We just finished our second week, and it was another successful one! We are over halfway to our target sample size and we managed to put out 2 of the remaining three satellite tags. We also managed to opportunistically disentangle a juvenile elephant seal who had rope wrapped around his neck. All in all….another great week on the island!
We now have have good locations from 5 juvenile sea lions. Our 6th tag is having some problems connecting with the satellites so I have been chatting with the company to try and find out what is going wrong. Two of our animals are still local, but three of them are up foraging off of Half Moon Bay and further north and have been hauling out (resting ashore) at the Farallon Islands, which are about 27 miles offshore from San Francisco Bay. I will post a new map each week so you can check out where they continue to go! Unlike adult females that would need to return to the rookery to nurse their pup, these guys are free to go where they choose so the sky’s the limit.
We are deploying two types of satellite tags on juveniles, a SPOT tag (stands for satellite position only tag) and a SPLASH tag (it collects both location and dive data). While I certainly don’t understand all of the ins and outs of how the tags connect to satellites, I understand the basic concepts. Both of the tags transmit data over the ARGOS system, which is attached to polar-orbiting satellites. When the animal surfaces, the tag attempts to connect with the satellites – if there are no satellites overhead then we will not get a position on the animal. Satellite coverage varies depending on where you are and the time of day – we are pretty luck in California as satellite coverage is good. If there are enough satellites overhead and the tag is able to connect several times with the satellites, then it is possible to get a position of the animal via the Doppler effect. Whenever we get an ARGOS location, we also get a location quality, which is simply a value that tells us the error (in km) about the position. ARGOS positions can be accurate from ~1 km to up to ~70 km. This error doesn’t matter as much for an animal, like an elephant seal, that travels thousands of km at sea, but for a sea lion that is coastal it can be a bit more of a problem.
We can use tags that are equipped with GPS technology to get more accurate positions. These of course come with a tradeoff as they are more expensive and can be larger/heavier. The SPOT tags are about $1400, whereas our GPS tags (that also collect dive data) are $5K. Because of the larger size, I was looking for an older juvenile that could carry the tag without it affecting his behavior. This is always a concern when deploying tags of any kind on animals as many researchers are attempting to measure the natural behavior of the animal. Attaching any sort of instrument to an animal is going to alter its streamlined shaped and create some sort of drag – our purpose is always to minimize this so we don’t affect the animal’s energetic expenditure or its ability to find food.
We deployed our first GPS tag on a juvenile male estimated to be around 3 years old (see left picture). The big green circle on the tag is the GPS sensor and the antenna is for connecting with the ARGOS system. The tag not only transmits locations, but will also archive and transmit a portion of the dive data. Because we are not planning on recovering the tag from the animal, we will only get a portion of the data that are collected as there is too much data to be sent over the ARGOS system. Above right is a map of his first trip to sea – all of the green locations are the ARGOS location and the pink ones are the GPS ones. Right away you can see the utility of having the GPS tag – our sea lion clearly didn’t wander into the Santa Cruz Mountains!
Año Nuevo Island
NMFS Permit # 17952, 17115