Habitat Quality and Prey Availability for Bottlenose Dolphins
By Elizabeth Berens, MSc, and Damon Gannon, PhD

Habitat and Prey Researchers in the field, Summer 2004.

Bottlenose dolphins of Florida inhabit some of the most urbanized coastlines in North America.  Currently, the habitat requirements of dolphins are poorly known.  We are looking to answer several different questions in the present study, including:

 -What qualities do dolphins look for when selecting habitat? 

-To what degree do the distributions of prey, predators, and competitors influence their habitat preferences? 

-How does the presence of humans affect dolphins’ use of coastal waters? 

 Answering these important questions will give us a better understanding of the habitat requirements  of dolphins in urbanized areas, an understanding essential for the conservation of dolphins within Sarasota Bay.

To address 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.  From June to September  we made 84 seine sets and 152 passive acoustic recordings, despite being tormented by Hurricanes Charlie, Frances, Ivan (twice), and Jeanne.  In total, 26,872 fish were caught, measured, and released.  Preliminary analyses have determined that the abundance of dolphin prey species in sea grass and mangrove habitats is at least two orders of magnitude higher than in sandflat, open bay, and shallow Gulf of Mexico habitats.   The average sizes of several important prey species—hardhead catfish, Arius felis; menhaden, Brevoortia spp.; pigfish, Orthopristis chrysoptera; pinfish, Lagodon rhomboides; spotted seatrout, Cynoscion nebulosus; sheepshead, Archosargus probatocephalus; and spot, Leiostomus xanthurus—differed significantly between habitats.  The sizes of the fish are important because dolphins tend to select prey within the size range of about 6 to 16 inches (15-40 cm).  Therefore, prey availability for dolphins appears to vary substantially among the habitats found within Sarasota Bay.  Since specific habitats may support more prey, dolphin conservation in Sarasota Bay may depend more on the conservation of a couple specific types of habitat.  Future analyses on the current data will test for statistical differences among habitats regarding species richness and the occurrence of dolphins.  Our sampling will resume in December and run through March 2005.  Collecting summer and winter seine data will result in seasonal comparison of how dolphins and their target prey species are distributed throughout Sarasota Bay over the course of a single year.

Coupling the fish data from this study with our long-term database documenting dolphin and human uses of Sarasota Bay will allow us to quantify habitat quality for bottlenose dolphins.  With this knowledge, a simple index of habitat quality will be created that will allow scientists and government managers to determine the quality of any habitat for bottlenose dolphins.  This index could greatly benefit the conservation of bottlenose dolphins worldwide by helping resource managers predict the consequences of altering different types of habitat.  In addition, our data on fish abundance and distribution will be used to create an ecosystem-based model of Sarasota Bay’s food web, which in turn can predict how changes to one part of the food web will affect every other part of the food web.  For example, this model will be able to predict how an increase in fishing activity would affect different fish species in the Bay, as well as dolphins, manatees, seagrass, and mangroves.  Predicting these changes in the Bay’s food web could potentially allow researchers and managers to focus on key ecological changes likely to occur from proposed coastal development projects and to focus further research efforts on specific factors that influence the Bay’s dolphin population. 

As a further benefit of the research, we are collecting our data for Sarasota Bay using the same techniques the Florida Wildlife Research Institute is using for the neighboring estuaries of Tampa Bay and Charlotte Harbor.  Thus, we will be able to fill in a geographical gap in the state’s ongoing assessment of fish stocks.   The present research project would not be possible without the help of many interns and volunteers who generously donate their time and effort to this ambitious project.  Essential funding was provided by NOAA Fisheries (U.S. Department of Commerce) and by Harbor Branch Oceanographic Institution’s Protect Wild Dolphins Program.

conserve any species, scientists and government managers need to understand that species’ specific habitat requirements. The bottlenose dolphins of Florida inhabit one of the most urbanized coastlines in North America. Despite intense study in recent decades, habitat requirements of bottlenose dolphins are poorly known. What qualities do dolphins look for when selecting habitat? To what degree do the distributions of prey, predators, and competitors influence the dolphins’ habitat preferences? And how does the presence of humans affect their use of coastal waters? These are just a few of the questions we hope to answer with our new investigation of dolphin prey.

As a new postdoctoral scientist with the Sarasota Dolphin Research Program, I will be coordinating an ambitious fisheries sampling program to determine how prey are distributed among the various habitats within Sarasota Bay and how the distribution of prey changes over the seasons. The sampling program will use both traditional tools of fisheries science (net sampling) as well as new techniques (passive acoustic monitoring of sounds produced by fish). By coupling the fish data with our long-term database on how dolphins and people use the Bay, we will be able to quantify habitat quality for bottlenose dolphins. With this knowledge, we can create a simple index of habitat quality that will allow scientists and government managers to quickly determine the quality of any habitat for bottlenose dolphins. Such a tool for assessing habitat could be of great benefit to the conservation of bottlenose dolphins all over the world by helping resource managers predict the consequences of altering habitats. The data on fish abundance in Sarasota Bay will also be used to make a model of Sarasota Bay’s food web, which could allow managers to predict how changes to one part of the Sarasota Bay food web will affect all other parts of the food web. For example, the model will be able to predict how an increase in fishing activity will affect each fish species, as well as dolphins, manatees, seagrass, mangroves, and everything else that lives in the Bay. This research is funded by NOAA Fisheries.