Fig. 4 Observations (dots) and model estimates (lines) of porpoise behavioural responses to an approaching boat (averaged across animals; data from phase 2 before the closest point of approach, i.e. CPA). X-axis values were back-transformed from their logarithms. In a and d, values on the y-axis were back-transformed from their cube root; in b, c and d, bars show observed frequencies of a specific behaviour at each distance range. Shaded areas show 95% confidence intervals. * denotes significant models (p <0.025).

Porpoise behaviour before and during exposure

The porpoises’ behaviour during the minute where the boat was closest did not differ significantly from their behaviour before the experiment started (their speed, absolute turning angles, and the probability of diving or of breathing, were the same; p >0.05 for all variables and both boat speeds; Fig. 5).