Habitat quality and prey availability for bottlenose dolphins
By Damon Gannon, PhD, and Elizabeth Berens, MSc

Bottlenose dolphins of Florida inhabit one of the most urbanized coastlines in North America. Currently, the habitat requirements of dolphins are poorly known. We are addressing several important questions regarding prey availability and habitat quality, including: 1) What qualities do dolphins look for when selecting habitat? 2) To what degree do the distributions of prey, predators, and competitors influence their habitat preferences? and 3) How does the presence of humans affect the dolphins’ use of coastal waters?

To answer these questions, we began studying Sarasota Bay’s fish community with the use of a large purse-seine net and a passive acoustic recording system in June 2004. From June 2004 to September 2005 we made 311 seine sets and 440 passive acoustic recordings. So far, we have caught, measured, and released 70,112 fish. In summer, the abundance of dolphin prey is at least two orders of magnitude higher in seagrass and mangrove habitats than in sandflat, open bay, and shallow Gulf of Mexico habitats. But during winter, many fish appear to leave the seagrass beds and mangrove forests and move to the deeper waters of the inlet, open bay, and dredged channel habitats. As you might expect, dolphins also tend to spend more time in these deeper habitats during winter.

As fish grow, their habitat preferences change. For many species, such as pinfish (Lagodon rhomboides, the most abundant bottom fish in Sarasota Bay and a common prey item in the diet of bottlenose dolphins), juveniles tend to be concentrated in the seagrass and mangrove forests, whereas adults often occupy deeper waters. The sizes of the fish are important because dolphins tend to select prey within the size range of about 4 to 16 inches (10-40 cm). Therefore, prey availability for dolphins varies substantially among habitats and the distribution of prey changes seasonally. In addition to predictable seasonal changes in prey availability, the occurrence of red tide also results in changes in prey availability for dolphins and these red tide events are not predictable (see below).

Winter and summer sampling will resume in 2006. This research project would not be possible without the help of interns and volunteers who generously donate their time and effort to this ambitious project. Funding is provided by the NOAA Fisheries and by Harbor Branch Oceanographic Institution’s Protect Wild Dolphins Program.