Risks to Aquatic Organisms from Use of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 120

fi sh; the value was 0.32 for Aphanius iberus sp. (Varo et al. 2002 ), which is not
indicative of biomagnifi cation. CPY is metabolized by a variety of aquatic organ-
isms and results of studies in model food chain cosms (Metcalf et al. 1971 ; Metcalf
and Sanborn 1975 ) did not suggest that CPY will biomagnify to a toxicologically
signifi cant extent in food chains found in surface waters. Because of the relatively
small BCF and BMF values, exposures of terrestrial wildlife that consume aquatic
food items were excluded from this risk assessment (Fig. 1 ). For the same reason,
exposures of fully aquatic organisms will mostly be via uptake directly from water,
although some dietary exposure to CPY might result from residues adsorbed to
food items such as algae, macrophytes, and invertebrates, or from ingested sedi-
ment particles. Exposures and risks to terrestrial birds through consumption of
granules and terrestrial food items are addressed in a companion paper (Moore
et al. 2014 ).

2.2 Toxicity of Chlorpyrifos

The mechanism of toxic action of CPY is through inhibition of acetylcholinesterase
(AChE) by the active metabolite, chlorpyrifos oxon (CPYO). The specifi cs of the
mode of action are discussed in greater detail in the companion paper (Solomon
et al. 2014 ). Inhibition of AChE by CPYO is reversible and, in the case of sublethal
exposures, recovery of AChE can occur. AChE is a key enzyme in the nervous systems
of most animals, and direct effects of CPY will occur at much smaller exposure
concentrations than in organisms that lack the target enzyme, such as plants. Insects
and crustaceans are generally more sensitive to CPY than are fi sh or amphibians
(Giesy et al. 1999 ). The primary focus of this ERA is CPY and CPYO. Other metab-
olites and breakdown products, such as trichloropyridinol (TCP), are much less
toxic (USEPA 2008 ) and are not addressed.
Effects of CPY on animals range from lethality to minor symptoms from which
animals recover. Most testing of toxicity of CPY to aquatic animals has used lethal-
ity as the measurement endpoint, with results usually expressed as the LC50 or the
no-observed-adverse-effect-concentration (NOAEC). Fish are an exception since a
number of studies have characterized sublethal and behavioral responses.

2.2.1 Sublethal Effects on Aquatic Animals

Several studies have reported effects of CPY on behavior of arthropods and fi sh.
Interpretation of these studies presents diffi culties, because it is not always clear if
the observed responses are alterations in normal behavior specifi cally induced by
the pesticide or changes in behavior in response to general stress or symptomology
of the toxicity. This distinction can be addressed in specifi cally designed tests such
as have been used to assess aversion to ingestion by birds (Moore et al. 2014 ). In
studies of the prawn Macrobrachium rosenbergii , effects on feeding were observed