City dolphins get a boost from better protection and cleaner waters

There are many places in the world where you can see bottlenose dolphins, but the dolphins swimming in the Port River estuary near Adelaide, Australia, are special. They gambol about in waters surrounded by factories, power stations and other signs of human habitation.

For much of the 20th century, there were no dolphin sightings in the inner estuary. Prior to European settlement in 1858, bottlenose dolphins were commonly seen by the local Kaurna aboriginal tribal group. But as the city of Adelaide was built, the dolphins disappeared. What changed that enabled their return? A combination of improved environmental conditions, a little bit of protection and some public education, researchers report October 24 in Marine Mammal Science.

“The future of these dolphins would appear to be as secure as any population of any species can be in this era of climate change,” says the study’s lead author, Mike Bossley of Whale and Dolphin Conservation Australasia in Port Adelaide, who has studied the area’s dolphins for 25 years.

As the city of Adelaide grew, the Port River grew to be an unfriendly spot for marine wildlife. People cleared away the marshes and mangroves, replacing them with sulfuric acid and soda ash producers, sugar refineries and power stations. Sewage and storm water flowed into the river. Boats and ships traversed the estuary, which had become the main shipping port for the state of South Australia. And no one reported a dolphin sighting between 1940 and 1980.

Scientists began field studies in 1989 and started finding bottlenose dolphins — and documenting threats to them. In addition to pollution, any dolphins brave enough to traverse the Port River had to deal with boat strikes, infections, entanglement with nets and other marine trash and even deliberate attacks. In response, the Adelaide Dolphin Sanctuary was established by law in 2005, setting aside a small patch of river for the resident dolphin population (about 30 live in the river; another 300 visit the area regularly) and establishing resources for public education about the dolphins. And over the last few decades, water quality has improved as some of the least environmentally friendly activities — such as sulfuric acid production, salt evaporation and coal-fired power production — have ended.

In 1990, Bossley and his colleagues started surveying the Port River dolphins. The researchers would take their boat out on a 40-kilometer journey through the estuary, following the same path each time and photographing any dolphins they saw. They used distinguishing marks, such as shape or notches, on a dolphin’s dorsal fin to identify the animal and ensure each was only counted once.

From January 1990 to December 2013, the researchers made a total of 735 complete journeys, averaging one survey every 11.7 days. Based on those surveys, and the near absence of dolphins in the 1980s, the team estimates that bottlenose dolphin sightings are increasing by about 6 percent a year.

“The trends in sightings provide compelling evidence of a large change in some aspect of relative abundance and occupancy and usage of the Port River estuary,” the researchers write. The increase could be the result of an increase in the resident population, in the number of visiting dolphins or a combination of both.

Improved water quality may be better for the dolphins or their prey. Plus, the establishment of the sanctuary gave the dolphins some protection against human activities. In addition, Bossley notes, “by designating the area as a sanctuary, the public is both more aware and more protective of the dolphins.” The dolphins still have to deal with human impacts — the river is not pollution free, an attack occurred as recently as 2014 and there’s a growing new potential threat in the form of tourism — but the dolphins appear to be able to cope.

The Adelaide dolphins offer a lesson in conservation, Bossley and his colleagues note. Corralling off large areas for wildlife from human activities isn’t always necessary. “People have to learn to live with wildlife,” Bossley says. And that “requires taking into account both the wildlife itself and its habitats.”