Diet analysis of bottlenose dolphins Tursiops truncatus from the Northwest Florida Panhandle in relation to a red tide Karenia brevis bloom

Bottlenose dolphins Tursiops truncatus in the Northwest Florida Panhandle are threatened by Karenia brevis blooms which produce brevetoxins that bioaccumulate in fish. The purpose of this study was to identify the diet of bottlenose dolphins from the Northwest Florida Panhandle and determine if diet changed during a K. brevis bloom. Frequency of occurrence and abundance of prey items were compared between dolphins that stranded prior to a K. brevis bloom (“non-bloom”) and those that stranded during and shortly after the bloom (“bloom”). Stomachs (N = 24) were collected from stranded bottlenose dolphins from November 2006 to March 2008. A K. brevis bloom occurred from September 2007 to early January 2008. Non-bloom strandings (November 2006 to August 2007; N=10) had the highest frequencies of occurrence for spot Leiostomus xanthurus, silver perch Bairdiella chrysoura, seatrout Cynoscion sp., and squid (all 40.0%). Bloom strandings (September 2007 to March 2008; N=14) had highest frequencies of occurrence for spot (50.0%), seatrout (42.86%), shrimp (35.7%), and pinfish Lagodon rhomboides (28.6%). Prior to the bloom, dolphins had higher abundances of Atlantic croaker Micropogonias undulatus (44.7%), silver perch (16.2%), and pigfish Orthopristis chrysoptera (14.0%). In contrast, bloom dolphins consumed a higher abundance of spot and pinfish (50.0% and 26.2%, respectively). Differences in prey species abundance were significant between bloom and non-bloom strandings (p > 0.001). Bloom strandings consumed a majority of spot, pinfish, and shrimp whereas non-bloom strandings consumed Atlantic croaker, silver perch, pigfish, and squid. Other explanations for these differences include the east to west spatial distribution and stranding locations either inside or outside the bays. Differences in diet may expose some bottlenose dolphins in the Northwest Florida Panhandle to higher levels of brevetoxins than others. This research highlights the importance of consistent sample collection in order to understand the effects of harmful algal blooms on marine mammals.