Page 726 - The Toxicology of Fishes

706 The Toxicology of Fishes

the same study it was clear from tissue contaminant analysis and lipid peroxidation measurements that
fish at the contaminated sites were suffering from toxic effects. It might therefore be preferable to choose
a condition index that is more closely related to the target organ for the pollutant of concern. Alternatively,
the condition factor may confirm the absence of toxic effects or indicate ecosystem recovery from
pollution. In a study on the chronic toxicity of sediments containing drilling fluids, Payne et al. (1995)
observed only minor changes in the condition indices of turbot which were supported by the absence
of organ pathologies and dose effects in blood parameters. In a study of remediation in acidified lakes,
Gloss et al. (1989) found an increase (recovery) in the condition factor of brook trout (Salvelinus
fontinalis) after liming.

Organosomatic Indices
These indices measure organ mass relative to body mass (e.g., hepatosomatic index, liver weight/fish
weight × 100%) and offer a big advantage over the condition factor as a biomarker in that they may
relate directly to toxic effects on the target organ of contaminant exposure. Thus, the hepatosomatic index
(HSI) might be an appropriate biomarker for substances that are toxic to the liver (e.g., cadmium) (Haux
and Larsson, 1984), while the gonadal somatic index (GSI) may be more appropriate for estrogen mimics
(Table 16.6) (Jobling et al., 1996). The hepatosomatic index is perhaps one of the most commonly applied
indices because of the central role of the liver in the detoxification of pollutants (see examples in Table
16.6). Goede and Barton (1990), however, pointed out that the HSI may decline in response to starvation
(glycogen depletion), but liver weight my increase due to pathological changes (e.g., hyperplasia asso-
ciated with pollutant exposure). The resultant effect could, in theory, be no net change in HSI even if
resultant histological examination of the tissue reveals toxic responses (Schwaiger et al., 1997; Teh et
al., 1997); nonetheless, the HSI has proven useful in field and laboratory studies with fish (Table 16.6).
Other organosomatic indices, including the splenosomatic index, viscerosomatic index (Goede and
Barton, 1990), and relative ventricular weight (Armstrong and West, 1994), have been applied to studies
on salinity, temperature, and seasonal change but remain to be validated as a technique for the toxicologist.

Growth
Growth rate has been used as an endpoint for many years in ecotoxicology. Measurements of growth
can be simple measurements of body weight over time to calculate weight gain or mean daily specific
growth rates (SGRs), or growth can be measured at the biochemical level in terms of changes in cell
protein content (Smolders et al., 2003) or RNA/DNA ratios (Benton et al., 1994; Pottinger et al., 2002).
The underlying assumption is that growth is reduced by pollutant exposure because energy intake has
decreased or energy expenditure has risen because of the costs of avoiding exposure, detoxification, or
tissue repair. The effects of contaminants on the growth of fish are inevitably measured during chronic
sublethal exposure conditions, usually over several weeks rather than acute exposures of a few days.
The effects of chronic dietary metal exposure on fish have recently been reviewed (Clearwater et al.,
2005; Handy et al., 2005), and the growth rates of fish have also been measured during exposures to
organic chemicals (Brown et al., 2002; Palm et al., 2003; Reiser et al., 2004). Clearly, growth rate is
influenced by many factors, including food intake, the energy content of the food, water temperature,
energy expenditure on swimming, and so on (Jobling, 1993). From the view point of biomarkers, we
would like to differentiate these normal biological effects on growth from those of pollutants. This is
relatively easy to do in the laboratory, and given sufficiently long enough time or high enough doses
the growth rate can be impaired (e.g., for metals) (Clearwater et al., 2005). Many reports, however, have
indicated that growth rates do not change very much during environmentally relevant exposures (Brown
et al., 2002; Campbell et al., 2002; Handy et al., 1999; Palm et al., 2003; Reiser et al., 2004). This may
be because the fish have altered their locomotion budget to save energy and preserve their growth rate
during pollutant exposure (e.g., dietary copper) (Campbell et al., 2002; Handy et al., 1999), or, alterna-
tively, the fish have made an endocrine adjustment that promotes growth. Enhanced metabolism of
thyroid hormone (T ), for example, may preserve growth rates during PCB exposure (Brown et al.,
4
2004b). This is particularly interesting as it suggests that the hormones that control growth rate, rather
than the growth rate itself, may be a potentially useful group of biomarkers in the future.