Techlink :: Tech Practice :: Electronics :: Tracking Eels Across the Oceans

Somewhere in the warm waters of the Pacific, possibly around Fiji or Samoa, there is a mysterious place where all of New Zealand's freshwater eels go to breed. Nobody knows exactly where. A project involving scientists from the New Zealand National Institute of Water and Atmospheric Research (NIWA) and the University of Tokyo has been trying to unravel the mystery.

New Zealand has two species of freshwater eel: Longfin eels (Anguilla dieffenbachii) and shortfin eels (Anguilla australis). The longfin eel is found exclusively within New Zealand; the shortfin eel is more widespread, and is found around southeastern Australia and other southern Pacific islands. Although both species can occur together, longfin eels are more common in inland waters; shortfin eels are more common near the coast.

When they mature at around 35-40 years, female longfins leave their homes and head for the sea. The breeding instinct is so strong the eels will leave water and navigate their way around obstacles, such as dams and weirs, blocking their path. The eels go through several changes before leaving: their heads become more streamlined, their eyes enlarge (with a blue ring appearing around them) and their gut degenerates, resulting in loss of appetite. The Māori have a special name for migratory eels, Tuna-heke, as they cannot be caught using bait because of this lack of appetite.

In a journey lasting months, the eels swim north, sometimes in warm surface waters, sometimes in the cold and dark, almost a kilometre down; when they reach the spawning grounds, they mate with longfin males who made the same journey slightly earlier and release as many as 20 million free-floating eggs. Then they die. Shortfin females make the same journey when they reach 20-25 years.

Eels hatch as leptocephali – small, transparent, leaflike larvae, which bear little resemblance to adults. These drift and swim about, making daily trips up and down the water column, feeding on plankton. After about 7-8 months, shortfin eels metamorphose into round-bodied young eels called elvers. Longfin larvae take a little longer before changing – about 10 months. Then, somehow, the tiny eels make their way to New Zealand. On reaching the coast, they swim up rivers and streams, feeding on bottom animals until they become black-and-silver-bodied adults, completing the cycle.

Eels have puzzled humans for a very long time. Worldwide, there are stories and legends about the origins of eels; one in particular gives the eel its scientific family name Anguillidae. This came from Greek mythology where the white armed-goddess Anguilla and the king of Greek gods, Jupiter, together produced eels. Māori call both the shortfin and the longfin eel Tuna. South Island Māori legend states the origin of the Tuna, is from the heavens known as Orukateraki. Due to the heat of the sun and the lack of water, Tuna's skin had been burnt and turned black which made him sad. After meeting Tawhaki, a man who had travelled from the earth to the heavens, Tuna descended to Papatuanuku, the earth, where he found a cool pool in which to live called Muriwaiowhata. Sometime later, Tuna was discovered by a woman named Hineturepo, who saw Tuna as a taniwha, or monster. The people of the area caught Tuna in a large hinaki (eel pot), killed him and cut him into pieces. The pieces of Tuna were scattered and became the various types of 'eel' we have today; the conger eel, the lamprey, the hagfish and the freshwater shortfin and longfin fin eels.

The mystery of eel migration spans the history of western science. Because fishermen never caught anything they recognised as a young eel, the animal's life cycle was an enigma. The Greek philosopher Aristotle (384-322 BC), who did the first known research on eels, believed they emerged spontaneously from mud – no fertilisation needed. Roman naturalist Pliny The Elder (c77AD) suggested they grew from horsehair. It wasn't until 1777, when an Italian, Carlo Mondini, identified the creature's gonads, that eels were proved to be fish. The migration and reproduction of freshwater eels remained a mystery until 1922 when a Danish scientist, discovered the spawning grounds of the Atlantic eel in the Sargasso Sea between Bermuda and Puerto Rico. While all 15 species of freshwater eel spawn at sea, the spawning grounds of only four species are known with certainty. Not included in the four are the two common New Zealand species.

Until very recently, the only way of figuring out where freshwater eels spawned was to tow fine-mesh nets at depths ranging from 50m to 300 m, and try to catch progressively smaller larvae - a time-consuming and costly business. The development of radio-tracking devices and satellite coverage has made the process easier and more accurate.

Over the past few years, scientists from NIWA and the University of Tokyo have tried tracking New Zealand longfin eels to their spawning grounds, using a system of electronic tags. The tags record and store environmental information along the journey. Data recorded includes water temperature and day-length figures, and the times of sunrise and sunset; the latter can be used to calculate the latitude and longitude of an eel each day and map the track of the migrating fish.

Tags are attached to the eels using a tether. At a specified date and time, the tag actively corrodes the pin to which the tether is attached, releasing the tag from the animal. The tag then floats to the surface and transmits summarised information to an Argos satellite. The Argos system also uses the transmitted messages to provide the position of the tag at the time of release. The transmitted data are forwarded on to the researcher.

Each tag has a small, pressure sensitive guillotine, so if an eel dies and begins to sink, the tag will be automatically cut adrift at the 1,800m mark. As the tags are comparatively large (75g, 175mm long), they are attached only to large longfin females (8-11kg). The eels used in the study were caught by commercial fishers in Te Waihora (Lake Ellesmere) near Christchurch and acclimatised in sea water before being released.

The first four eels were released in May 2000. Two of the tags were programmed to release and ascend after two months, followed by the other two a month later. The data they transmitted included enough daily location data to construct swimming tracks for all four eels. Three of them showed an eastward movement along the Chatham Rise. The data from the fourth eel's tag had a gap for the first three weeks (by which time the eel was in Hawke Bay), but then she swam south, also to the Chatham Rise.

While information from these tags did not shed much light on possible spawning areas, the daily temperature variations indicated the eels were moving significant vertical distances through the water column, and the scientists were able to calculate that they swam 26 to 31km per day.

A second batch of ten eels was released from Te Waihora in May 2001. Tags for these fish included a pressure sensor to record swimming depth. Unfortunately, good information was received from only three of the tags, which stayed attached for periods ranging between 26 to 161 days. One tag surfaced 160km northeast of New Caledonia; indicating the spawning grounds for longfins was is in the tropics, something the scientists had assumed but were unable to prove.

As the three eels all swam too deep for the tags to record surface light, scientists were unable to accurately determine their swimming tracks; however, they did obtain some very interesting information on swimming depths. The scientists were surprised to find that all three eels showed regular daily vertical movements through the water column - two of the three frequently swam to depths of 800m or more, with the greatest depth recorded being 980m. Why would migrating eels spend energy on daily dives of many hundred metres? Perhaps to avoid predation, as various sharks, swordfish, and toothed whales are known to make regular dives to similar depths. Even in the tropics, water temperatures encountered by the eels at these depths were only 5-6°C – too low for efficient metabolism. When the eels ascended during the evening to shallower and warmer waters, it was presumably to warm up. So, while the scientists didn't find the spawning grounds of longfins, they did obtain very interesting information on swimming speeds, depths and behaviour of migrating eels.

In May 2006, three more eels were released from the outlet of Lake Ellesmere. All three tags are due to ascend in early December. The scientists hope that the eels will show some consistency in swimming direction, and converge on the possible spawning area.

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.