Will large ships and their tugs influence Harbour Porpoise behaviour in Head Harbour Passage? We may soon know the answer as researchers continue to investigate this concern.
While it's a bit heavy duty for the average folks, the following is one of the papers currently being published. Important stuff really, but it will be nice to see results emerge from the depth of jargon.
Kastelein, R.A.*, and P.J. Wensveen. 2008. Effect of two levels of masking noise on the hearing threshold of a harbor porpoise (Phocoena phocoena) for a 4.0 kHz signal. Aquatic Mammals 34(4):420-425.
The 50% detection hearing thresholds of a harbor porpoise for a 4.0 kHz narrow-band FM signal, presented at the background noise level in a pool and with two masking noise levels, were measured using a go/no-go response paradigm and an up-down staircase psychometric method. The masker consisted of a ⅙-octave noise band with a center frequency of 4.25 kHz. Its amplitude declined at 24 dB/octave on both sides of the spectral plateau. The absolute hearing threshold of the porpoise, found previously, was confirmed. The animal's auditory system responded in a linear fashion to the increase in masking noise. Since the narrow-band noise was off-center of the test frequency, the critical ratio of a harbor porpoise for 4.0 kHz tonal signals in white noise can at present only be estimated to be between 18 and 21 dB re: 1 μPa.