Biomanipulation is the deliberate alteration of anecosystem by adding or removing species, especially predators.
Changing the fish population of bodies of water as a part ofwatershed management can facilitate desirable changes inaquatic ecosystems suffering fromeutrophication characterized by domination byphytoplankton aidingecosystem restoration, an application ofrestoration ecology. In ponds or lakes alternative stable conditions, one with high algae populations, little other plant life, and turbid water, another with low algae populations, a diverse plant population, and clear water, may exist. In addition to prevention of excess nutrients such as phosphorus and nitrates, removal of certain fish species adapted to turbid water may facilitate change from one steady state to the other, an application ofdynamical systems theory.[1] Fish species may be removed by means of poison, harvesting, or introduction of predatory species. As a different fish community will result from the process it will affect recreational and commercial fishermen whose cooperation or opposition is important.[2]
Lake Zwemlust, a hypertrophic pond used as a swimming pool in The Netherlands with an area of 1.5 hectares and an average depth of 1.5 meters, was treated in March 1987. The initialSecchi disk transparency was only 0.3 meters, less than the 1 meter maximum permitted for swimming pools in The Netherlands. In the first summer Secchi disk transparency increased to at least 2.5 meters, the maximum depth of the lake.[3] The lake was drained and 1,500 kilograms of planktivorous and benthivorous fish such ascommon bream were removed byseining andelectrofishing. The pond was stocked with 1500northern pikefingerlings and some maturerudd whose offspring served as food for the pike.Willow branches,nuphar lutea roots, and starts ofChara globularis were added as vegetation and shelter.[4] Expenses were met by the local water authority which was compensated by increased patronage by swimmers.[2] In the summer of the second year, 1988, there was considerable plant growth and, possibly due to lack of predation bycarp or minnows, an explosion in the number ofsnails, includingRadix peregra var. ovata a host ofTrichobilharzia ocellata the cause of schistosome dermatitis,swimmer's itch.
In addition to grazing byzooplankton the lush growth ofmacrophytes removed sufficient nutrients from the water to preventalgal bloom byphytoplanktons.[3]
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