Bletchley Park called the entire family of German teleprinter cipher traffic “fish.” The Lorenz SZ40/42 machine’s traffic specifically became “Tunny” (tunafish); the rival Siemens T52 was “Sturgeon.” Every individual radio link got its own fish name.

The dates are operational visibility into when the fish were biting. In this case Rommel was dispatching orders. Perch is the notable one, as it was among the earliest and most-read Eastern Front links, and the kind of traffic that justified building Colossus to attack the wheel settings at speed. Each fish gave the British a window into a different slice of the German high command’s communications.