Interpretation of the Outputs from the Interim PCoD Scenarios
The interpretation of the results of implementing the interim PCoD protocol for North Sea harbour porpoises and European minke whales using the hypothetical disturbance scenarios outlined above is relatively straightforward. The forecast effects are relatively small (a very high probability that the population will decline by less than 1% over the period of construction) and these predicted effects are not sensitive to assumptions made about the relative vulnerability of individual animals within the population to the effects of PTS and disturbance.
The results for coast east Scotland bottlenose dolphins suggest that there is a relatively high probability (more than 50%) that construction scenario we have used as an illustration could result in a short term decline of 1% or more. This would be classified as "significant" using the UK approach for assessing conservation status under the Habitats Directive (Joint Nature Conservation Committee, 2007). However, the sensitivity analysis indicates that these forecasts are affected by the choice of assumption about the way in which the vulnerability of animals to PTS varies over time. This suggests that mitigation measures designed to reduce the number of animals that experience PTS could substantially reduce the risk of a significant decline in the size of this population. Developers could therefore be encouraged to propose such measures and estimate their likely effectiveness in reducing the risk of PTS. The population consequences of these mitigation measures could then be assessed by re-running the interim protocol using the revised values.
The results for both Moray Firth harbour seals and Moray Firth grey seals indicate that the disturbance scenarios we have chosen could result in substantial (around 20% in the case of harbour seals and 10% in the case of grey seals) short term declines in abundance. These are a consequence of the high proportion of the population that is predicted to experience disturbance and PTS on each day of construction. These high proportions are themselves a consequence of the fact that Southall et al. (2007) recommend the use of a substantially lower sound threshold for the onset of TTS and PTS in seals than for cetaceans. The use of different values for these thresholds, and the implementation of mitigation measures to reduce the risk of PTS would reduce the calculated risks of substantial population decline. For example, if the estimate of the number of harbour seals that may experience PTS on one day of construction is reduced to zero, but the estimate of the number of animals that may experience disturbance remains the same, then the median forecast decline in the Moray Firth population at the end of construction is reduced from around 20% to less than 10%.
The interim PCoD approach can provide forecasts of the possible size of a population many years after any disturbance associated with a particular development ceases. However, these forecasts are unlikely to be realistic because they assume that the vital rates within a population that has been reduced in size will not change as a result of density dependent processes. Therefore, simulated populations do not show any recovery once the effects of disturbance and PTS have ceased. In practice, because of the factors described in the preceding section on density dependence, there is likely to be some increase in vital rates, provided that there are no other threats to the population. With current information, it is not possible to predict or model these changes with any confidence. Even if we chose to use a standard function for density dependence, such as the generalised logistic equation used by Thompson et al. (2013) for harbour seals in the Moray Firth relationships, we would have to make a large number of arbitrary choices about which parameter values to use to define the shape of this function. This process would have to be repeated for each demographic rate that might show a density dependent response for each species. In addition, we would have to arbitrarily define a carrying capacity for each MU of each species. Figure 9 of Thompson et al. (2013) demonstrates that the value chosen for carrying capacity can have a substantial effect on the predicted trajectory of a population that is subject to disturbance from a renewable energy development.
Nevertheless, if is possible to draw some broad conclusions about the long term responses of marine mammal populations to disturbances of this kind. Seal populations are likely, on average, to recover relatively rapidly (probably within a decade following a 10-20% reduction) from the effects of short-term disturbance, provided all other factors affecting the population (such as deliberate killing, prey availability and other environmental conditions) remain unchanged. This is because of their potentially high maximum population growth rate and the fact that disturbance is likely to have a greater effect on young animals than on adult females, resulting in a population with a higher than normal proportion of adult females. Cetacean populations are also likely to show the same changes in population age structure as seals, but their maximum population growth rate is likely to be substantially lower than that of an equivalent seal population (Wade 1998) so they will take longer to recover from the effects of disturbance.