Hydrobiologia 468: 45–52, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
45
Impact of mass mortality of a mosquito fish, Gambusia affinis on the
ecology of a fresh water eutrophic lake (Lake Naini Tal, India)
Surendra S. Nagdali & P. K. Gupta
∗
Department of Zoology, D. S. B. Campus, Kumaon University, Naini Tal-263 002, India
Fax: 5942-35576
(
∗
Author for correspondence)
Received 22 August 2000; in revised form 14 August 2001; accepted 5 October 2001
Key words: Gambusia affinis, fish mortality, Lake Naini Tal, top-down control
Abstract
Between 28th March and 4th April, 2000 a fungal infection killed >80% of the most abundant planktivorous fish,
Gambusia affinis in Lake Naini Tal, Uttaranchal, India. In response to this mortality, planktonic communities and
some eutrophication-related parameters viz., primary productivity, phosphate–phosphorus, nitrate–nitrogen and
transparency of the water, were considerably changed. Total zooplankton number more than doubled, phytoplank-
ton number reduced nearly to half, primary production and phosphate-phosphoruswas dramatically reduced, while
nitrate–nitrogen and water clarity increased. The phytoplankton decline was caused by increased zooplankton
grazing (top-down control) rather than phosphorus deficiency (bottom-up control). After 3 months, Gambusia and
planktonic communities and nutrient levels reverted back almost to their pre-mortality state. Thus removal of G.
affinis could improve water quality of Lake Naini Tal.
Introduction
Lake Naini Tal, a national lake of India, has fascin-
ated the limnologists over the last several years (Pant
et al., 1979, 1980, 1981, 1985a, b; Pande & Das,
1980; Gupta & Pant, 1983a, b, 1986, 1990; Sharma
& Pant, 1984: Gupta & Shukla, 1996, 1998; Gupta
et al., 1999, etc.). The lake is an important source of
drinking water, an avenue of recreation, and a tour-
ist site. The climate of the area is largely controlled
by this lake. However, due to various human activ-
ities, the lake has experienced declines in fish fauna,
both in terms of catches and species diversity, and de-
terioration in water quality. These changes have been
attributed mainly to rapid urbanization and deveget-
ation of the catchment which have led to increased
siltation and loss of suitable habitats for the fauna,
and increased input of nutrients through domestic
effluents causing eutrophication. These factors have
threatened the availability of clean drinking water for
future, biodiversity and climatic changes which in turn
have impacts on social, medical and economic fronts.
State and national efforts are being made to manage
these problems through National Lake Conservation
Plan (NLCP). The measures include preventive, cur-
ative and legislative; however, these still need to be
implemented in full, perhaps due to scarcity of funds.
In the last decade, a new problem emerged for the
lake: a mosquito fish, Gambusia affinis invaded the
lake. Before 1990, this fish was not seen. It is believed
that the fish was introduced by Malaria Control De-
partment (although there is no threat of malaria in this
region), but no one takes responsibility of doing so.
Very soon, the fish population tremendously increased
and the whole littoral zone of the lake experienced
almost a monospecific population of the fish. Since
this is a larvivorous fish, and prolific breeder, we hy-
pothesized that it must have an adverse effect on the
eco-functioning of the lake. We were planning to study
the impact of this fish on the lake ecology under NLCP
programme but in the mean time a large scale natural
mortality between 28th March and 4th April, 2000
eliminated more than 80% of the population. This be-
came a burning news for local news papers; even the