By integrating different types of technology we can answer key questions about the movement and behavior of sharks and other pelagic creatures.

As a network, each research team uses compatible technology so that data can be shared and sharks which move between study regions can be detected.

For further information about the manufacturers of tags, see www.vemco.com for ultrasonic telemetry and www.wildlifecomputers.com for satellite tags.

Ultrasonic tags and sea floor receivers

The shark is tagged, and the signal from the tag is picked up whenever the shark swims close to one of the detectors. These detectors are strategically places in Marine Protected Areas across the East Tropical Pacific. The data is collected from the listening devices.

Ultrasonic telemetry essentially consists of two components – a tag and a receiver.

Tags are small, simple devices, which emit a coded train of pulses at a known frequency (usually 69 KHz). They may also be fitted with sensors to measure depth, temperature or other parameters. Each tag emits a unique signal which allows it to be identified (there is usually a random delay between signals – in our studies this varies from 45-120 seconds – to prevent repeated signal collision between tags).

The tags have a lifespan of several years and can be placed inside the peritoneal cavity of a shark that is brought onboard, or attached externally behind the dorsal fin of the shark by free divers using pole spears.

Teams of free divers sweep the area and dive down onto the sharks to attach the tags. By holding their breath, they avoid creating bubbles which can frighten the sharks. The attachment process is quick, and although the shark feels the impact of the dart, it quickly resumes its normal swimming among the school.

Underwater receivers are essentially passive devices that detect and store the time and date of any tag which comes into range. It is important to carry out range tests when starting a new study, because the detection range of a tag will depend not only on the power of the tag, but also on the physical environment around the receiver.

We have found that our receivers have a detection range of between 100-200 metres.

The receivers are deployed on concrete bases with mooring cables and buoys. We usually place them at the limit of normal diving depths (just below 30 metres). Every six months, we send a team of divers down to retrieve the receiver and download the data.

By compiling a database of the detections from each shark at different sites throughout the region, we can determine whether sharks display site fidelity, diel behavior, group fidelity and movements between sites.

Ultrasonic tag and vessel monitoring

A different type of tag is used in this case, which emits a stronger signal that can be traced by a special detector just under the surface for 48-60 hours. Following the tagged shark in this way gives a very precise measurement of its whereabouts.

To track a shark, or any other animal, continuously over a time period takes a lot of effort and dedication.

To do this, we use an ultrasonic tag with a temperature and depth sensor, but unlike the coded tags described above, this tag operates on a unique frequency and emits a signal every second. We can attach the tag either by free diving or by attracting the shark to the surface with bait.

Once the shark is tagged, a motorboat is ready to follow it using a directional hydrophone which picks up the signal from the tag. This piece of equipment has an inbuilt GPS but it is important for the tracking team to stay as close to the shark as possible.

We usually work in eight-hour shifts, and record other data such as seabed depth, sea surface temperature and salinity, and sea state, as we track. Every hour, we take vertical salinity and temperature profiles with a CTD. To date, Migramar researchers have followed Galapagos sharks and hammerheads for periods of approximately 48 hours.

Satellite tracking 1: PAT tags

A PAT tag lasts for a number of months, recording depth and other data. After that time, it detaches itself and floats to the surface where a satellite can pick it up and download its information.

Pop-up Archival Tags (or PAT, for short) store information and are programmed to detach from the host creature at a fixed time and pop up to the surface, from where they send their information to satellites.

These tags can be equipped with a variety of sensors (usually depth and temperature), but they have the added advantage over ultrasonic tags in that they can determine the location of the animal in question.

They do this by means of a light sensor and an internal clock. The highest light intensity is attributed to midday, and the time is compared with Greenwich Mean Time (GMT) to determine longitude. For latitude, the light sensor measures the daylength, defined as the time between the maximum increase in light (dawn) and the maximum decrease (dusk).

There can be a fair amount of error in these estimations, but these tags give important information on long distance migrations. The error can often be corrected by comparing surface temperature for the tags with satellite imagery. We have used these tags on hammerheads, but they seem to be able to shed them quite quickly. We are also using them on whale sharks.

Satellite tracking 2: SPOT tags

Tags that emit a signal strong enough to be picked up by a satellite must be fit on to the shark on its dorsal fin, which means capturing the animal for a short while. Unfortunately, the signal can only reach the satellite from above or very close to the surface.

SPOT tags are attached to the dorsal fin of the shark, and emit signals whenever they are on the surface.

They are therefore not much use for animals which do not use the surface. However, both hammerhead and Galapagos sharks can often be seen swimming with their dorsal fins sticking out of the water.

The signals from the tag are received by a satellite which calculates the position of the shark using the Doppler effect. However, the drawback is that the animal must be captured in order to attach the tag. These have been placed on Galapagos sharks and hammerheads.

We work with local fishermen to catch sharks using vertical lines attached to surface buoys.

Our hooks are barbless so that we can remove them without harming the shark. Once the shark is caught, it is brought to the mother vessel and lifted aboard using a sling.

We immediately immobilize the tail, cover the eyes and place a tube in the shark’s mouth and pump seawater through its gills. While the shark is being measured and sexed, four small holes are drilled into the dorsal fin, and the satellite tag is mounted.

Once the tag is attached, and the hook has been removed, the shark is lifted over the side and lowered into the water. It swims out of the sling and back into the ocean.

Underwater visual censuses

In December 2007, a workshop involving shark researchers from all over the Eastern Tropical Pacific was carried out in the Galapagos Marine Reserve, in order to develop a standardized technique for identifying, counting, and estimating the size of sharks. A shared database was developed as an online tool, and training of local dive guides began in early 2008.

Cut-out sharks of different sizes were built and used in underwater training exercises to determine the range of size estimates at different distances between divers and sharks. Dive guides are asked to carry a small PVC armband on which they can note down shark abundance per species, sex and size of individual sharks, and physical aspects of their dive (dive time, currents, visibility, depth and position of thermocline).

Each dive guide has an access code in order to enter the data in our online database. The results are therefore continually updated, and will serve as a baseline from which changes can be monitored in the future. Currently, the online database is being redesigned.