We implemented a model of killing rate similar to a classical formulation
of attack rate × number of prey, with attack rate partitioned into an encounter
rate (e) defined as the probability that a group of killer whales would encounter
a particular individual prey, and vulnerability (v) equal to the probability
of being killed by the group once encountered (i.e, expected kills per day
equals e × v × number of prey). To make this dependent on group size (x),
we used a simple logistic function with a user-defined maximum vulnerability
and logistic parameters a and b: v = vmax * exp(a + b × x) / (1 + exp(a +
b × x)). A sample vulnerabilty curve and resulting expected kills per whale
by group size is shown below.