Seismic surveys and humpback whales

From September to November 1996 scientists working in waters to the north-west of Exmouth in Western Australia and in the waters of Exmouth Gulf, conducted research into the effects of commercial seismic surveys on the behaviour of humpback whales.

The project was initiated and funded by the Australian Petroleum Production and Exploration Association (APPEA) and the Energy Research and Development Corporation (ERDC). It follows and independent scientific review published by these organisations in 1994. Additional funding and support to expand studies into humpback whales and seismic surveys was provided by WMC Resources Ltd. Petroleum Division.

Seismic surveys use very short and high energy pulses of sound directed downwards into the Earth's crust. These pulses are most commonly produced by arrays of "air-guns" which generate an impulsive noise signal by rapidly releasing high pressure air into the water column (similar to the noise produced by bursting a balloon). These noise pulses travel downward into the sea floor with some energy reflected back to the surface every time the signal encounters a change in the type of rock strata. These returning pulses are detected, stored and then processed to give geological profiles of the sea floor, commonly in depths of ten kilometres.

There has been concern that these pulses, which lie toward the lower end of the range of human hearing, may adversely affect the thousands of migrating whales traversing the Western Australian coast. In March 1996, the Centre for Marine Science and Technology at Curtin University, Western Australia, began a three year program to study the environmental consequences of offshore seismic survey techniques.

As part of this project, fieldwork in late 1996 was carried out to study the response of humpback whales to seismic survey devices. This work comprised two separate by related projects comprising observations about and from a seismic survey vessel working an area in which migrating humpbacks were believed to be crossing, and exposure experiments in which the behaviour and movement patterns of selected pods of humpback whales was monitored before, during and after an approach by a vessel with a single operating air-gun.

Commercial seismic operations are subject to the "National Parks and Wildlife Conversation Act", the "Whale Protection Act", the "Endangered Species Act", and "The Petroleum (Submerged Lands) Act", amongst others. The exposure experiments were carried out according to Commonwealth and State permit procedures.

Observation of whale movements about the 75 metre seismic survey vessel "Geco Resolution" comprised aerial surveys before and during the survey, observations made by an observer from the vessel, and noise measurements and whale movements and behaviour in the vicinity of the "Geco Resolution", made from a smaller vessel, the "Blue Horizons". The "Geco Resolution" carried out a survey covering approximately 500 square kilometres over 34 operating days (excluding down time to bad weather). The vessel used a 2678 cubic inch air-gun array, operated at an approximate eight second interval to give a pulse every 12.5 metres across the ground. Four, three kilometre strings of hydrophones towed adjacent to each other at 100 metre separation were trailed behind the "Geco Resolution" to record the air-gun pulses.

Julie Murdoch, the marine researcher from Curtin University placed on board the "Geco Resolution", was responsible for spotting whales. Their positions were plotted by recording range and bearing from the ship and their general movements and behaviour in the vicinity of the vessel were noted. Many sightings were fleeting or isolated and thus accurate positions could not always be obtained.

Over a sighting period comprising 270 forty minute observation blocks, Julie sighted 51 whale pods made up of 95 whales of 86 adults and 9 calves. The aerial surveys spotted 93 whales in waters east and north-east of Exmouth Cape, while observations made from the "Blue Horizons" included 23 pods sighted of 28 adults, on juvenile and seven calves. Four "follows" were made from the "Blue Horizons", in which pods of whales on interception courses with the operating "Geco Resolution" were followed by the "Blue Horizons" from 3-10 sea miles north of the "Geco Resolution", to a similar range south of the vessel.

Although a thorough analysis of the data is yet to be finalised there did not appear to be a gross displacement of migrating humpback whales in the study area. Some whales did appear to avoid the general vicinity of the operating seismic vessel, generally altering their course to pass behind the vessel and resuming course when the vessel had passed at some distance. On some occasions whales did approach the operating vessel quite close and on at least two occasions the vessel stopped working as whiles were within the self imposed three-kilometre limit. These instances involved length (and costly) turns to realign the vessel along its set trackline.

The exposure experiments carried out within Exmouth Gulf involved a vessel firing a single, small air-gun approaching select whale pods from 5-6km away. These results are presently being finalised, and are expected to be analysed by mid 1997. Seven successful trials were carried out, with reactions of whale pods ranging from an apparent near reversal of whale course on approach of the air-gun, to slight course changes by whale pods so as to pass the air-gun vessel at 500 metres. On three occasions single animals broke off from the main pods, swam to the vessel with the air-gun operating, circled the vessel at a few hundred metres and then swam off.

The independent scientific review of the environmental implications of seismic surveys published in 1994 stated that "The fact that the Western Australian humpback population, which has traversed the region of greatest seismic activity in Australia for many years, has an annual rate of increase comparable with the relatively unexposed east coast population, suggests that the population status of these animals is little threatened by seismic survey activity". The work initiated by the APPEA and ERDC, further supported by WMC Petroleum and carried out my Curtin University, will shed a significant amount of further light onto the relative truth of this statement. But, there needs to be some caution in interpreting results, as although any effects seen on the behaviour and movements of migrating whales may appear to be comparatively insignificant, their translation to aggregations of whales actively involved in breeding, mating or calving activities, or on the staged resting of calves in sheltered bays during the southern migration, may have more serious consequences.