Here’s the Japanese news post by the Sumida Aquarium, which includes the Email addresses to FaceTime the eels: https://www.sumida-aquarium.com/news/details/2236/ A shame that The Guardian’s article did not include a backlink to it.
It's absurdly common with news articles. They never link the stats/paper/organisation. I often wonder about the amount of collective time spent by individuals simply trying to find the data summarised in an article. For all the lamenting about the media's decline there's plenty of amateur blogs out there that manage to link their sources when writing an article.
It might just be a policy to not add links except if it's to articles on the same site, which is obviously stupid from the user's point of view.