A dynamic model of group-size choice in the coral reef fish Dascyllus albisella

We developed a dynamic programming model of group size choice for settling coral reef fish to help understand variability in observed group sizes. Rather than calculating optimal group size, we modeled optimal choice and calculated the acceptable group sizes that arose fi om this choice. In the model, settling individuals weigh the fitness value of settling in a group against the expected fitness of searching another day and encountering other groups, choosing the option with the higher value. Model results showed that individuals settling on any given day in the settling season have several acceptable group sizes in which they can settle. The range of acceptable group sizes also changes across the season. Early in the season, when there is still adequate time to grow large groups (with higher survival) have the highest fitness. Late in the season, when the ability to grow fast becomes more important, small groups, which convey fast growth rates (although riskier), have higher fitness. Thus, according to our model, even when fish all make the same, simple decisions, a variety of outcomes are possible, depending on the specific options encountered and temporally changing ecological pressures. Even when all fish behave optimally, initial variability in group sizes will persist