Douglas H. Cato

GEOGRAPHIC VARIATION IN SOUTH PACIFIC HUMPBACK WHALE SONGS

Every winter, (male) humpback whales (Megaptera novaeangliae) produce long complex songs. Song content is dynamic and singers incorporate changes as they occur, thus song is shared through cultural transmission. We compared songs recorded in winter migratory termini in Tonga, New Caledonia, Eastern Australia, and on migration paths off Eastern Australia and New Zealand, in the winter of 1994. Seven themes were shared by all regions, with an additional two themes shared by all but Tonga. Differences in regional variants were most pronounced between Tongan and Eastern Australian song. New Caledonian and Kaikouran song were more similar to songs from Eastern Australia rather than Tonga. These regional differences were stable across the season. The results suggest some migratory exchange among widely separate wintering regions of Area V, consistent with tag recovery data, but the time and location at which song sharing occurs remains speculative.

The status of humpback whales Megaptera novaeangliae in east Australia thirty years after whaling

Abstract The portion of the Area V (130°E–170° W) humpback whale Megaptera novaeangliae stock that migrates along the east Australian coast is estimated to have numbered 1900 ± 250 in 1992 compared with a population possibly as low as 100 at the conclusion of whaling in 1962. The average annual rate of increase from 1984 to 1992 is estimated to have been 11·7% with a 95% confidence interval of 9·6–13·8%. This level of recovery suggests that, in the post whaling period, there have been no environmental factors detrimental to this particular stock.

Multivariate analysis of behavioural response experiments in humpback whales (Megaptera novaeangliae)

SUMMARY The behavioural response study (BRS) is an experimental design used by field biologists to determine the function and/or behavioural effects of conspecific, heterospecific or anthropogenic stimuli. When carrying out these studies in marine mammals it is difficult to make basic observations and achieve sufficient samples sizes because of the high cost and logistical difficulties. Rarely are other factors such as social context or the physical environment considered in the analysis because of these difficulties. This paper presents results of a BRS carried out in humpback whales to test the response of groups to one recording of conspecific social sounds and an artificially generated tone stimulus. Experiments were carried out in September/October 2004 and 2008 during the humpback whale southward migration along the east coast of Australia. In total, 13 ‘tone’ experiments, 15 ‘social sound’ experiments (using one recording of social sounds) and three silent controls were carried out over two field seasons. The results (using a mixed model statistical analysis) suggested that humpback whales responded differently to the two stimuli, measured by changes in course travelled and dive behaviour. Although the response to ‘tones’ was consistent, in that groups moved offshore and surfaced more often (suggesting an aversion to the stimulus), the response to ‘social sounds’ was highly variable and dependent upon the composition of the social group. The change in course and dive behaviour in response to ‘tones’ was found to be related to proximity to the source, the received signal level and signal-to-noise ratio (SNR). This study demonstrates that the behavioural responses of marine mammals to acoustic stimuli are complex. In order to tease out such multifaceted interactions, the number of replicates and factors measured must be sufficient for multivariate analysis.

Against the humpback whale sonar hypothesis

A rebuttal is presented to the article of L.N. Frazer and E. Mercado (ibid., vol. 25, pp. 160-182, 2000), who presented a sonar model for humpback whale song, is presented. This rebuttal considers the noise-limited form of the sonar equation, current understanding of humpback whale behavior and the characteristics of humpback whale songs, along with arguments from an evolutionary perspective. Arguments from all of these different aspects do not support the model of Frazer and Mercado.

Physical Mechanisms of Noise Generation by Breaking Waves — A Laboratory Study

Breaking waves are recognised as a significant source of ambient underwater noise in the ocean yet the details of the causal physical mechanisms are poorly understood. This contribution describes the initial findings of a detailed laboratory study aimed at elucidating this process using simultaneous high speed photography and sound measurements under controlled conditions. It was found that the dominant noise occurred in the form of discrete tone bursts which appeared to be associated with the formation of bubbles and coalescing or splitting of bubbles.

Ambient sea noise dependence on local, regional and geostrophic wind speeds: Implications for forecasting noise

Abstract Measurements of ambient sea noise in two regions near Australia are presented as a function of wind speed (a) measured locally (at a buoy, distance about 120 m), (b) measured at weather stations (distances up to 51 km) and (c) predicted from the geostrophic air flow. Estimates of geostrophic winds from atmospheric pressure contours are used in forecasting wind speeds. Good correlation of noise with local wind speed was observed. Poorer correlations were observed between noise and regional wind speeds or those predicted from geostrophic wind speeds: trends were similar but the spread of the data points was greater. Correlations were better when the wind field had minimal influence by the presence of land upwind. The variation of geostrophic wind speed with time generally followed that of the local wind speed but peaks and troughs were sometimes displaced in time by a few hours, accounting for some of the spread of the data. These results suggest that geostrophic wind, and thus forecasts of wind speeds, would be useful in forecasting sea noise.

Acoustic surveying for beaked whales in the Coral Sea as a mitigation measure for naval exercises

Beaked whales have been over-represented in whale strandings that have occurred at similar times and places of some naval exercises in the northern hemisphere. Although whale strandings are common, it is unusual to find beaked whales stranded in such large numbers. Consequently, the environmental management of naval exercises requires mitigation measures to avoid potential impact on beaked whales. This requires some knowledge of beaked whale distributions and behavior, but little is known about beaked whales, less than for all other whales. Beaked whales are small whales that inhabit deep water. They are particularly elusive and rarely seen at sea. Much of what little is known about them results from the few that have been washed ashore. Visual surveying has proved to be generally ineffective. Beaked whale vocalizations are, however, sufficiently distinctive to allow passive acoustic detection and classification [1]. The sounds are sufficiently distinctive to distinguish them from those of other toothed whales, though these distinctions are subtle. All toothed whales produce echolocation clicks or pulses with a small number of cycles and most energy at ultrasonic frequencies. Beaked whale clicks can be distinguished by the frequency range, duration and number of cycles but other species have clicks that are close in one or other of the characteristics.

AMBIENT NOISE AND ITS SIGNIFICANCE TO AQUATIC LIFE

Underwater ambient noise is the background noise from all sources, although it usually does not include sounds from close sources that are individually identifiable, such as a passing ship or a nearby singing whale. Ambient noise provides a basic limitation on the use of sound by aquatic animals because they have to detect acoustic signals of interest against the ambient noise background. Because ambient noise levels vary over a wide range, the distances at which sources are audible also vary substantially. Ambient noise plays a significant part in the acoustic behaviour of aquatic animals. Natural ambient noise is the baseline noise exposure experienced by aquatic animals, and comparison with the underwater noise from human activities may help us to understand the effects of anthropogenic noise.

Addressing challenges in studies of behavioral responses of whales to noise

Studying the behavioral response of whales to noise presents numerous challenges. In addition to the characteristics of the noise exposure, many factors may affect the response and these must be measured and accounted for in the analysis. An adequate sample size that includes matching controls is crucial if meaningful results are to be obtained. Field work is thus complicated, logistically difficult, and expensive. This paper discusses some of the challenges and how they are being met in a large-scale multiplatform project in which humpback whales are exposed to the noise of seismic air guns.

Noise Generated by Motion of the Sea Surface — Theory and Measurement

This paper discusses a model of noise generation by sea surface motion and describes measurements in a controlled experiment to test the predictions of the theory. The model is a development of Lighthill’s (1952) theory of noise generation, applied to the case where there is a moving density discontinuity, like the sea surface, within the volume of the fluid. The results show that the sound field is equivalent to that produced by distributions of monopole and dipole sources over surfaces of density discontinuity, in addition to the expected volume distributions of quadrupoles. For a complex sea of linear surface gravity waves, both horizontal and vertical dipoles are shown to dominate the generation of sound at frequencies of 0.1–5 Hz.