Carrying on with Marine Scotland EPS guidance (there is a lot of it!):
1.2.4 What is a significant effect on local distribution or abundance of a species?
Regulation 39(1)(b)(v) specifies disturbance which is likely ‘to significantly affect the local distribution or abundance of the species to which it belongs.’ The Commission Guidance suggests that, for a significant effect on the local distribution or abundance of a species to occur, disturbance would need to produce more than a transient effect. Although the effect of the disturbance does not need to be permanent, it would need to have a significant effect at the time to fall within Regulation 39(1)(b)(v) of the Habitats Regulations.
In the offshore marine environment, there will usually be adjacent areas for marine EPS to move to that are within the natural range of their populations, and hence compensate for the loss of, or displacement from, a particular area of habitat. This cannot be assumed to be the case for inshore waters where the coastline could act as a physical barrier to other areas. Nor can it be assumed that displaced animals will fare as well in some other part of their range, since adjacent areas might be already populated, potentially resulting in increased competition, or might be of lower habitat quality. This would occur, for example, if a number of animals became displaced from an area used frequently, for a period longer than they would normally be absent. The significance of the duration of the potential displacement would have to be assessed on a case-by-case basis, depending on the spatio-temporal patterns of the species occurrence in the area affected.
So, could dolphins distribution be altered by STS? Quite possibly. If you stick 3 tankers and associated tugs in the dolphins favoured habitat and make lot of noise they could become stressed and move away. Maybe their food sources would move (fish!). To show that they would not be displaced would required detailed assessment of patterns of species occurrence in the area affected then detailed noise modelling (based on the sound power level of each ship involved) to show that disturbance would not occur either to cetaceans or their prey. Difficult to prove no impact. This would also need to be undertaken when carrying out the Appropriate Assessment which looks at the impact on the integrity of the SAC. Don’t forget that this would have to be proved beyond all reasonable scientific doubt.
