Freshwater Drum, Aplodinotus grunniens
The Freshwater Drum, Aplodinotus grunniens, whose common Spanish name is roncador de agua dulce, is a member of the Croaker or Sciaenidae Family, known collectively as berrugatas and corvinas in Mexico. This fish is also known as the Sheephead. It is the only global member in the genus Aplodinotus and is found throughout the freshwater systems of mainland Mexico.
The Freshwater Drums are large fish with laterally compressed deep bodies and high arched backs. Larger fish have a significantly greater body depth anterior of the first dorsal fin. They vary in color being gray or silvery in turbid waters and bronze-brown in clearer waters. Their anal fin has a posterior straight margin and two spines, the second spine being significantly larger than the first, and six or seven rays. Their caudal fin is rounded or triangularly-shaped. Their dorsal fin is deeply notched with nine or ten spines and 28 to 31 rays. Their pectoral fins have 15 or 16 rays and their pelvic fins have one spine and five rays. Their head has a rounded snout and a subterminal blunt mouth equipped with numerous small sharp teeth on each jaw and small round teeth on the pharynx used for crushing and grinding prey. Their lateral line is prominent and continuous into the caudal fin. They are covered with scales.
The Freshwater Drums inhabit freshwater their entire life, making them the only North American member of the Sciaenidae Family to do so. They are normally found demersal in clear water over sand and gravel substrates but can also tolerate turbid and murky waters. They are found in deep pools in medium to large rivers and at varying depths up to 60 feet in large lakes. Females grow faster and become larger than males. They reach a maximum length of 1.22 meters (4 feet) and weight of 27 kg (60 pounds). They are known to congregate in large schools for feeding and breeding. They are nocturnal predators consuming aquatic insect larvae, bivalve mussels, crayfish, and small fish. They are believed to help control minimally the invasive Zebra Mussel in northern lakes and rivers. In the northern part of their range they associate with the Walleye, Sander vitreus. Juveniles are preyed upon by numerous predatory fish including Smallmouth Bass, Walleye, and birds. They migrate to warmer shallower waters for spawning during the summer as they require water temperatures in excess of 18oC (65oF) for this activity. Reproduction is oviparous with females broadcasting 34,000 to 67,000 eggs into the water column which float to the surface and are then fertilized randomly by males. Eggs and larvae are pelagic without parental care and subject to significant predation. Mature males emit sounds which are believed to be linked to the spawning process. They are long-lived and have a lifespan of up to 74 years based on extensive otolith measurements.
The Freshwater Drums are one of the most wide-ranging species in North America being found from the Hudson Bay in the north to Guatemala in the south. In Mexican waters they are found in all freshwater systems that drain into the Atlantic Ocean.
The Freshwater Drum is straightforward to identify and not easily confused with any other species due to its two anal spines, the second of which is large, and its prominent lateral line that extends into the caudal fin base.
The Freshwater Drums are important to the commercial fishing from the Mississippi River with the majority being taken as a by-catch of targeted higher value fisheries. They are harvested at a level of 1 million pounds per year in North America and considered a popular meat in some areas. From a conservation perspective they are considered of Least Concern being abundant with stable populations throughout a vast range. Current adult population levels are estimated to exceed 100,000 individuals. At present they are without any form of protection. They are of modest interest to traditional recreational anglers and are known for their great fighting ability and large size. Due to their abundance they are also a target of bowfishermen. They are also used on a limited basis as live bait for other fish. They have exceptionally large inner ear bones or otoliths which are called “lucky stones” and collected for good luck. The otoliths have been found in old Indian settlements and used in trade and as jewelry.
Freshwater Drum, Aplodinotus grunniens. Fish caught from the Gatineau River, Gatineau, Quebec, Canada, June 2014. Length: 33 cm (13 inches). Catch, photo, and identification courtesy of Kenneth Tse, Toronto, Canada.
Freshwater Drum, Aplodinotus grunniens. Fish caught from coastal Canadian waters of Lake Ontario, near Toronto, June 2014. Weight: 4.5 kg (10 pounds). Catch, photo, and identification courtesy of Kenneth Tse, Toronto, Canada.