Foraging sperm whales spend the majority of their time underwater and are only available at the surface for relatively brief periods (10mins) interspersed by long absences (+40mins) underwater (Papastavrou et al. 1989; Gordon and Steiner 1992). This can mean that sperm whales are unrewarding subjects for whale watchers and operators may feel the need to approach at great speed whales which surface at some distance, which is likely to disturb them. The general solution to this problem has been to use passive acoustic tracking methods to locate vocalising whales during their dives and to predict their surfacing location (Whitehead and Gordon 1986; Gordon et al. 1992). It was recommended that vessels watching sperm whales use visual/acoustic tracking to locate the whales and that surface intervals be monitored.
Surface intervals are relatively easy to measure and have been monitored during some studies of whale watching. Dive time data are much more difficult to obtain as the animal must be seen soon after it returns to the surface and must be reliably identified. Both Norwegian and New Zealand studies found significant decreases in surface times for approached versus non-approached whales (Gordon et al. 1992; Eberhardt 1993) . It is suggested that because sperm whales feed at great depths (perhaps at the physiological extremes of what is possible for mammalian divers) even modest reductions in the time available for recovery on the surface may be deleterious. For example, sperm whales which feed at great depths invest a significant proportion of each dive in travelling to and from these foraging areas. Thus, a reduction in dive length, caused perhaps by being forced to dive early before full recovery, would result in a proportionally larger decrease in time spent usefully feeding at depth.
Many aspects of sperm whale feeding behaviour remain a mystery and it is very difficult to obtain any direct measures of feeding success. Managers and operators should be aware that disturbance could potentially be affecting this crucially important activity without them being aware of it. Collecting data on surface times and dive lengths should be one priority of any monitoring scheme.
There is no audiogram for the sperm whale, however, a number of observations give some indication of the likely range of greatest acoustic sensitivity. One researcher told the group that in his experience sperm whales responded most strongly to noise sources below 28kHz. For example, one instance was reported of sperm whales being affected by sonar of 8kHz. Further, the anatomy of the sperm whale ears suggests that they are not as well adapted to very low frequencies (below 1 kHz) as baleen whales (Ketten 1992).. It is not simply the level and frequency of noises which cause disturbance. Sudden increases in noise, unusual noises and rapid movements of the noise source may all be particularly alarming.
When sailing craft are being used near silent approaches to whales are possible. However, this can lead to severe disturbance when whales suddenly find a vessel very close to them. In these situations it was felt important that the boat make some noise, for example the engine should be left running in neutral at minimum idling speed, so that the whales would be aware of where the boat was. Field experience has shown that it can be extremely alarming for sperm whales suddenly to find that they are very close to a silent vessel.
Two sorts of directional hydrophones are commonly used. Single hydrophones mounted in reflective cups, or short arrays, are usually deployed from stationary vessels to give bearings on whales. With a streamlined housing to shield the hydrophone from water noise, such directional hydrophones can be used when moving at less than about 7 knots. Towed stereo hydrophones give less information, only whether the whale is ahead or behind, but can be monitored while the vessel is moving (Leaper et al. 1992).
When only a few whales are present in an area and the vocalisations of single animals can be determined, precise tracking can be conducted so that whales may surface within a few hundred meters of a boat. Sperm whales click more sporadically towards the end of dives and stop clicking a few minutes before surfacing which makes their appearance at the surface more predictable. In larger groups, when many whales can be heard on different bearings, the situation can become confusing. In these situations, precise tracking is rarely possible and acoustic techniques are mainly useful for keeping the boat in the centre or local areas of high sperm whale density.
Listening to sperm whale vocalisations, especially the clicks made at the beginning of dives and the interesting codas and social noises heard from socialising groups, should enhance the whale-watching experience for the public. Non-directional hydrophones connected to the boat's speaker system are most appropriate for this.
Sperm whale vocalisations are some of the most obvious and easily recorded behavioural cues for this species. They may offer the only feasible means of measuring potential effects of whale-watching on underwater behaviour including feeding (Gordon et al. 1992). In addition, sperm whale vocalisations can very easily be recorded, if necessary, by nonspecialists or remotely, and there is very considerable scope for automated analysis (Gillespie, 1996).
Sperm whales may have the lowest reproductive rate of any mammal. Females begin to breed at around 9 years of age, gestation may take as long as 18 months and calves are born about once every 5-6 years. Calf mortality may be quite high (Best et al. 1984). Statistics agreed by the IWC suggest that the maximum growth rate for sperm whale populations is 0.86% (Calculation by Whitehead based on IWC (1982)).
Clearly a species with such a low reproductive rate is very vulnerable to any number of human activities causing disturbance, which might tend to depress it further.
Female groups of sperm whales inhabit warm, clear tropical or temperate waters, which is an inviting environment for humans who want to swim with or make underwater observations of whales.
Relevance to whale-watch activity
The popularity of swimming with wild cetaceans is of especial concern in the case of sperm whales. Attempts to swim with whales are likely to be more disturbing than other types of encounters because such activity involves close approaches by boats and humans. Attempts at swimming with whales are usually made with socialising/resting groups. Solitary calves at the surface are also especially accessible, and therefore vulnerable, to whale-watchers wanting to swim with whales.
Swimming in the open ocean is always a risky activity for humans, and the risk is heightened when many people of varying swimming abilities are simultaneously in the water. Moreover, swimming with other wild cetaceans (e.g., pilot whale, bottlenose dolphin) has proven to be dangerous to humans, resulting in serious injury and even death (Shane et al. 1993). Although there are no reports of aggression towards humans by sperm whales (except in the context of traditional whaling), human swimmers are at risk of injury by whales, either intentionally or inadvertently. Sperm whales are, after all, the largest of the toothed whales.
Swimming with whales is a rare privilege and it would be misleading of operators to suggest to the public that it was one they were likely to experience as a matter of course. One suggestion was that underwater observations should only be allowed with observers holding on to a drifting vessel. However, the consensus was that swimming with whales should not form part of normal whale-watching activities.
If swimming with whales was felt to be necessary for scientific or film-making reasons then it should only be attempted under a special permit granted to appropriately experienced groups.