BOSTON (AP) — The chief executive of the New England Aquarium is stepping down after overseeing the completion of the facility’s new central tank and the expansion of critical ocean research and conservation initiatives.

Bud Ris (riss) announced Wednesday he will step down in May after nine years on the job and will be replaced by Nigella Hillgarth, the executive director of UC San Diego’s Birch Aquarium at Scripps, part of the Scripps Institution of Oceanography.

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The Boston aquarium is the second most visited attraction in New England with more than 1.3 million visitors per year.

During Ris’ tenure, he helped raise $43 million to rebuild infrastructure, enhance mission programs and developed new exhibits including the renovation of the 26-foot deep, 200,000-gallon Giant Ocean Tank, which holds more than 1,000 sea creatures.