On The Origin Of The Term "Killifish"
According to the Oxford English Dictionary, killifish is a composite word "commonly supposed" to have been created by the suture of "kill," the Dutch word for stream, and “fish.”
As a boy growing up in New York City, killifish meant only one thing, a bait fish, the mummichog, and a word from which I was protected until I reached my majority– a fish, common though it may be, with a tortuous nomenclatural career. That fish is either Fundulus heteroclitus heteroclitus or F. h. macrolepidotus, for both are found in the waters of Long Island. The former never forms freshwater colonies, the latter does. Only recently, a freshwater population was discovered in the Bronx River, Samaritan & Schmidt (1982). Fundulus heteroclitus macrolepidotus is the fish that gave killifishes their name.
The earliest reference to the term killifish I have found is Schoepf (1788) spelled "killfish.” Schoepf's "killfish" was specifically applied to what we refer to today as F. h. macrolepidotus. In 1792 Walbaum described Cobitis killifish (= Fundulus h. macrolepidotus). By 1815, killifish appears as a common name in Mitchill's "The fishes of New York."
But why does our etymological resource, the Oxford English Dictionary, hedge a little when it tells us the "commonly supposed" origin of the term killifish? Well, as it turns out, there is another theory for its origin, one no more or no less plausible, related to the use of killifishes as bait fish.
According to the Oxford English Dictionary, based on documented usage from 1681 until 1880, a very effective bait was referred to as "killing," as in As killing a Bait as any whatever, and Fishing with the young frog is a very killing method of fishing for chub. The transition from "killing bait" to "killing fish" to "killfish" to "killifish" is believable.
Geographically, the term "killifish" arose in the New York City area where killifishes are not stream fish. In the south, Fundulus heteroclitus heteroclitus was called the "mudfish." The term "kill" was not always used with great precision, however. The water between Staten Island and New Jersey, known as the Arthur Kill, is not a stream, while Great Kills Harbor opens only into the Atlantic Ocean and these are areas where Fundulus heteroclitus macrolepidotus would abound.
The often complicated nomenclatural history of the two subspecies of Fundulus heteroclitus may be found under their respective entries. Recognition of the subspecies macrolepidotus has been checkered until recently.
To some, the bait fish theory is bound to be a more interesting explanation, if only because it is so different, so unexpected an alternative. Either way, those who have used the New York saltwater killifish, Fundulus heteroclitus macrolepidotus, to catch blue claw crabs or fluke or bluefish know it to be a killing bait indeed.