**2.3 Interaction between agents**

**A risk screening is carried out for Common Guillemot based on the oil spill model** results by computing the spatial overlap of the oil slick and the simulated species distribution. Each oil particle represents a given area with a given slick thickness and relevant properties such as oil viscosity and depth below the water surface. The latter determines whether the oil floats on the surface or the oil droplets are dispersed into the water column due to wave action.

For each time step in the oil spill simulation period, it is tested if the position of the individual sea bird agent particle is within the oil area represented by the individual oil spill particles. If this is true, the sea bird agent particle is flagged as being in contact with oil. However, it is further tested before flagging if the following parameters exceed threshold levels for the sea-bird in question.


**Figure 3.**

*Schematic of the calculation of the affected number of individuals based on the overlap area of the oil- and bird species particle.*

Each sea bird particle/agent represents a large number of individuals, i.e. it is a so-called super individual. This must be considered when assessing whether or not individuals of a species have been in contact with a drifting oil slick. In case of super individuals, the point position of the agent/particle is replaced with an area around the particle. It is assumed that the individual birds represented by the super individual particle are uniformly distributed within this area. If there is a spatial overlap with the area defined by any oil particle, the affected number of individuals corresponds to the relative overlap area of the bird species particle. The area covered by the bird species particle is then reduced proportionally or, when no unaffected individuals remain, the particle is removed from the analysis. This means that the number of affected birds serves as a worst-case estimate (**Figure 3**).

The risk screening for the oil spill is then expressed in terms of the number of birds exposed (flagged) to oil above the threshold values during the oil spill simulation period. The number of birds exposed is also expressed as a fraction of the average population size with the model domain during the oil spill simulation period. The oil spill simulation will typically continue 30 days after the spill has terminated.

The above approach is based on pure Lagrangian ABM results for birds and oil, respectively, which in turn provide consistent results independent of the Eulerian grid mapping (i.e., results that are independent of the grid resolution).
