Abstract
Bioenergetic models have been used to infer changes in an individual’s energy stores with behavioural state or as a consequence of disturbance, and have been widely used to investigate potential impacts of disturbance on marine mammals at both individual and population level. Dynamic Energy Budget (DEB) theory provides a mechanistic framework that predicts the consequences of an organism’s acquisition of environmental resources for energy demanding traits, such as growth and reproduction, via internal physiological functions. The equations in a DEB model describe the life history processes of a cohort of organisms, based on energy fluxes. Resources assimilated from the environment are allocated to maintenance, growth and reproduction via a reserve compartment. The overall objective of this project was to explore how DEB frameworks can be used to model the link between disturbance and population vital rates for five UK species of marine mammal to potentially improve marine mammal assessments for offshore renewable developments.
The report describes in detail the different parameters that are required by a DEB model, which of those are likely to be found in or derived from the literature and which are unknown and require subjective judgement. Based on an extensive literature search, we provide suggested parameter values for harbour porpoise, bottlenose dolphins, minke whales, harbour seals and grey seals in the UK, noting in each case the literature that was used to derive the parameter and where they were estimated by subjective expert judgement. While recommended parameters were collated for each of the five species, a full DEB model was created for harbour porpoise only. Therefore, the focal species in this report is the harbour porpoise. The text details the results of the literature search for harbour porpoise and exactly how each parameter value was calculated or estimated. Similar detail is provided for bottlenose dolphins, minke whales, harbour seals and grey seals in Appendix 1-4.
Where values required subjective expert judgement or where a range of values are estimated in the literature, sensitivity testing was conducted to understand the sensitivity of the DEB model to plausible ranges of these parameters. Sensitivity testing showed that varying the age at which the calf’s foraging efficiency is 50% made significant changes to calf survival and consequential reproductive success.
The report then illustrates how the DEB model can be used to investigate the potential effects of disturbance that causes a reduction in energy intake and subsequent effect on vital rates (individual survival and birth rate), using harbour porpoise as an example. The scenarios explored here used a theoretical assumption that an individual’s energy intake would be reduced by 25% on the day it was disturbed. The effect of this disruption, experienced during different time periods in the year, when females are in different reproductive stages was investigated. The model highlighted that nursing female harbour porpoise are particularly susceptible to disturbance between the time the calf is born until it is able to acquire at least some food independently. The results showed that disturbance after the mother has begun to reduce milk provisioning and post-weaning is unlikely to markedly affect survival rates.
Finally, the report explores if DEB models could potentially be integrated into the interim PCoD model to replace the dependency on transfer functions derived using expert elicitation approaches that are currently used to the links between disturbance and subsequent effect on vital rates. One potential issue with this approach is that some of the parameters which potentially have a large effect on susceptibility to disturbance cannot be measured directly and, therefore, have to be chosen subjectively. There is, therefore, a requirement for experts to agree on plausible ranges for the relevant model parameters; however, once these are agreed, the incorporation of DEB models into the iPCoD code can be done without major structural changes.
See corresponding fact sheet: Developing marine mammal Dynamic Energy Budget
models and their potential for integration into the iPCoD framework for more information.