British Antarctic Survey (BAS) has chosen acoustic release transponders from Sonardyne International Ltd (Stand No. 926 OI 2008) to support the second year of its studies of the calls of whales in the waters around South Georgia and the Scotia Sea. The organisation has bought 10 Lightweight Release Transponders (LRT) that are being used for the seabed deployment of specialised recording devices in waters up to 500 metres deep and in temperatures between 0.2 and 5 degrees Celsius. Using the Sonardyne LRTs, BAS has now successfully completed the second year of its research which entails listening for whale calls in frequencies below 500 Hz so that their movement, populations and feeding habits can be studied.
The ongoing study programme uses monitoring periods that range from three to 12 months when typically six Sonardyne transponders are deployed with the instrument packages. The data is collected using MARUs (Marine Acoustic Recording Units) developed by Cornell University in the United States. These are widely used for studies in waters around the United States and, because they are not sold commercially, are leased as part of a data sharing research agreement.
The unique recording devices were designed to collect acoustic data in a way that consumes the least power. This is achieved by storing five minutes of recorded data on a buffer then transferring it to a hard drive which is only activated at intervals to conserve power. The MARUs were in use on a year-round rotation when they also picked-up other acoustic events including earthquakes, volcanic eruptions and distant seismic survey operations.
At the end of the study period, the transponders, the instrument packages and the valuable data they contain are recovered by transmitting an acoustic command that activates the LRT's release mechanism that ensures a positive action to overcome marine growth and enables the unit to return to the surface. The data are then analysed at the British Antarctic Survey headquarters in Cambridge where the acoustic files are converted into sonograms.
Tony Martin, BAS section head of marine mammal studies explained, "Each whale species has a distinctive call and by carefully analysing the sonograms we are able to identify the species and work out how many there are and how far away. We can also work out how much they eat based on our knowledge of their calorific requirements and the food available in the area. We are looking at the whales' recovery since commercial whaling was banned in the 1960s and when South Georgia was the world centre for the industry. Happily we have found that all species are recovering, even the blue whale which is slowly returning from a 99 per cent depletion," said Martin.
The use of the Sonardyne LRTs in the research has found the BAS scientists particularly satisfied with the ranging and release of their MARUs. The LRTs enable a quick turnaround of recording systems and optimise the time of the RRS James Clark Ross from which they are being deployed. The ship was already fitted with a Sonardyne Fusion USBL (Ultra-Short BaseLine) acoustic positioning system and it was the crew's familiarity with this that encouraged Tony Martin to consider the use of Sonardyne LRTs.
The Sonardyne LRT is unique among low-cost releases in that it has the ability to both receive and transmit acoustic signals. This provides the operator with confirmation that the release mechanism has been activated. It also allows slant ranges to be measured for accurate positioning purposes and for relocating it prior to activation of the release mechanism. The LRT is depth rated to 500 metres at which slant ranges of 750 metres are typical and this makes it suitable for use in most continental shelf waters. Constructed from high strength plastics that offer excellent corrosion resistance, the transponder has a long operational life. Field replaceable alkaline or lithium battery packs give the LRT a listening life of 18 months or 4½ years respectively. This feature is particularly important to BAS because bad weather or the early return of sea ice can result in a MARU being trapped on the sea bed for much longer than anticipated. Should this happen, it is important that the LRT and its valuable data package can still be recovered when the ice retreats in the Spring. The surface control system consists of a portable deck unit and a dunking transducer. The deck unit is used to program the transponder with its unique, high security acoustic identity which can be chosen from one of hundreds available.