The freshwater drum, Aplodinotus grunniens, is a fish endemic to North and Central America. It is the only species in the genus Aplodinotus. The freshwater drum is a member of the family Sciaenidae, and is the only North American member of the group that inhabits freshwater for its entire life. Its generic name, Aplodinotus, comes from Greek meaning "single back", and the specific epithet, grunniens, comes from a Latin word meaning "grunting". It is given to it because of the grunting noise that mature males make. This noise comes from a special set of muscles within the body cavity that vibrate against the swim bladder. The purpose of the grunting is unknown, but due to it being present in only mature males, it is assumed to be linked to spawning.
The drum typically weighs 5–15 lb (2.3–6.8 kg). The world record was caught on Nickajack Lake in Tennessee, and weighed in at 54 lb 8 oz (24.7 kg). The freshwater drum is gray or silvery in turbid waters and more bronze or brown colored in clearer waters. It is a deep bodied fish with a divided dorsal fin consisting of 10 spines and 29–32 rays. It is also called shepherd's pie, gray bass, Gasper goo, Gaspergou, gou, grunt, grunter, grinder, wuss fish, and croaker, and is commonly known as sheephead or sheepshead in parts of Canada, the United Kingdom, and the United States.
Freshwater drum are the only North American member of their family to exclusively inhabit freshwater (freshwater family members in genera Pachyurus and Plagioscion are from South America, while Boesemania is Asian). Their great distribution range goes as far north as the Hudson Bay, and reaches as far south as Guatemala. Their longitudinal distribution goes as far east as the eastern Appalachians and stretches as far west into Texas, Kansas, and Oklahoma. Freshwater drum are considered to be one of the most wide ranging species in North America.
The freshwater drum prefers clear water, but it is tolerant of turbid and murky water. They prefer the bottom to be clean sand and gravel substrates.
The diet of the freshwater drum is generally benthic and composed of macroinvertebrates (mainly aquatic insect larvae and bivalve mussels), as well as small fish in certain ecosystems. Freshwater drum show distinct seasonal differences in their diet. In April and May, the drum feeds on dipterans. During these months, dipterans make up about 50 percent of the freshwater drum's diet. In August through November, they tend to eat fish (which are primarily young-of-the-year gizzard shad). The percentage of fish in their diet at this time ranges from 52-94 percent. Other items in the drum's diet are mollusks and crayfish. Freshwater drum tend to hang out with walleye.
The freshwater drum competes with several organisms. During its early stages in Lake Erie, it has been shown to compete with yellow perch, the trout-perch, and the emerald shiner. During its adult lifetime, it competes with yellow perch and silver chub in deep water, and competes with black bass in the shoal areas.
Predators on drum include humans and other fish. During its first year, the freshwater drum serves as a forage fish for many species of predatory fish. These include smallmouth bass, walleye, and many other piscivores. After its first year, the primary predators on freshwater drum are humans. The drum is an important commercial crop on the Mississippi River, but in other areas it constitutes only a small portion of the commercial catch. Consistent with other Sciaenids, freshwater drum are strongly nocturnal with the bulk of most catches being derived from night angling/sampling. Commercial fisheries are present for this species, although market price tends to be quite low. Thus, many freshwater drum are harvested as bycatch from targeted higher-value species.
There has been some research on the freshwater drum's impact on the invasive Zebra mussel in northern lakes and rivers. Zebra mussels are consumed by freshwater drum once they reach a length of 25 cm (9.8 in), but drum under 35 cm (14 in) in length only eat small mussels and reject the larger ones. The fish larger than 35 cm (14 in) exhibit less selectivity and consume mussels relative to their availability in lakes. These larger fish are not restricted by their ability to crush the zebra mussels, but they are restricted by the size of the clumps that they can remove. Though the drum do eat zebra mussels, they are not having an impact on the spread of this invasive species. Though they do not control the population of zebra mussels, they do contribute to a high mortality in the zebra mussels.
During the summer, freshwater drum move into warm, shallow water that is less than 33 ft (10 m) deep. The freshwater drum then spawn during a six to seven-week period from June through July when the water reaches a temperature of about 65 °F (18 °C). During the spawn, females release their eggs into the water column and males release their sperm. Fertilization is random. Males generally reach sexual maturity at four years, whereas females reach maturity at five or six years. Females from six to nine years old have a clutch size of 34,000 to 66,500 eggs and they spawn in open water giving no parental care to their larvae. The eggs then float to the top of the water column and hatch between two and four days. Due to the broadcasting of eggs in open water and lack of parental care, many eggs and larvae fall victim to predation upon hatching, the pro-larvae average 3.2 mm (0.13 in) long. The post larval stage begins about 45 hours after hatching and a length of 4.4 mm (0.17 in) is attained.
Females grow at a faster rate than the males and adult characteristics start to form at a length of 15 mm (0.59 in). Females continue to outgrow the male throughout their lives reaching a length of 12 to 30 in (30 to 76 cm). Usually the freshwater drum weighs 2–10 lb (0.91–4.54 kg), but they can reach well over 36 lb (16 kg). Freshwater drum are long-lived and have attained maximum ages of 72 years old in Red Lakes, Minnesota and 32 years old in the Cahaba River, Alabama. Using sectioned otoliths from archaeological sites near Lake Winnebago, Wisconsin, freshwater drum have attained the age 74 years. Though they can reach a very old age, the average age of a freshwater drum is between 6 and 13 years.
There are not currently any management practices that could be found for the species. The freshwater drum is not federally or state listed by any states. In all of its range, the freshwater drum is abundant and in no danger of overharvest. Although the commercial harvest is up to 1 million pounds per year, they are in no danger of overharvest. In the Mississippi River alone, the commercial catch has reached about 300,000 lb (140,000 kg) in recent years. Due to its abundance, many states allow bowfishing and other non-conventional means to harvest the fish.
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- Minnesota Department of Natural Resources. Fishes of Minnesota: Freshwater Drum (Sheepshead). Minnesota Department of Natural Resources 2011. http://www.dnr.state.mn.us/fish/freshwaterdrum.html.