Tracking Eels Across the Oceans
Extra Methods
Besides tracking the animals with the tags, scientists will use surface-dispersal modelling and data from satellite measurements of surface currents to solve the spawning mystery. The computer models created from the data collected will indicate whether prevailing surface currents could enable eel larvae of both species to arrive off the New Zealand coast by a combination of drifting and swimming from a range of possible spawning areas in the South Pacific. While preliminary data indicate this is a promising option, the transport model must include active swimming by the larvae themselves.
The American-made tags used by the programme were designed to track the migrations of large oceanic fish such as tunas, sharks, and swordfish, and, as such, are little too large to be ideal for eel work, but, in the future, smaller versions of the pop-up tags may be produced. Much smaller archival tags are available, but using them means the eels would have to be recaptured to retrieve the tags and their stored data.
The tag's electronic components are cast in a tube 21mm in diameter. The added float is 40mm in diameter at its widest point. The overall length of the tag, not including the antenna, is 175mm, with a total weight of 75g at one atmosphere. The cast tube and float are tested to withstand 2,000m of pressure.
A lithium battery supplies enough power for the tag to sample data for at least a year and make 10,000 32-byte transmissions over the course of about seven days. Researchers can tailor the scale of the transmitted data so that it is appropriate for the deployment length (for example, fine-scale for short-term deployments). The deployment length currently is limited by the challenge of keeping a tag on the study animal.
The transmitter inside the tags generates 0.5W of radiated power output. The tags have 16 mb of flash memory to store archival and summarised data; enough memory to store all the summarised and sampled data for most deployments. Data are retained even if the battery is exhausted. This means archival data can be recovered if a tag is found after it pops off. High-accuracy depth and temperature readings are provided by 12-bit analog-to-digital converters. A 10-bit analog-to-digital converter is used for light-level and battery voltage readings, as well as other housekeeping chores. Depth and temperature sensors are calibrated to provide an accuracy of 1% of the measurement. Depth and light level are temperature-compensated to provide consistent readings through temperature variations. Depth can be measured down to 1,000m with a resolution of 0.5m. Temperature can be measured from -40°C to +60°C with a resolution of 0.05°C.
Light level is measured as irradiance at a wavelength of 550nm, and dawn and dusk can be discriminated at depths up to 300m in clear water conditions. Using light level recordings, researchers can calculate longitude with an accuracy of ±1 degree. Latitude accuracy depends on both the latitude and the time of the year. The tags have a wet/dry sensor and probability software to determine the ideal time to transmit; that is, when it has the highest likelyhood of remaining dry over the duration of the transmission. This reduces the number of corrupted messages sent to the Argos satellite.
