Monitoring the Occurrence of Large Whales Off North and West Scotland Using Passive Acoustic Arrays

Abstract

C.W. Clark, Cornell University, M.L. Tasker, Joint Nature Conservation Committee, M. Ferguson, Amerada Hess Limited, J.P. Hartley, Amoco (UK) Exploration Company, I. Fletcher, Arco British Ltd., A. Whitehead, BP Exploration Operating Company, A.A. Duff, Chevron UK Ltd., J.F. Appelbee, Conoco (U.K.) Ltd., C. Pickton, Deminex UK Oil and Gas Limited, J. Spink, Elf Enterprise Caledonia Limited, C. MacDuff-Duncan, Esso Exploration and Production UK Ltd., S.J. Knight, Kerr-McGee Oil (UK) PLC, A.H. Walls, Mobil North Sea Ltd., A. Onder, Shell UK Exploration and Production, J. Urbanus, Texaco North Sea UK Company, and I. Buchanan, Total Oil Marine PLC. Abstract It is well known that baleen whales produce a variety of low-frequency sounds. The song of the humpback whale, Megaptera novaeangliae, is the most popular example, while the sounds of other species such as blue Balaenoptera musculus, fin Balaenoptera physalus, and minke Balaenoptera acutorosfrata whales remain less familiar. Assessment of the seasonal occurrence and the relative numbers of whales in an area has relied almost exclusively on visual survey methods. Such methods depend entirely on whales surfacing to breath and cannot be used at night or when visibility conditions are poor, and are only good for distances within a few miles of the sighting platform. Because of the limitations inherent in any visual survey method, broad-based, ocean-area surveys are relatively expensive and cannot adequately sample the habitats frequented by whales. Recent access to fixed arrays of bottom-mounted hydrophones has provided a novel mechanism for simultaneously documenting the presence of whales throughout large areas of the Atlantic Ocean. Such coverage would be impossible using traditional visual methods. A recently initiated project is now underway to systematically describe the seasonal occurrences, distributions, and relative numbers of blue, fin, humpback and minke whales in the UK region using passive acoustic techniques. Data from this study are intended to provide information to both document the presence of and reduce the potential impact of oil and gas development and operation activities on whales. This paper presents some existing results from the western North Atlantic and very briefly describes preliminary results from the UK region. Introduction In 1971 Payne and Webb first postulated that several species of large baleen whales, the blue and fin whales, might acoustically communicate over great distances. Their evidence in support of this hypothesis was based on the fact that the sounds from these animals are infrasonic (ca. 20 Hz), loud (ca. 188 dB re 1 Pa at 1m), and highly patterned. Using the narrowband sonar equation and a simple spherical-cylindrical transmission loss model, they estimated that in the ocean prior to the advent of modern engine-driven ships, whales could hear each other across thousands of miles of ocean. In the age of modern shipping came a resultant increase in ambient noise in the frequency band below 100Hz due primarily to propeller cavitation noise. As a result of this increase in ambient noise levels, Payne and Webb estimated that the range of communication for whales was reduced to several hundreds of miles. This long-range communication hypothesis has never been tested due to the difficulties of demonstrating communication for whales swimming in the open ocean. However, there is every reason to support the conclusion that whales rely heavily upon sound production and perception for survival. Their inner ears are remarkably well adapted for detecting and encoding low frequency sounds. They are acoustically prolific throughout the oceans during all months of the year. P. 23^