Seahorses lead the charge in a teeming Thames

Rare birds and sea creatures are turning up in record numbers, writes Ellen Himelfarb

Two weeks ago, as I read to my daughter from a picture book about a nubbly seahorse, I wondered: Why must children's books abound with creatures their audience is unlikely ever to see? The very next day, I had to eat my words when news broke that a small colony of short-snouted seahorses (Hippocampus hippocampus) was living just a few kilometres away in the sludgy Thames estuary. The Zoological Society of London had kept schtum about the discovery for 18 months, while it lobbied to have the species protected under the Wildlife and Countryside Act. Finally, this month, the act was modified to include the spiny seahorses, along with the water vole, angel shark and Roman snail.
Time was London's waterways were about as savoury as a Victorian privy. Now, the same river that 50 years ago was declared biologically dead - depleted of oxygen and reeking of sulphur - is clean enough to house some of our most threatened storybook characters.
The rehabilitation of the Thames began in the 1960s, when London started investing in water treatment and ended the dumping of raw effluent into the water. Today, with tighter regulations on riverside industry, water quality has pretty much plateaued, so the sudden appearance of seahorses cannot be readily explained.
"We're not quite sure why they're here," says Neil Dunlop of the Environmental Agency, "but there hasn't been a massive improvement [in the Thames] since the seventies and eighties."
He attributes the seahorse sighting to improved monitoring rather than filtering.
Nevertheless, the return of river life has been increasingly evident. In 2004, a piranha and a seal were found within days of each other near the Thames shore. Soon after, there were sightings of oysters, Dover sole, cockles and salmon, searching for a stream in which to spawn.
Even more exciting, in 2006, was the appearance of an 11-year-old northern bottlenose whale that had lost its way in the frigid Thames. Rather than join its pod in the Atlantic, it ended up skirting the North Sea, finding the river's mouth and visiting Chelsea instead. It died shortly after a rescue crew tried to lead it back out - it was not the Thames that killed her, but dehydration, likely caused by the lack of suitable food.
"The whale came down the wrong side of Britain and followed her natural instinct to go back to the west coast," says Alison Shaw, an environmentalist at the Zoological Society. "If it [the Thames] was really revolting, she might have turned up her nose and swum away."
Londoners are clearly enjoying the river's bounty. On a recent trip along London's Regent's Canal, I happened by two Sunday fishermen hauling out of the murky water a bass that must have weighed 16 pounds or more. The way they were congratulating each other, and the way the masses soon gathered, it seemed unlikely they had ever experienced a catch that impressive. Nowadays, fishermen are five to a yard along the canal, fishing for bass as well as perch, carp, bream, chub and pike - though it's not just fish that have returned.
Along the New River, a waterway constructed in North London during the 17th century, it has become commonplace to spot herons or egrets - a sight equivalent to spotting a flamingo in New York's Washington Square Park. And no fewer than three esteemed London restaurants source their fish - including sea bass and salmon - from within the city's ring road, the M25.
I recently took my daughter to the London Aquarium, and we saw the seahorses on display there. She loved it. But with more than 100 species - from porpoises and dolphins to otters - now inhabiting the nearby Thames, it occurred to me: We could probably now find a greater level of biodiversity in the wilds of Central London than at the city's aquarium - which is exactly as it should be.