Off the coast of California near Santa Cruz earlier this year, a small team of scientists aboard a research vessel witnessed a group of bottlenose dolphins attacking a small harbor porpoise near the beach of Pajaro Dunes. They could be seen drowning it, chasing it, beating and ramming into it.
It was on this trip on Sept. 16 that Dr. Daniela Maldini, the chief scientist at Okeanis, a small non-profit organization dedicated to research and understanding of marine life, witnessed the alarming behavior. Despite the morbidity of the situation, Maldini was excited to witness one of the rare attacks.
“There were clues that this was going on, but nobody had ever actually seen a bottlenose dolphin doing it,” Maldini said. “Now we have a confirmation of that.”
Since 2004, residents have noticed harbor porpoises being swept ashore with various injuries including broken ribs and spines, and with bruise marks on the bodies. Bottlenose dolphins had long been suspected of being responsible for the attacks.
According to researchers from UC Santa Cruz’s Long Marine Laboratory, in the last year alone 74 porpoises were washed ashore dead.
“We’ve been studying the bottlenose dolphins since 1990 so we know the population very well, and the animals have been around for many years,” Maldini said. “I think that it’s a natural phenomenon.”
Scientists are trying to pinpoint the biological reasons for the dolphins’ odd behavior. Okeanis looks at the social structure of bottlenose dolphins, sex ratios, survival rates of calves and the contaminant levels in the blubber of the animals, including mercury. By exploring all these factors, Maldini hopes to narrow the possibilities down.
“At this point, I’m not sure what level of concern we should have for the harbor porpoises because we do not know what the impact on their population has been,” Maldini said. “Aggression between species has been reported before, it’s just never been seen before in California, so certainly there may be environmental stressors that we have to consider.”
Moss Landing Marine Mammal Center is a local non-profit organization dedicates itself to saving the porpoises from being killed off the coast. While Okeanis’ main goal is research, Moss Landing Marine Mammal Center takes a more hands-on approach.
“When an animal comes in still alive, we are the primary responders. We put it into a pool and try to rehabilitate it,” said Susan Lambrecht, a researcher at the center.
On the same day that Okeanis researchers witnessed the attack, Moss Landing Marine Mammal Center launched a rescue mission for a harbor porpoise being harassed by the same pod of bottlenose dolphins. Lambrecht’s team was the first to respond.
“We picked up the porpoise, but it died on the way to the rehabilitation pool,” Lambrecht said. “We tried our best to let it live.”
The porpoise was turned over to the Long Marine Laboratory to be used as further evidence.
This and other pieces of evidence are being compiled and will be well into the future. Researchers are eager to find answers but also remain focused on the long term results.
“We’ve been out there for the past 20 years and we will continue to be out there for the next 20 years,” Maldini said. “We don’t plan on quitting anytime soon.”
