 81-98.

[3] Watkins, William A., et al. “The 20‐Hz signals of finback whales (Balaenoptera physalus).” The Journal of the Acoustical Society of America 82 (1987): 1901.

Fin whales: where are they?

When I first started learning about fin whales, I found it really hard to believe that we (humans, scientists, animal lovers, etc.) still don’t really know where they hung out.  What their migration patterns are. If they even migrate!  As you know, these animals are enormous.  The second largest in the world.  How can we not know everything there is to know about the second largest animal in the world??

Well, the answer is pretty straightforward, actually.  Firstly, they spend most of their time underwater.  Where we can’t see them.  Second, the move fast – did you know their nickname is the “greyhound of the sea”  – they swim at speeds in excess of 20 knots!  It just boils down to the fact that they are really tough to observe for long periods of time.

This is a problem for us – humans did, after all, basically cause the severe endangerment of most large whale species, including fin whales.  And now that commercial whaling has been banned, we want to be able to protect them from further harm arising from human activities.  Large baleen whales are still at risk from ship strikes, low frequency noise (from ships or active seismic experiments), and entanglement in fishing gear.  If we want to protect them, it’s important that we have an idea of where they are in the ocean at any given time.

from http://lsiecosystem.org

Probably the most common method that scientist use to assess fin whale distributions is by using visual surveys.  This is a whole topic unto itself, a very interesting one, but I’m going to jump past it fairly quickly in this post.  Suffice to say that visual surveys are hampered by a couple of things:  they are restricted by weather conditions, you can only see whales at the surface, and in order to conduct a thorough survey, it can become very expensive.

Enter, acoustics! (as if you didn’t know that was coming)

Passive acoustic monitoring (PAM) is popular because you don’t have to bother the animals, you can record continuously for weeks or months, maybe even years.  You don’t need to worry about weather.  Some things you *do* need to worry about:  how often do they call? Which animals are calling? Is it only males producing calls as a breeding display?  Does calling activity increase near increased food sources?

Up until this point, I had been focusing on a little dataset up on the Endeavour segment of the Juan de Fuca ridge.  A teensy little dataset consisting of 8 instruments, each collecting 128 samples per second continuously for three years (that was sarcasm, it’s a ridiculously large dataset).  This little dataset (see a bit more here) has been great – I have used it to develop a couple of automatic detectors, and also to measure source levels.  But there’s a new dataset on the horizon – the Cascadia Experiment (dun dun!!).  This dataset consists of 70 OBSs, spanning the entire length and width of the Juan de Fuca plate, from Vancouver Island to Cape Mendocino, and out to the boundary with the Pacific plate.  As far as I know, it is the largest offshore seismic network ever deployed by the United States.  The image below shows a map of the coverage.

I’ll try to post some more blog posts discussing a few more details about how we plan on using this dataset to learn about what fin whales are doing – more specifically, what controls fin whale distributions in this area – is it food sources?  Is it water depth? Is it related to specific breeding or calving areas?  Hopefully we’ll be able to answer some of these questions!