The New England Aquarium announced Wednesday that president and CEO Bud Ris is stepping down and will be succeeded by Nigella Hillgarth, the longtime executive director of UC San Diego’s Birch Aquarium at Scripps, part of the Scripps Institution of Oceanography. Ris, who’s been with the Aquarium since 2005, is credited with overseeing dramatic improvements to the physical plant and also helping to boost the institution’s financial health. Together with Aquarium COO Walter Flaherty and the former president of the board of trustees Bill Burgess, Ris was instrumental in raising $43 million over a six-year period that saw the creation of several new exhibits, including the redesign of the Aquarium’s Giant Ocean Tank, the opening of the shark and ray touch tank, the construction of a marine mammal center, and a multimillion-dollar sea turtle hospital and quarantine/holding space, located at the former Quincy Shipyard. Ris, who in the 1980s was a leader of the Union of Concerned Scientists in Cambridge, is also well known as an expert on the issue of climate change, consulting with everyone from former Mayor Tom Menino to former British Prime Minister Tony Blair. The Oxford-educated Hillgarth, meanwhile, has been at the San Diego aquarium since 2002 and before that headed up the Tracy Aviary in Salt Lake City, the largest bird park in the US. A native of Ireland, Hillgarth has conducted research on penguins and other birds, traveling to the Arctic, Antarctica, Argentina, the Amazon, and the Galapagos, among other exotic locales. Hillgarth said her interest in research drew her to New England. “It has such a strong conservation and research arm,” she said, of the aquarium. “I think understanding that will be my first priority.” Hillgarth said she is not put off by our weather, despite her years in California. Ris said that one of the first days that Hillgarth came to interview, the city was covered in snow and it was 5 degrees. “She still took the job,” he said.