Evaluating the costs and benefits of intervening when dolphins face life threatening entanglements

Some of the most popular stories we tell are about the successful rescues we’ve undertaken to save wild dolphins with life-threatening injuries from entanglement in fishing gear or other types of debris. Social media posts about these heartwarming saves get thousands of ‘likes’ and ‘shares.’

But mounting such rescue efforts can be extremely challenging.

Rescues are expensive to undertake, they are often logistically difficult to organize and conduct, even with Southwest Florida’s strong network of trained responders stretching from Clearwater to Marco Island, and they can be dangerous for dolphins and people.

While we think it’s important to save each individual animal we can, as scientists we know that it’s also important to evaluate the costs and benefits of these rescues and to determine how they impact an overall dolphin population.

The ultimate goal for each intervention we undertake with approval from the National Marine Fisheries Service, which is tasked with protecting wild dolphins, is that we want the animals to remain in or return to wild populations to survive and reproduce. This goal also has two complementary objectives:

  1. To help individual dolphins survive and heal from their injuries;
  2. To allow animals that would have otherwise died, to remain as functioning members of local populations contributing to the community’s stability and survival in the face of many concurrent and cumulative threats.

So how do we determine success?

One key factor, of course, is by looking at how long an animal survives after an intervention. Previous studies — including those led by our program’s Director, Dr. Randy Wells — indicate that survival for six weeks after an intervention is a good benchmark for understanding the success for an individual cetacean. But what about the impact on a larger population?

Fortunately, the Chicago Zoological Society’s Sarasota Dolphin Research Program has spent more than 50 years monitoring the Sarasota Bay dolphin population and works closely with other nearby dolphin studies. This rich, long-term dataset allowed us to take a step back and look at dolphin interventions from a wider perspective — 35 years to be exact.

For the study “Staying Alive: Long-Term Success of Bottlenose Dolphin Interventions in Southwest Florida” published in the journal Frontiers in Marine Science, we evaluated the outcomes of 27 rescues conducted to save free-swimming dolphins from life-threatening entanglements that took place from 1985 through 2019 and whose cases included direct observations from follow-up monitoring beyond the initial six-week success criterion.

We found that:
  • Nearly all rescued individuals — 92 percent — survived longer than six weeks post-release, with 13 still observed frequently within their prior resident communities, in good physical health, and engaging in normal behavior.
  • Approximately 75% of rescued dolphins in our study survived over multiple years.
  • Survivorship rates did not decline substantially between one and five years post-rescue, meaning that survival beyond one year may be a useful benchmark of long-term success.
Importantly, all living animals remained in their local communities, and rescued females that reached reproductive maturity have gone on to produce offspring — that’s 12 post-intervention calves to date just in Sarasota Bay! Together these findings strongly support the idea that interventions to save individuals with life-threatening anthropogenic — human induced — injuries provide benefits not only to the welfare of those individuals, but also to the stability and growth potential of their local populations. In other words, success!

The study’s lead author is Katie McHugh, of the SDRP. SDRP Co-authors include Aaron Barleycorn, Jason Allen, Kim Bassos-Hull, Carolyn Cush, and Randall Wells. Additional co-authors: Gretchen Lovewell, Mote Marine Laboratory; Denise Boyd and Anna Panike, Florida Fish and Wildlife Conservation Commission; Blair Mase, National Marine Fisheries Service; Robert Lacy, Chicago Zoological Society; Michelle Greenfield, Cornell University Daniel Rubenstein, Princeton University; Ann Weaver, Good-natured Statistics Consulting, ; Abby Stone and Lisa Oliver, Clearwater Marine Aquarium; Kent Morse, The Dolphin Study.

  • Read the full study https://doi.org/10.3389/fmars.2020.624729
  • Citation: McHugh KA, Barleycorn AA, Allen JB, Bassos-Hull K, Lovewell G, Boyd D, Panike A, Cush C, Fauquier D, Mase B, Lacy RC, Greenfield MR, Rubenstein DI, Weaver A, Stone A, Oliver L, Morse K and Wells RS (2021) Staying Alive: Long-Term Success of Bottlenose Dolphin Interventions in Southwest Florida. Front. Mar. Sci. 7:624729. doi: 10.3389/fmars.2020.624729

This work was funded by a series of grants from the Disney Conservation Fund and NOAA’s John H. Prescott Marine Mammal Rescue Assistance Grant Program, as well as support from the Charles and Margery Barancik Foundation, the Batchelor Foundation, the National Fish and Wildlife Foundation, Save the Manatee Trust Fund, and institutional support for the members of the stranding network involved in the interventions, rehabilitation, and post-release follow-up monitoring, including the Chicago Zoological Society, Mote Marine Laboratory, Florida Fish and Wildlife Conservation Commission, Clearwater Marine Aquarium, NOAA’s National Marine Fisheries Service, and the University of Florida College of Veterinary Medicine. The Vortex population modeling software is made available by the partners of the Species Conservation Toolkit Initiative (scti.tools).

In 2016, we received reports of an entangled dolphin off Nokomis Beach. We found then 10-year-old Bill entangled in a crab trap line and tethered in place with his blowhole barely above water. With our intervention, he survived and we continued to see him through August of last year.

This picture shows four long-term resident mothers and their calves in 2017. Over the decades, we had assisted three of the four moms. Without our intervention these moms’ contributions to the Sarasota Bay dolphin community would have been lost.