Fish
Freshwater Drum
Aplodinotus grunniens · also called sheepshead, gaspergou, croaker
The drum and croaker family is a saltwater group, built around fish that grunt and boom on the sea floor. One species broke the rule. The freshwater drum lives entirely in rivers and lakes, still makes the noise, and leaves behind an ear bone that people have carried as a charm for thousands of years.
Overview
Aplodinotus grunniens is the only member of the croaker family that lives entirely in fresh water, and it ranges further than any other North American freshwater fish — from Hudson Bay to Guatemala — over sand and mud in big rivers and lakes. It is a bottom feeder, shifting with age from insects and crayfish to snails and mussels, including invasive zebra mussels.
The drum, and the crushing teeth
Both sexes, especially spring males, grunt by vibrating muscles against the swim bladder. To eat shellfish it has large flat crushing teeth in the throat, strong enough to grind mussel shells.
The lucky stone
Its inner ear holds a large, hard, ivory-white ear bone marked with an L-shaped groove. These have been used as good-luck charms, beads, and trade items for millennia and still wash up on Great Lakes beaches.
Status
Least Concern and abundant across a huge range, tolerant of muddy water, and only lightly fished thanks to its reputation as a coarse table fish.
Freshwater Drum: common questions
Why is the freshwater drum called a sheepshead if there is already a marine sheepshead?
The name sheepshead has been applied to several unrelated fish because of a blunt, sheep-like profile and prominent teeth. On the Atlantic and Gulf coasts, sheepshead usually means Archosargus probatocephalus, a porgy with human-looking front teeth. In the Great Lakes and the Mississippi valley, sheepshead means the freshwater drum. They are not related; the drum is in the croaker family and the marine sheepshead is in the porgy family. Regional names for the freshwater drum also include gaspergou, from a French phrase, and croaker, grunter, and grinder, all referring to the sound it makes.
How does a fish make a drumming sound?
The freshwater drum has a pair of specialised muscles attached to the body wall near the swim bladder. When the fish contracts and releases these muscles very rapidly, they vibrate the taut wall of the gas-filled swim bladder, which resonates like the head of a drum and broadcasts a low grunting, rattling, or purring sound into the water. Males call most in the spring spawning season, apparently to attract females and coordinate spawning, but both sexes can produce the sound year-round. It is loud enough that people in boats, and sometimes on the bank, hear it clearly, which is how the fish and its whole family got their name.
What is a lucky stone?
It is one of the ear bones of the freshwater drum. All bony fish have otoliths, dense structures in the inner ear used for balance and hearing, but the drum's are exceptionally large, hard, smooth, and ivory-coloured, roughly the size and shape of a large bean, with a distinctive shiny L or J-shaped mark on one face. People have collected these for millennia; they turn up in Native American graves and trade caches far from any river, have been used as beads, gaming pieces, and good-luck tokens, and beachcombers on the Great Lakes still hunt for them. Each drum has two.
Are freshwater drum good to eat, and are they good for controlling zebra mussels?
The flesh is white, firm, and mild, and it is perfectly good eating when the fish is fresh, well handled, and not too large, though it has a poor reputation partly because bigger drum can be coarse and partly from snobbery about a fish that is easy to catch. On the ecological side, the freshwater drum is one of the few native North American fish that regularly eats zebra and quagga mussels, and in some lakes drum have been found with hundreds of crushed mussel shells in their stomachs. They do not control an invasion on their own, but they are a genuine native check on it.