With the advent of robust, miniaturized electronic devices, an increasingly common method of studying wild animals involves temporarily attaching data-logging sensors directly to them. Some readers might have seen point-of-view video footage obtained with National Geographic's "Crittercam," for instance, or heard about the study where the migratory routes of Arctic terns were determined by putting tiny light loggers on the birds. Now, a consortium of scientists from nine European research institutions have tagged cod fish with mini-thermometers, to find out how they will be able to cope with rising ocean temperatures.
Cod are generally thought of as cold water fish, so could be particularly vulnerable to warming seas. The Codyssey project scientists wanted to establish just what temperature range cod can actually tolerate. So, between 2002 and 2005, they attached the temperature gauges to 3,000 of the fish from eight different North Atlantic stocks. Once an hour for a year, the devices measured and stored data on the water temperature surrounding each fish.
To date, 902 of the cod have been recaptured by fishermen, and their thermometers sent back to the researchers. Partially due to the fact that some of the devices stopped working before a full year was up, a total of 384 thermometers were used to collate the data.
The results showed that the fish inhabited water as cold as -1.5C (29F), but also as warm as 21C (70F), which was a surprise. Different cod stocks, however, are presumably used to different temperatures, so couldn't necessarily survive in each other's habitats. It was also discovered that they all sought out water that was between 1 and 8C (34 and 46F) for spawning, suggesting that the eggs and larvae do require cold temperatures.
Additional electronic tags that were used in Codyssey recorded data such as depth and geographical location.