By Bill Kearney, South Florida Sun-Sentinel
Recent research conducted by Florida International University reveals the remarkable adaptability of South Florida’s bottlenose dolphins in the absence of seagrass, a vital component of their food source and habitat.
During the study, which is yet to be published, scientists compared the dietary habits of two distinct dolphin populations: one in southern Biscayne Bay with access to seagrass meadows, and the other in the more urbanized Northern Biscayne Bay, which has suffered significant seagrass die-offs since 2011.
The dolphins with access to abundant seagrass meadows primarily consumed bottom-dwelling sea creatures and organisms lower in the food chain. In contrast, the dolphins in Northern Biscayne Bay, where seagrass had disappeared, had to adapt by feeding on larger fish higher up the food chain, which are less abundant. The impact of the seagrass loss on the dolphin population size is not yet clear.
However, the dolphins in Northern Biscayne Bay have found alternative strategies to overcome the loss of seagrass. Lead researcher Jessica Carde explains that these marine mammals are highly social and often work together to herd schools of larger fish, trapping them against seawalls where they are easier to catch. Additionally, the dolphins rely on the seasonal migrations of mullet for sustenance.
The researchers used chemical analysis of the dolphins’ blubber and skin to determine the source of their food and its position in the food chain.
Carde points out that the northern and southern populations of dolphins are distinct. The southern dolphins range from Key Largo to Elliott Key, including Biscayne National Park, while the northern dolphins inhabit the urbanized North Bay between Haulover Inlet and Key Biscayne.
In the past, the northern dolphins had access to extensive seagrass meadows on both sides of the Tuttle Causeway, connecting Miami Beach to the mainland. However, these meadows began to deteriorate in 2011, resulting in a 90% reduction in seagrass presence. Consequently, the bay bottom in their territory transformed into mud, gravel, and algae, which typically support less marine life.
Carde suggests that the difference in diet between the two dolphin populations is related to the seagrass decline. While she lacks data on the northern dolphins’ pre-2011 diet when seagrass was abundant, similar studies in Sarasota Bay demonstrated a similar pattern of dietary change when seagrass disappeared.
Notably, many of the dolphins in the North Bay are old enough to have experienced the bay’s transformation. One member of the population is at least 36 years old. Carde speculates that the dolphins communicate and share knowledge with each other, aiding their ability to adapt and survive in a changing environment.
The research also suggests that the dolphins’ social behavior has intensified in recent years, potentially as a response to declining habitat quality. Carde believes that larger social groups possess more collective knowledge, facilitating resilience and adaptation.
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