用鲢鳙直接控制微囊藻水华的围隔试验和湖泊实践

Direct control of microcystis bloom through the use of Planktivorous Carp-closure Experiments and Lake Fishery Practice

为了控制水体富营养化所形成的水华,国外经典的生物操纵论者提倡放养食鱼性鱼类以控制食浮游生物的鱼类,藉此壮大浮游动物种群,然后借助浮游动物遏制藻类。按照这条思路,以浮游生物为食的鲢和鳙应该是清除的对象。本文作者认为我国湖泊中危害性最大的是微囊藻水华,而浮游动物根本不能摄食这种水华,不如直接利用鲢鳙进行控制,因而1989-1992年间在武汉的东湖进行了三次原位围隔试验,2000年再度重复了一次围隔试验,结果证明,迄今在东湖中已消失18年的微囊藻水华,重新出现在不养鱼的围隔里。在养入一定密度的鲢或/和鳙的围隔中,就没有水华出现。已经出现水华的围隔,放入适当密度的鲢或/和鳙后,在短期内水华又复消失;而放入草鱼的围隔,则水华始终持续,不受影响。说明鲢和鳙能有效地遏制微囊藻水华。鲢、鳙遏制水华的有效放养密度(亦即有效生物量)为46-50 g·m-3。东湖湖泊中鲢、鳙的生物量,直到1985年才达到这个水平,所以1985年湖里反常地没有出现水华。往后的年份,东湖的鱼产量越来越高,也就是湖里鲢、鳙的生物量越来越大,所以迄今没有微囊藻卷土重来的机会。东湖的水质和其他非生物条件仍适于蓝藻水华的发生与发展,如果东湖鲢、鳙的产量下降到阈值以下,亦即降到1985年以前1 000 t水平。

For the sake of controlling algal bloom caused by the eutrophication of water bodies, the orthodox biomanipulation approach advocates the stocking of piscivorous fishes to suppress the planktivorous fish so as to bolster the zooplankton population, which is entrusted to suppress the algal community through its grazing pressure.. According to this line of thinking, the planktivorous silver carp (Hypophthalmichchys molitrix) and bighead carp (Aristichthys nobilis) should be the target of elimination from the water bodies concerned. Authors of the present paper, however, consider that the most undesirable water bloom in Chinese lakes is the cyanophycean or cyanobacterial Microcystis bloom, which can hardly be consumed by zooplankton, accordingly, it seems more reasonable to employ the silver carp and bighead carp directly for the control of Microcystis bloom. Thus, during the years 1989 to 1992, three in situ enclosure experiments were undertaken in Lake Donghu of Wuhan city, and the experiment was further repeated in 2000. The results of all these experiments corroborated that Microcystis bloom, vanished from Lake Donghu for 18 years already, did resume in the enclosures devoid of fish. In those enclosures stocked with adequate densities of silver carp or/and bighead carp, Microcystis bloom failed to occur. In those fish-less enclosures where Microcystis bloom already occurred, the introduction of silver carp or / and bighead carp at an adequate density did wipe out the bloom within a short period of time; whereas the stocking of grass carp (Ctenopharyngodon idellus) in these enclosures brought no effect at all-the bloom persisted throughout the experiment. The above results demonstrated consistently that the planktivorous silver carp and bighead carp could reliably suppress Microcystis bloom; the effective stocking rate or biomass of these fish for the elimination of Mycrocystis bloom has been estimated at 46~50 g per cubic meter of water. The biomass of these two fishes in Lake Donghu was not up to this standard until the year of 1985, hence the exceptional absence of this bloom from the lake that year. In the succeeding years, the fish yield of the lake has been on the rise year after year, implying a steady increase of the biomass of these fishes in the lake, hence there has been no opportunity for the Microcytis bloom to recur. Evidently, water quality and other non-biological environmental factors of the lake are still vulnerable to the formation and development of this bloom, and if the annual fish crop declines to below the threshold level (i.e., the 1015 t fish yield of the lake in 1985), the recurrence of Mycrocystis bloom will be inevitable. The doctrine and practice of the orthodox approach of biomanipulation could in no way check the outburst of this bloom; on the contrary, the direct suppression of this bloom through the phytoplanktivorous fishes silver carp and bighead carp has its experimental grounds and has been proved effective in the fishery practice of Lake Donghu.