Millions of pilchards--five-inch-long fish related to sardines-- died off the coast of New Zealand and southern Australia this past year, apparently of a viral infection. On some beaches, piles of rotting fish stretched for 60 miles, and long streaks of dead pilchards could be seen at sea as well. It was the biggest fish kill ever recorded in Australia and perhaps in the world. Fortunately for the Australian fishing industry, which uses pilchards primarily as bait, this die-off--amounting to an estimated 10 percent of the pilchard stock--is not expected to have a serious economic impact. But the sheer numbers of dead fish are reason enough for concern. Tests showed that a new type of herpes virus was probably the culprit; the virus destroys cells lining the gills and suffocates the fish. The biggest mystery about the virus--aside from where it came from--is how it spread among the pilchards. It seems that somehow the virus can actually spread against ocean currents and can travel faster than pilchards can swim, says Michael Hine, a marine pathologist at the New Zealand National Institute for Water and Atmospheric Research. Why did the epidemic hit now? So far, says Hine, no one’s come up with a plausible explanation.