Scientists recognize two bottlenose dolphin ecotypes (forms): coastal and offshore. In the northwest Atlantic, bottlenose dolphin coastal and offshore ecotypes can be differentiated by skull and body measurements as well as by characteristics of their blood.
In general, the coastal ecotype seems to be adapted for warm, shallow waters. Its smaller body and larger flippers suggest increased maneuverability and heat dissipation. These dolphins frequent harbors, bays, lagoons, and estuaries.
In general, the offshore ecotype seems to be adapted for cooler, deeper waters. Certain characteristics of its blood indicate that this form may be better suited for deep diving. Its larger body helps to conserve heat and defend itself against predators.
In northwest Atlantic bottlenose dolphin studies, researchers determined that dolphins within 7.5 km (4.65 mi) of shore were coastal ecotypes. Dolphins beyond 34 km (21 mi) from shore were offshore ecotypes.
The worldwide population of bottlenose dolphins is unknown. Specific bottlenose dolphin populations in a few areas have been estimated.
U.S. National Marine Fisheries Service (NMFS) surveys estimate 243,500 bottlenose dolphins in the eastern tropical Pacific.
Japanese surveys estimate 316,935 dolphins in the northwest Pacific.
NMFS surveys in the northern Gulf of Mexico estimate as many as 45,000 bottlenose dolphins from the coast to about 250 km (155 mi.) offshore. This area includes the Gulf's bays and sounds, coastal waters (about 17,600 individuals), and continental shelf waters (about 25,320 individuals).
In U.S. waters of the western North Atlantic, average abundance estimates for the coastal population are 9,206 from the summer survey and 19,459 from the winter survey. The western North Atlantic average abundance estimate for the offshore population is 29,774 individuals.
The Mediterranean population is estimated at less than 10,000.
Average abundance estimates for the U.S. west coast include the California coastal population of 206 individuals and the California-Oregon-Washington offshore population of 5,065 individuals.
Chromosome banding techniques have proven useful in bottlenose dolphin population studies. In some areas, scientists can identify individuals and determine relationships among dolphins in a group.