Telemetry includes any device that emits a signal. We use two basic type: a) Acoustic telemetry and b) satellite telemetry.
a) Acoustic transmitters emit high frequency sound waves. There are two basic types
i) Active tracking. A transmitter is attached to the animal and the signal is detected by a hydrophone/receiver on a boat or kayak. You then follow the animal around. Active tracking provides high spatial resolution data (you know exactly where the animal is while tracking) but you can only track for short time periods (days). Any longer and human fatigue becomes too much of an issue.
ii) Passive telemetry. These transmitters emit an acoustic signal every few minutes. The acoustic signal is then detected by strategically placed listening stations. We collect and download the listening stations every few months. Each transmitter has a unique identification code so that we can identify individual animals. We therefore get a list of which animals were detected where and when. This data can have low spatial resolution (with some exceptions) and more importantly, if we don’t detect the animal on any receiver, then we don’t know where it was. However, the transmitters can have battery lives of many years so you can get a good understanding of movements over long time periods.
Both active and passive acoustic transmitters can also be integrated with sensors that measure depth, temperature, acceleration, and even pH (of the stomach, to look at feeding).
b) satellite telemetry is used with animals that move over large distances. Again there are two basic types. Pop-up archival tags (PAT) record depth, temperature and light. At a pre-programmed time, the tag detaches from the animal and floats to the surface where the data is downloaded via satellite. We then get sent emails containing at least some of the data. The light level data can be used to estimate geographic position of the animal, but there can be large errors associated with these estimates. Basically, you can get very useful horizontal movement data if the animal moves a large distance, but less useful results if it stays in a small area. You will also get long term records of swimming depth and water temperature. Pop-off satellite tags are most useful for animals migrating large distances over long time periods.
Another type of satellite transmitter is a SPOT tag. SPOT tags are generally attached directly to the dorsal fin. Every time the fin breaks the surface of the water it will start transmitting its location to satellites. The location of the animal will be estimated, although accuracy will depend on how many satellites detected the tag during the time when the animal was at the surface. It is possible to obtain geo-location estimates with an accuracy of < 250 m. You can get high spatial resolution tracks over periods of months to years….however, if the animal doesn’t surface then you don’t get anything.
Bio-loggers are devices we attach to animals that contain many different sensor types, and which store data to memory. Data is not transmitted which means we have to get the data-logger back in order to retrieve any information. However, because the data is stored to memory, we can sample at very high frequency and get a lot more information back (for example our acceleration sensors sample 10 times per second). We use a variety of data-loggers to understand shark behaviour including acceleration and swim speed (to understand patterns of activity or energy expenditure), gastric pH and motility (to record feeding events and digestion) and even video cameras.
3) Stable isotopes
The old saying goes “you are what you eat”. Stable isotopes are rare forms of common elements (e.g. carbon and nitrogen) which all animals assimilate in their tissues. These isotopes are assimilated from the tissues of the animal’s prey, and we can use the ‘isotopic signature’ to estimate the animal’s trophic position (where it is located in the food web) as well as determine where it may be foraging (e.g. coastal vs pelagic habitats). Stable isotopes can provide a lot of information especially when combined with telemetry and movement data. We collect small muscle biopsies from the sharks we tag and use that to study their feeding habits.
4) Technical diving
Recreational diving requires that the diver not go into decompression. This means at any one point, a diver can ascend directly to the surface. The deeper you go, the shorter your “no-decompression” time. At 40 m you only have about 7 minutes before you must start to head back to the surface. There are other issues as well. At high pressure, the nitrogen in the gas we breath starts to become narcotic and has similar effects to being drunk. The deeper you go, the stronger the effect. For these reasons, most studies by marine ecologists on coral reefs have been from the surface to 40 m depth, but the reefs don’t stop at those depths (in many tropical habitats there is still plenty of light at 100 m). We call these deeper areas, mesophotic reefs, and for the most part they have been under-studied. To work at these deeper depths we have to use technical diving techniques. We breath what are known as mixed gases (a mixture of helium, oxygen and nitrogen) to combat the effects of nitrogen narcosis. Our dives require us to go into decompression which means we cannot go directly to the surface. We makes stops on the way up, which sometimes can be on the order of several hours. We are also using closed-circuit rebreathers which are devices that recirculate our gases. This enables a small tank of gas to last a long time. Other benefits include the fact that rebreathers produce no bubbles so are less likely to startle animals. They are also gas mixing-machines and can make the whole decompression process much more efficient.