This article was first published in the March, 2007 edition of Semaphore, the magazine published by the Seapower Centre of Australia and is reprinted here with their kind permission.
The extent to which marine mammals are affected by human-created underwater sound, particularly active sonar, has been a topic of growing public concern in recent years. This Semaphore will explore the complex issues surrounding the effects of underwater sound on marine mammals and the importance the Royal Australian Navy (RAN) places on environmental management, to ensure long term access to vital offshore training areas.
Australia is fundamentally a maritime nation, potentially vulnerable to any efforts to block key trade and supply routes from above or below the sea. Maintaining a credible RAN anti-submarine capability remains important in a region that has seen significant growth in submarine forces. In addition, the increasing focus on littoral operations, linked partly to the need for maritime amphibious capabilities, means ships will need to operate in areas where conventional sonar technology is challenged by poor seawater transmission characteristics and complex sea floor structure. Modern conventional submarines are quieter through better design, and are therefore more challenging to identify by passive means. With no viable alternative technology, the RAN will continue to rely on a combination of passive and active sonar for detection of submarines. This requires regular and realistic sea-going training of personnel and maintenance of equipment to meet this complex and multi-faceted challenge.
Australian waters are populated or visited by around forty species of whales and dolphins, ranging in size from dolphins to the Blue Whale (up to 30 metres in length). Unlike other parts of the world, Australian marine mammal population levels are almost uniformly stable, or recovering, and are not under threat from human activity. Depletion of some species through whaling and other human causes such as pollution and by-catch has strengthened community resolve to ensure their protection. In parallel, development of a whale watching industry with prospects for employment and wealth generation in regional areas has highlighted the economic value of marine mammal conservation.
All marine mammals have adapted to use sound as a primary tool for communication, identification and hunting prey. As a result, any human activity that produces underwater sound has the potential to impact on or disrupt these vital communication processes. Underwater sound from RAN vessels can be emitted by explosives, ship and boat engines, underwater communication systems and active sonars.
The impact of sound disturbance on marine mammals can be manifested in a number of ways, including:
- masking of important biological sounds (sounds of prey or communication with other members of the pod)
- changing behaviour (dive patterns, movement, abandonment of activities such as hunting prey)
- stress (fright, flight)
- physical injury to hearing mechanisms
- tissue damage leading to injury or death.
The scale of impact is a function of the source sound output level (loudness), transmission reflection and absorption characteristics of the water column and sea floor, and distance from the source to the animal. Equally important is the auditory capability of the animal (can the species hear the transmission frequency?) and the animal’s propensity to react to the sound (is it easily startled?). Scientists and regulators are particularly interested in managing `biologically significant’ sounds, specifically those that affect important activities such as feeding, breeding and migration.
Recent articles have highlighted the challenges faced by navies worldwide in dealing with these issues. For the RAN, the conduct of vital training activities in realistic conditions at sea is essential to maintaining necessary operational skills. Offshore training areas are concentrated close to the major fleet bases on the east and west coasts, to ensure ready access and minimise transit times between harbour and sea. These areas are also frequented by increasing whale populations.
For example, the West Australian Exercise Area, west of Fremantle, is inhabited by various species, including Blue whales which feed in the Rottnest Trench in summer and autumn. Humpback whales migrate through the area twice each year between their winter breeding areas in the tropical north and summer feeding grounds in the waters of Antarctica. Beaked whales are also seen in deep offshore waters over summer. Increasing numbers of marine mammals can therefore be expected to be encountered in the area regardless of time of year, reinforcing the need for RAN exercise planners and individual ships, submarines and aircraft to remain alert to possible whale interactions.