Brookfield, Ill. – Wild dolphins are known for their complex social lives, but new research shows those social networks can be influenced by human activity.
A new study published in Animal Behaviour found that bottlenose dolphins in Sarasota Bay, Florida, that engage in risky human-centric foraging behaviors, such as taking bait or catch from fishing gear, scavenging discarded bait or fish, or approaching humans for food tend to associate more with other dolphins that use similar tactics. The study also found that severe harmful algal blooms, commonly known as red tides, altered the relationship between these foraging behaviors and dolphin social structure.
The research was led by Kyra Bankhead of Oregon State University, with co-authors Mauricio Cantor of Oregon State University, Katherine McHugh and Randall Wells of the Sarasota Dolphin Research Program, which is operated by Brookfield Zoo Chicago.
"Dolphins are highly social animals," said Kyra Bankhead, lead author. "This study shows that human activities can do more than change where dolphins feed or how they find food. They can influence the social fabric of a dolphin community, especially when that community is already experiencing environmental stress."
Using more than 20 years of observational data from Sarasota Bay's long-term resident bottlenose dolphin community, researchers examined how dolphins' social networks changed before, during and after a period of intense red tide events. The Brookfield Zoo Chicago's Sarasota Dolphin Research Program has studied this dolphin community for more than 56 years, making it the longest-running study of a wild dolphin population in the world.
In Sarasota Bay, some dolphins have learned to take advantage of food sources associated with people. These behaviors may appear to offer an easy meal, but they come with serious risks to dolphins, including boat strikes, entanglement in, hooking by, or ingestion of fishing gear, injury, illegal feeding, reduced reproductive success and even death.
"From a human perspective, feeding or interacting with a dolphin may seem harmless or even helpful," said Katherine McHugh, deputy director, Sarasota Dolphin Research Program, Brookfield Zoo Chicago. "But for dolphins, these interactions can have lasting consequences. They can change behavior, increase injury risk and, as this study suggests, potentially affect how dolphins associate with one another."
This study focused on a subset of Sarasota Bay dolphins observed consistently from 1995 through 2012. Researchers divided the data into three six-year periods: before, during and after intense harmful algal blooms that affected the bay in the early 2000s. These blooms depleted many of the fish species dolphins rely on, forcing dolphins to search more widely for food and, in some cases, increasing opportunities for interactions with humans as they vied for the same few remaining fish.
Researchers found that dolphins with similar levels of human-centric foraging were more likely to associate with one another. That pattern remained even after accounting for other factors known to influence dolphin associations, including sex, age and home range overlap. The study also found that the proportion of dolphins engaging in human-centric foraging increased over time, from 12 percent before the intense red tide period to 22 percent during it and 41 percent afterward.
However, during and after the red tide period, the relationship between human-centric foraging and social association weakened. Researchers suggest that during periods of prey scarcity, dolphins may have been driven to aggregate around remaining natural prey resources, temporarily reducing the role that shared human-centric foraging tactics played in shaping associations. Afterward, the continued increase in human-centric behaviors may indicate that these tactics spread more broadly through the social network.
"This gives us a more nuanced picture of how environmental change and human influence can interact," said McHugh. "It is not simply that dolphins respond to people or respond to red tide. These pressures can overlap, and together they may shape how animals forage, learn and maintain social relationships."
The findings are especially important as harmful algal blooms and other environmental disruptions are expected to become more frequent or severe in many coastal ecosystems. For long-lived, highly social animals like dolphins, changes in social structure can have implications for learning, survival and resilience.
The research also underscores the importance of public education and responsible behavior around wild dolphins. In the United States, feeding or attempting to feed wild marine mammals is illegal. Boaters and anglers can help protect dolphins by never feeding them, never discarding bait or catch near dolphins, reeling in lines when dolphins are nearby, and waiting until dolphins leave or by moving away before casting again.
"This study reinforces a message we have shared for decades: the best way to help wild dolphins is to let them remain wild," said Wells. "Long-term research allows us to understand not only the immediate dangers of human interaction, but also the deeper, more complex ways those interactions may affect dolphin communities over time."
