What is "mackerel guide"

Garfish \Gar"fish`\, n. [See Gar, n.] (Zo["o]l.)

1. A European marine fish ( Belone vulgaris); -- called also gar, gerrick, greenback, greenbone, gorebill, hornfish, longnose, mackerel guide, sea needle, and sea pike.

2. One of several species of similar fishes of the genus Tylosurus, of which one species ( T. marinus) is common on the Atlantic coast. T. Caribb[ae]us, a very large species, and T. crassus, are more southern; -- called also needlefish. Many of the common names of the European garfish are also applied to the American species.