Fig. 4. (A) The boat contained the computer and other equipment together with necessary battery power and cable storage. The hearing test platform (HTP) was suspended by a Kevlar line and by an electrical cable. The cable carried power for lights, video, preamplifier, projector and hydrophone lines. A projector below the HTP delivered tones. A hydrophone in front of the whale station received the projected tones, the whale whistles and background noise. A video camera above the HTP allowed us to observe the whale to ensure correct positioning. (B) An amplitude/time series of a typical whistle employed by each whale as a response to tones it heard. MUK’s whistle lasted approximately 100 ms, while NOC’s whistle lasted 300–400 ms. (C) Typical spectra of whistles (all levels are received levels). The only response from NOC at 300 m was a pulse train, and this was not included in the whistle spectral analysis.