1. Introduction
The Nile Tilapia (Oreochromis niloticus) is currently the most
commonly cultivated fish species in Brazil. It is cultivated in 22
Brazilian states, with an annual production of 30–40 thousand
tons. Tilapia was one of the first farmed species to be processed
and commercialized as frozen fillets [1]. In aquaculture, Tilapia is
constantly at risk of being attacked by predators or parasites and
organophosphate pesticides are used to help prevent this [2]. In
addition, the application of pesticides in agriculture and in areas
located close to wetlands has resulted in toxicity to many nontarget
species such as fish and aquatic invertebrates [3–5].
Folidol 600 is a commercial product widely used in Brazilian
agriculture and contains methyl parathion as the active ingredient.
It is used in aquaculture ponds to eliminate the larvae of aquatic
insects that are predators of fish larvae [2,6]. This product usually
results different forms of environmental contamination, as well as
accidents involving man and native fauna [6,7]. Thus, contamination
by pesticides is a serious water pollution problem, which
may cause an environmental imbalance and increase the incidence
of poisoning of fish and other aquatic species [5,6,8].
Studies to evaluate the toxicity of chemicals released into water
are useful to assess the impact they may have on the aquatic biota.
Several kinds of physiological endpoints can be used for the study