Underwater Loggers Aid Discovery of Prehistoric Fish
In April 1998, Dr. Mark Erdmann began using StowAway TidbiT temperature loggers from Massachusetts-based Onset to record seawater temperatures at several locations and depths off Manado, Sulawesi.
Originally, he was monitoring coral reef health in northern Indonesia using marine crustaceans as bioindicator species. Temperature data is important for interpreting changes in species abundances and distributions. TidbiTs were deployed at several locations from the intertidal to a depth of 40 meters.
However, his research took a dramatic turn last year when he and his wife noticed a large lobe-finned fish in the fish market at Manado. It was a coelacanth (pronounced “Seal-a-canth”) belonging to an ancient lineage of fishes that are closely related to the first land vertebrates.
This group of fishes was thought to have become extinct 80 million years ago, but in 1938 a living coelacanth was caught in a net off East London, South Africa. At the time, it was heralded as one of the greatest zoological discoveries of the 20th century. Fourteen years later, another coelacanth was caught off the Comoro Islands in the Mozambique Channel. Since then, nearly 200 coelacanths have been caught in the Comoros, and the animals have even been filmed using a submersible vehicle.
This is thought to be the world's only population of coelacanths and, unfortunately, the population is declining rapidly. It is estimated that only 200 to 300 fish remain. The discovery of another population 10,000 km away would spell hope for coelacanths, while opening up numerous evolutionary and biogeographical questions. However, Dr. Erdmann had been unable to acquire the fish that he saw in the market. All he managed was to take three photographs of it. Was it really a coelacanth? Did it come from Indonesian waters? Was it a different species? Was there a population of coelacanths living in this area?
Dr. Erdmann immediately began a search for the fishermen who caught this fish to learn where it came from and if others had ever been caught. His hope was to eventually obtain another coelacanth that could be preserved and studied. After interviewing dozens of fishermen from villages around northern Sulawesi, he found two men living on a small island off Manado who claimed to have caught coelacanths in the past. One, in fact, was the fisherman who caught the “animal” that Dr. Erdmann saw in the Manado fishmarket. They were both shark fishermen who made their livings setting large gill nets at the base of a young volcanic island. Often the nets were placed at a depth of 200 to 300 meters.
In the Comoros, coelacanths are usually caught at depths between 100 and 300 meters. These fish also seem to prefer water temperatures between 12º and 18°C. Dr. Erdmann attached TidbiTs to the fishermen's nets to determine if they were placing their nets at a depth that would have the appropriate temperature for these fish. Indeed they were. Temperatures were determined to be around 15°C. For several months he continued to monitor the temperature at the depths at which the fishermen were setting them, but no coelacanths were caught. Then on the morning of July 30, 1998, a crew of fishermen drove their boat up to his house and announced that they had caught a live coelacanth. Dr. Erdmann photographed the fish both in and out of the water.
Due to the warm temperature of the water near the surface and the trauma of capture, the fish died within a few hours. Tissue samples were taken for study and the 1.2 meter, 25 kilogram fish was frozen. Later it was thawed, preserved, and put on display in Jakarta.
The discovery of this Indonesian coelacanth was announced in the September 24, 1998 issue of Nature Magazine, where a photograph of it appeared on the cover. It was also featured in the December, 1998 issue of National Geographic Magazine, and it was included in Discover Magazine's list of top science stories of 1998. Currently the molecular genetic analysis of the tissue taken from this fish is being completed and compared to data from coelacanths taken in the Comoros Islands. These results will be submitted to Nature Magazine for publication this winter.
Dr. Erdmann is continuing to monitor water temperatures around Manado as well as periodically placing TidbiTs on the shark fishermen's nets to track water temperatures at the base of Manado Tua (a volcanic island.) Eventually, he hopes to be able to determine if the fish are most likely to occur within a certain temperature range. They are also planning a submersible mission next spring in an attempt to observe these fish in their natural habitat and hopefully estimate their numbers.
Since publishing this finding, Dr. Erdmann has received reports of sightings of coelacanths in other parts of Indonesia. He plans to visit many of these areas to determine if the habitat is similar to that which is believed suitable for coelacanths. One measurement that will be taken is temperature at depths of 100 to 300 meters. This will generally be done by attaching temperature recorders to fishing lines or nets. Because of their dependability, small size, and relatively low cost, TidbiTs are ideal for this type of survey work since there is little logistical support, and conditions can cause instruments to be frequently lost.
For his monitoring work on coral reefs, the TidbiTs are left in place to record temperature for one to three months at a time. For use on fishing nets, the devices are attached to the nets, which are deployed at dusk and retrieved at dawn. Ultimately, the coral reef monitoring research should help us all to understand what types of pressures reefs are under, and how to better manage and protect them. For the coelacanth research, Dr. Erdmann hopes to determine if other populations exist and learn more about the habitat in which they occur.
Dr. Erdmann noted that he has been extremely pleased with the TidbiT recorders, and especially the BoxCar Pro software. The TidbiTs installed at 3 meter, 20 meter, and 40 meter depths around Bunaken Island since June 1998 have provided an excellent temperature time-series dataset to track La Nina-related changes in water temperature, which have resulted in massive coral bleaching in the area since November.
For more information about data loggers, please contact Onset at 1-800-LOGGERS, or visit www.onsetcomp.com.