Silver Carp, Hypophthalmichthys molitrix
Image Caption: Juvenile silver carp. Credit: Mirko Barthel/Wikipedia (CC BY-SA 3.0)
The silver carp is a freshwater fish native to north and northeast Asia. It has been introduced or spread into at least 88 other countries around the world for reasons of aquaculture. It was imported into North America in the 1970’s to control algae growth at wastewater treatment facilities, soon after, the silver carp escaped into the wild and spread rapidly. By 2009 the population was abundant from the gulf to the Great Lakes.
The natural habitat for the silver carp are rivers where the water levels fluctuate, swimming just below the surface. The silver carp has also been introduced in lakes and ponds. Several countries have reported adverse ecological effects after the introduction of the species.
The body of the silver carp is a pale silvery greenish grey above and whitish below, with 1 – 3 Dorsal spines having 6 – 7 soft rays, and 1 – 3 anal spines having 10 – 14 soft rays. The lower body is scale-less from the pectoral area to the anal area. This species can grow to 3.5 feet in length and weigh 40 or more pounds.
It is sometimes called a flying carp, when startled in can leap 10 feet out of the water. Because of this it is considered dangerous to boats, jet skis, or water skiers near where this species inhabits. People have been injured by this fish leaping from the water and colliding with the person in the water craft, on skis, or on other devices used for water recreation activities; in one instance a teenager received a broken jaw while being pulled in an inner tube, and in 2003 a woman on a jet ski received a broken nose.
The silver carp is strictly a filter feeder capable of filtering food particles as small as 4 micrometers. Mucus is released from an organ that assists in trapping the small particles as water is forced through the filter by a pump within the mouth. The silver carp has no stomach and is thought to feed constantly on phytoplankton, zooplankton, and algae.
This species has been used for controlling water quality, especially with blue-green algae. However, certain types of this algae can pass directly through the silver carp and pick up nutrients while in the gut, and the algae bloom is made worse after being released from the silver carp. In turn toxins are stored within the flesh of the fish making it hazardous to eat.
The adult silver carp breed in rivers, tributaries, and flooded water ways in shallow rapids with a gravel or sandy bottom and having a high current flow. When water temperatures range between 65 – 78°F., and have high oxygen content, spawning begins as large schools are formed. The school will migrate long distances as the water level increases. If the conditions change, spawning will cease. After spawning the adult will migrate to foraging habitats, and in autumn, the adult will move into deep river depths.
Fertilization is done externally, the eggs are scattered and left unguarded. The larvae will drift downstream and settle in flooded lakes, and shallow shorelines, where there is little or no current.
Dams and restrictions on the rivers have prevented the natural migration of this species causing a reduction in spawning and adult population. Due to this, the fish has been listed as near threatened on the endangered species list.