It sounds like something out of a dystopian novel: The fresh fish in many of Beijing's biggest supermarkets simply disappeared late last month, as if summoned to another realm. Social media buzzed with alarm and paranoia. The Beijing News placed a photo of an empty aquarium and an underemployed fishmonger on its front page.

The truth turned out to be something earthier and more familiar. According to state media, someone had tipped off the supermarkets about an impending health inspection. Rather than risk a problem, the stores — including Wal-Mart and Carrefour outlets — unloaded their fish. So far, at least, nobody seems to know where they went.

The incident suggests a few underlying problems. One is that a huge amount of food production in China remains small-scale and untraceable. A 2006 government census found more than 200 million individual agricultural holdings. Some consolidation has taken place since, but not nearly enough. According to state media, the vast majority of Beijing's freshwater fish supply comes from thousands of independently owned ponds in neighboring provinces, some within an hour or two of the city. During a recent road trip through rural Guangdong province, I saw small fish farms located willy-nilly throughout the countryside, sometimes next to factories.