In our last post (click here if you missed it!), we chatted about tools scientists use to monitor and estimate wildlife population size. While we mentioned several techniques that can be used, we ended with the question:
Can Argos satellite transmitter tags be used for electronic mark-resight studies?
The short answer: probably yes.
The long answer is one we are seeking in conjunction with a collaborative study led by Dr. Michelle St. Martin with the US Fish & Wildlife Service (USFWS).
The tags we’re investigating are actually Life History Transmitters – a very specialized type of Argos transmitter. LHX tags are implanted into animals and monitor their temperature and collect other data while the animal is alive. Once the animal dies, the tag emerges and starts communicating with Argos satellites to share the data it collected. {Check out how ASLC scientists have used LHX tags in other animals here, here and here}
So if LHX tags know whether an animal is alive or dead, we should be able to use them to track populations or survival….but, it is complicated. The big problem is that while the animal remains alive, LHX tags cannot reach a satellite – the signal is weakened too much by the animal’s body.
They can however reach a nearby receiver at a distance of maybe 100 to 500m–which is where our study comes in.
Under funding from the North Pacific Research Board, we have assembled automated shore-based data receiving systems – largely from modified off-the-shelf components, that can pick up these transmissions. We are calling this the VitaLink system. In the case of our collaborative study with the US FWS, we are using this novel VitaLink system to see if we can receive occasional transmissions from LHX tags implanted into sea otters.
Field Testing
In the summer of 2019, the USFWS – together with a veterinary team from Monterrey Bay Aquarium – implanted 20 sea otters in Kachemak Bay, AK, with one LHX tag and one VHF tag each. (Remember the importance of dual tagging?–here)
We then set up VitaLink systems at five different locations, and [spoiler alert] discovered we could pick up transmissions from LHX tags in live animals! VitaLink can then either store any received transmissions in memory until we download it during a service visit, or it can actually re-transmit anything it has received via an Iridium satellite data link. That means we can get data in near-real time.
However, we did not receive all transmissions the LHX tags were programmed to do. That is because we can apparently only receive them when the otters are resting or ‘rafting’ at the surface, and in close proximity to a VitaLink station. Ocean water blocks radio transmissions, and thus when the otters are actively diving and moving, any transmission will be blocked.
The probability of picking up a transmission, (an electronic resight) is also affected by how often the LHX tags are programmed to transmit. We can try to improve the amount of data we receive by programming the tags to transmit more often, but that means they will exhaust their batteries quicker. Even though the LHX tags are projected to have a battery life of 10 years, if we reduce that too much, that will affect the main reason for using LHX tags, which is to determine date, time, location and causes of mortality in host animals.
So in selecting VitaLink sites in the future, we can consider these trade-offs (the number and distribution of VitaLink stations, the transmission rate tradeoff with battery life, and, more importantly, the behavior of the LHX tagged animals) to maximize our chance of data recovery.
Future Goals
There is one other goal we hope to accomplish with VitaLink stations. LHX tags are programmed to detect parturition in female hosts: in other words, if a female sea otter carrying an LHX tag gives birth, the tag will detect that and subsequently try to transmit that to VitaLink stations. That way we receive additional critical data from female otters hopefully at least once a year: how many pups has it given birth to. If we start with very young otters, we will also find out when they had their first pup, what we call ‘age at primiparity’ in geek speak.
Ultimately, if all this pans out as we hope, we will be able to use LHX tags combined with VitaLink stations in remote locations to collect essential vital rate metrics from multiple species we need to monitor. Not just sea otters, but also harbor seals, and maybe sea lion and fur seals as well. This important data would allow determination of age-specific survival rates, when where and why animals die, and for females, age at primiparity and the rate at which pups are born.
Early indications from this pilot project are promising, and we are all excited to keep working towards the goal of collecting such important data in the Aleutian Islands, where populations of marine mammals are struggling to recover or are declining.
Written by: Markus Horning, PhD
