Shark tagging

Spied: a year in the life of a sleeper shark

Markus Horning

Markus

November 17, 2023
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This summer we recovered several of the miniPAT telemetry devices we had attached to sleeper sharks in Prince William Sound in 2022.

 

A shark is in the water next to the low deck of a small boat. Only the central part of the body of the shark is visible. Next to the dorsal fin of the shark, a grey pear-shaped telemetry transmitter is floating in the water, attached to a skin anchor via a flexible lead.
A miniPAT Pop-up Archival Transmitter is attached to the shark with a subdermal anchor connected via a flexible stalk. This telemetry transmitter records sensor data until at a pre-programmed time it detaches itself from the shark, floats to the surface and transmits stored data in a summarized format. If a tag is found and phycially recovered, we can actually download much more detailed, high-resolution original data from the device.

 

The tags were attached mostly in August, and we recovered them exactly a year later after they detached themselves from the sharks on schedule, floated to the surface and began to transmit. Once we recovered the tags, we were able to download a bonanza of data these tags had recorded over the preceding year. These represent the first year-long records we now have from Pacific sleeper sharks tagged in Prince William Sound.

 

On this x-y line chart three data channels are shown over the duration of an entire year on the x-axis. A blue line shows the depth as it varies across the year, ranging from as shallow as 50 meters to as deep as almost 500 meters, but most values are between 275 and 400 meters. A pink line shows the water temperatures that are remarkably constant throughout the year, around 6 degrees Celsius, with just a few short episodes of warmer water in November. A black line shows the y-axis of the tri-axial accelerometer to indicate movement. Values range from -1 g to zero. There is quite a bit of variation indicating more frequent movement or activity, and a period of much less movement or activity from January through June.
Depth (blue), water temperature (pink) and movement (black) are shown for a 1-year period for one particular sleeper shark we tagged in Prince William Sound.

 

The shark shown in the example above spent most of the year at depths ranging from as shallow as 50m, to as deep as 450m, though most of the time the depth was between 250 and 350m. Remarkably, the ambient water temperatures during almost the entire year stayed near 6 degrees Celsius, with just one short period in November when warmer water temperatures up to almost 10 degrees C were recorded by the tag. Another very interesting aspect of this record is the prolonged period of somewhat reduced activity from January through June that this shark exhibited, as indicated by the accelerometer y-axis variation (black line).

 

Now we can ‘spy’ on our tagged sharks and get an idea what they are up to and how that may vary over the year: how deep are they, what water temperatures do they prefer, how active are they? These are just some of the questions we are seeking answers to!

 

Here is one of the more interesting tidbits we had noticed in some earlier records from different sharks, that did not last a whole year, but that were recorded at a higher resolution (sampling rate):

 

Two panels with x-y plots show depth of a shark varying a little between 100 and 150 meters over a 15 to 20 minute period. Three distinct lines of three different colors show the output of each axis of a tri-axial accelerometer that all show some variation of a few tenths of a g, while during a specific moment one of the three axis outputs shows a strong peak, while at the same time the depth all of a sudden decreases. A pictogram illustrates that during this event the shark is changing from a horizontal orientation with an external floating tag pointing mostly straight up, to an orientation where the shark is pointing and moving upward, with the external tag trailing behind the shark and thus being kicked back. The legend points out that in the two panels the peak depth change rate maxes out at 1.2 and 1.4 meters per second, respectively.
Two examples of Momentary Acceleration Events (MAEs) recorded from two different sharks. The dark blue line shows the depth of the shark during the MAE, and the red, light blue and black lines show the three distinct axes of the tri-axial accelerometer.

 

These are two examples of what we call Momentary Acceleration Events (MAEs) – these are instances where accelerometer data from the miniPATs show that the tags were ‘pushed back’ from their usual upright orientation of floatation on the slowly moving sharks, to a horizontal, trailing orientation resulting from the shark speeding up for 20 to 40 seconds. During the same time, the depth decreased at a much greater rate than the typical 0.1 m/s to as much as 1.2 or 1.4 m/s (as shown here). Thus, they can move faster! We believe these MAEs may be when the sharks could be in pursuit of prey.

 

We are now digging into the analysis of mountains of data – we’ll post here when we have new exciting findings!