Grey Seals of Sable Island: Seal Biologists Collect Data Beneath the Waves

The Ocean Tracking Network’s grey seal bio-probe project equips seals with tracking technology, giving researchers an unprecedented view of their behavior and environment. Part One of Three. Click here for Part Two and Part Three of the series.

grey-seal

Grey seal outfitted with tracking device on Sable Island. Image Credit: Damian Lidgard

The Ocean Tracking Network (OTN) is a worldwide aquatic animal tracking network headquartered at Dalhousie University in Halifax, Nova Scotia, Canada. OTN’s mission is to generate knowledge on the behavior, movement and distribution of aquatic animals to advance the conservation and sustainable use of marine and freshwater species. OTN is tracking more than 100 keystone, commercially important and endangered species, including marine mammals, sea turtles, squid, benthic crustaceans (lobsters, crabs) and fish, including sharks, sturgeon, eels, tuna, salmonids and cod.

OTN is built upon the use of aquatic acoustic telemetry, a recent development in animal tracking whereby animals carry acoustic tags that can be detected by fixed receiver lines or by mobile receivers carried by free-ranging aquatic top predators (e.g., seals) and autonomous marine gliders. OTN deploys Canada’s largest operational fleet of autonomous marine gliders in support of its animal tracking and oceanographic research. At the same time, geographic and oceanographic data are collected allowing OTN to study habitat use and the drivers of fish and marine mammal movement. Acoustic detection records and associated data are stored in the OTN Data Warehouse, a repository for data collected by OTN researchers worldwide containing more than 145 million detection records.

Using top predators as bio-probes to collect biological and oceanographic data is one of OTN’s key research projects. Dr. Damian Lidgard, a research associate at Dalhousie University is one of the leaders of the grey seal bio-probe project.

The grey seal is a marine top predator that is distributed throughout the northwest Atlantic and Gulf of St. Lawrence with its core population on Sable Island. Due to its wide-ranging behavior and interactions with key commercial fish species, it is an ideal bio-probe candidate.

The project has two key objectives. The first is to examine encounters between grey seals and potential prey, most notably Atlantic cod. In the northwest Atlantic, stocks of Atlantic cod are at historical lows with many showing few signs of recovery. Due to the presence of cod in their diet and large population size, predation by grey seals is thought to be one of several potential causes limiting the recovery of these depleted stocks. The second objective is to examine how seasonal oceanographic features, such as thermoclines and currents, influence grey seal movement and foraging distribution. Together, these two closely linked studies will help to predict how climate variability and long-term change may affect top marine predators and alter their impact on continental shelf ecosystems.

Since 2009, Lidgard has been working closely with Fisheries and Oceans Canada (DFO) to deploy bio-logging instruments on grey seals on Sable Island and in Atlantic cod. Sable Island represents the core of the northwest Atlantic grey seal population and has an established DFO grey seal program. Thus, it is the most suitable location to conduct the bio-probe study.

During the summer of each year between 2009 and 2015, two bio-logging instruments were deployed on grey seals. The first was a GPS Argos-linked satellite tag that collects data on the movement and diving behavior of the seal in addition to temperature and light profiles for use in oceanographic models.

The second was a Vemco Mobile Transceiver (VMT), a relatively new bio-logging tool that provides researchers with the rare opportunity to study interactions between species and individuals in space and time. Each VMT has a unique identity that is repeatedly transmitted as an acoustic code, and also archives the presence of other VMTs and acoustic transmitters deployed in fish by recording date-stamped detections of this code. Through examining the pattern of seal-cod detections one can discern whether grey seals and cod share the same habitat and whether grey seals predate upon cod. A predation event will exhibit a unique pattern of detections; if a seal consumes a whole tagged cod, the tag will continue to transmit its unique code within the digestive tract of the seal until it passes through, and these transmissions will be recorded by the VMT. Alternatively, if the seal predates upon the cod but fails to consume the tag, the tag will fall to the sea floor and continue to transmit while the seal remains in the area. The pattern of detections will appear as a cluster with a variable time width depending upon whether the tag was consumed. To date, instruments from 104 grey seals have been recovered and show that 71 seals visited areas known to be used by cod. Of these, 25 percent of VMTs recorded acoustic detections from 45 individual cod but there were no clear indications of predation. Encounters with cod were brief and the seal appeared to be traveling rather than engaged in typical foraging behavior.

“From our system, it doesn’t look like seals are predating on cod, but there are unknown factors, such as how many seals are coming into contact with tagged cod but are not being detected,” says Lidgard. “Seals are likely using the same habitat, but it’s certainly not making us think they are a major predator of cod.”

Grey seal pops his head out of the water, sending data to the Argos satellite system. Image Credit: Damian Lidgard

Grey seal pops his head out of the water, sending data to the Argos satellite system. Image Credit: Damian Lidgard

One of the drawbacks of the system is that the VMT was designed as an archival tag and lacks the ability to communicate with the Argos satellite system for the remote collection of data. Instrumented seals must therefore be recaptured during the subsequent breeding season to recover the data. Although the rate of recovery of tags on Sable Island is high (90 percent), data on encounters between grey seals and cod are restricted to the Eastern Scotian Shelf where the majority of grey seals from Sable Island forage.

However, interest in seal-cod interactions is greatest in the Gulf of St. Lawrence where cod stocks remain in a depleted state. Due to the large geographical area of the Gulf and the fact that much of it is ice-covered during the grey seal breeding season, unlike Sable Island it is simply not possible to recapture individuals in this area and recover instruments and data.

To overcome this difficulty, the seal research team collaborated with the Sea Mammal Research Unit in Scotland and VEMCO in Canada and developed a Bluetooth link between the VMT and GPS-Argos satellite tags. Now, when fish detections are recorded by the VMT the data are transferred to the satellite tag via Bluetooth and then transmitted to the Argos satellite system for retrieval. Over three years, 18 seals were deployed in the Gulf of St Lawrence with Bluetooth VMT and GPS tags and data successfully recovered. Interestingly, the results are the same as those for the Scotian Shelf; 25 percent of seals that visited areas used by cod detected them and there were no clear signs of predation.

Although the data from the VMTs suggest that grey seals and Atlantic cod encounter one another at some frequency, without direct observations the behavior of the seals during these encounters can only be speculated. To tackle this problem, Lidgard and the research team collaborated with Maritime Biologgers, a small local business that builds accelerometer tags for deployment on terrestrial and marine animals. These instruments record fine body movements up to 50 times per second on three axes, up-down, side-side and forward-backward. Thus, through mounting an accelerometer to the head of the seal, the team can determine whether seals are foraging (i.e., lots of head movement) or engaged in another behavior such as resting (very little head movement). Preliminary data from accelerometers deployed on seals in 2014 and 2015 are very encouraging and suggest one can distinguish different behaviors, such as foraging and resting, during dives.

“We look at these data and relate them to the data from the GPS and VMT tags to give us a better idea of when the seals are most likely foraging,” explains Lidgard.

The accelerometer data may suggest to us that the seal is foraging, but without observing the behavior of the seal we cannot be certain. So, the next step is to corroborate the data from the accelerometers with observations of the seal in its natural environment. To achieve this, the team are deploying seals with video cameras, capable of recording several hours of footage when the seal is most likely engaged in foraging behavior (i.e., at night at depths below 25m). Collating these various data will allow the team to better understand the behavior of grey seals when encountering Atlantic cod and contribute toward understanding the importance of cod in the grey seal diet.

Despite the limitation of tracking technologies, the information gathered from projects such as this one provide critical information on animal interactions and ocean conditions where traditional monitoring equipment cannot be deployed, as scientists work to answer critical questions about animal movements, behavior and ecology.

Anja Samardzic is the communications officer for Ocean Tracking Network. She’s passionate about sharing OTN’S research with a variety of audiences through writing, design and creative content development.

Damian Lidgard is a research associate at Dalhousie University and an investigator on the OTN grey seal bio-probe project. He also enjoys spending his time behind his camera lens, capturing the beauty of wild places and sharing them with the science community and beyond.

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