Herring, chasing a different:

Herring, chasing a different: 1. Pursuing a fish of a different kind. 2. Aquatic analogy gone AWOL; loose grasp of piscatorial basics; extraordinary ability to distinguish between two different types of soft-finned teleost fish – a skill beyond most of us. (see AWOL, go; Bark into the wind, up the wrong tree; Fish up a tree, he looks like a; Sprat to catch a mackerel)