Sublethal end-point

The definition of the acute to chronic ratio (ACR) has been one of the methods used to predict the threshold concentration at which a toxicant does not produce noticeable effects during a chronic exposure. This ratio is based on the same concept as the application factors, but its numerical value is the inverse (Stephan, 1982). The ACR is the ratio between chemical concentrations exerting a lethal versus sublethal toxic effect and describes the ratio of a lethal to sublethal end-point ... 

CBs, like OPs, can cause a variety of sublethal neurotoxic and behavioral effects. In one study with goldfish Carrasius auratus), Bretaud et al. (2002) showed effects of carbofuran on behavioral end points after prolonged exposure to 5 pg/L of the insecticide. At higher levels of exposure (50 or 500 pg/L), biochemical effects were also recorded, including increases in the levels of norepinephrine and dopamine in the brain. The behavioral endpoints related to both swimming pattern and social interactions. Effects of CBs on the behavior of fish will be discussed further in Chapter 16, Section 16.6.1. 

The uses and end points measured for enclosure studies are similar to mesocosms, but here a portion of the natural environment is enclosed and manipulated, rather than constructing an artificial system. Manipulations include adding previously unexposed organisms to an enclosure in a contaminated environment, or applying test chemicals to an enclosed portion of a noncontaminated environment. Terrestrial enclosures are usually corrals fenced in by wire or plastic mesh or impermeable barriers such as metal or plastic sheets. They may range from < 1 m to more than a hectare. Aquatic enclosures may include a section of the shoreline fenced off by plastic curtains (littoral enclosures) or boxes made of flexible plastic sheets suspended in open water (limno-corrals). Small enclosures are used to monitor acute toxicity or sublethal effects, while larger enclosures may study population, community, or ecosystem level end points. 

An index based on the sum or average of bioassay end-points is the simplest to devise. In some instances it may be desirable to combine tests of acute lethality with sublethal tests in order to include a spectrum of organisms and/or responses. Indices are easier to construct if toxicity end-points are first translated into toxic units. The numerical values then can be summed like the chemical properties of a sample. An alternative would be to classify results on an ordinal scale (e.g. 0-10) based on the observed severity of effect. The approach is more subjective, but at least it incorporates expert judgement that should enter the assessment of data at some point. A ranking scale allows any kind of environmental measurement to be included in the index. 

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