A 2002 study examined 12 artificial reefs built off Miami Beach using limestone boulders, concrete-gravel aggregate and concrete mixed with waste tyre chips. Divers surveyed the reefs every two months from October 1998 to February 2001, recording fish species, numbers and sizes, along with spiny lobsters. They counted 97,826 fish from 146 species. The researchers found no significant difference in fish abundance, biomass or species richness between the three reef materials. However, comparisons before and after deployment showed that the artificial reefs increased local fish abundance and species richness.