There are 22 Herd Management Areas (HMAs) in California covering 7.1 million acres of public land, with an additional 2.3 million acres of non-BLM land.
In the wild, horse herd sizes increase on an average of 20% a year. At this rate of increase, excess horses need to be removed every two or three years.
When the Appropriate Management Level (AML) in a Herd management Area is exceeded, then a gather plan is developed for the removal of the excess animals. Once the plan has been approved, the trap site is set up and the gather operation begins.
Most California gathers take place during the fall, thus allowing the spring foals to be large enough to be moved to the trap site with their herd.