We developed a dynamic programming model of group size choice for settling
coral reef fish to help understand variability in observed group sizes. Rat
her than calculating optimal group size, we modeled optimal choice and calc
ulated the acceptable group sizes that arose fi om this choice. In the mode
l, settling individuals weigh the fitness value of settling in a group agai
nst the expected fitness of searching another day and encountering other gr
oups, choosing the option with the higher value. Model results showed that
individuals settling on any given day in the settling season have several a
cceptable group sizes in which they can settle. The range of acceptable gro
up 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 h
ighest fitness. Late in the season, when the ability to grow fast becomes m
ore important, small groups, which convey fast growth rates (although riski
er), have higher fitness. Thus, according to our model, even when fish all
make the same, simple decisions, a variety of outcomes are possible, depend
ing on the specific options encountered and temporally changing ecological
pressures. Even when all fish behave optimally, initial variability in grou
p sizes will persist