Bringing Oysters Back to Town

"I suppose that when [Henry] Hudson sailed through the harbor, you could see right through to the bottom," says Mark Kurlansky, author of "The Big Oyster: History on the Half Shell," the tale of New York City's long relationship with the mollusks. Their absence, he says, is "a symbol of how badly we've cared for New York."

By the 1930s, oysters were deemed too dangerous to eat in New York. A few decades later, they were ecologically extinct from the city's waterways. Thanks to the Clean Water Act, conditions have improved a lot since the 1970s when, as Mr. Kurlansky recalls, the water was black "with this sort of mother-of-pearl purplish green thing on the surface." Says Cervino, "I'll go swimming in this." As an adolescent, he'd studiously avoided it. Any restoration effort faces some serious obstacles.

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"Once you mess around with nature – if you remove something from the food chain – that space isn't reserved for it to come back," says Mr. Kurlansky. "It's very difficult to reverse these things because the absence has had all sorts of repercussions in nature."

He points to cod's nonrecovery after lengthy closures of Georges Bank and Canada's Grand Banks to cod fishermen. "To have a drastic moratorium like that and have nothing come back is pretty scary," he says.

Oysters need hard surfaces to attach to and access to minerals to form their shells, for example. When reefs are abundant, the shells of previous oyster generations serve as both a hard substrate and mineral source. Reefs self-perpetuate. But when oysters disappear, their habitat goes as well. With oysters gone, the area becomes less oyster-friendly.

In relatively enclosed waterways like Chesapeake Bay, high nutrient runoff from fertilizer and livestock, combined with the loss of filter feeders has led to more extensive algal blooms. That leads to low-oxygen conditions that, in turn, suffocate what bottom-dwelling filter-feeders remain, further exacerbating the problem.

In the 1950s, Asian oysters brought to the Eastern Seaboard for aquaculture also carried two parasites that American oysters had little resistance to. Native oysters succumbed easily. The bacteria linked to cholera in humans also exacted a toll. The hurdles facing oyster restoration quickly began to look insurmountable. The abiding question: How can you tip the ecosystem back to a more oyster-friendly state?