Atlantic cod stocks had once been so plentiful that early explorers joked about walking on the backs of the teeming fish. Today, cod stocks are at historically low levels and show no signs of imminent recovery, even after drastic conservation measures and severely limited fishing. Fishermen often blame the diminishing stocks on seals, which prey on cod and other species, but scientists believe that decades of overfishing are to blame. Studies on fish populations have shown that cod disappeared from Newfoundland at the same time that stocks started rebuilding in Norway, raising the possibility that the cod had migrated. Still no one can predict whether and when the cod will return to the Grand Banks.