Page 684 - The Toxicology of Fishes

664 The Toxicology of Fishes

effects are a result of the properties of the chemical, mode of action of the chemical, and metabolic
competency of the organism to change the form of the chemical. Some effects are reversible in that
damaged tissue may be repaired or recovery may occur following narcosis (i.e., nonselective toxicity).
A distinction in toxicological effects may also be made on the basis of site of action of a chemical
(Chapter 5). Local effects occur at the primary site of chemical contact (e.g., gill inflammation). In
contrast, systemic effects of a chemical require absorption and transport in the blood to a site distant
from the contact or entry site.
Certain chemicals may produce nonselective toxicity as a result of general disruption of different cells
and membranes. Narcosis is a baseline or minimum effect that every organic chemical can produce
except that it may be masked by a more specific effect acting at lower concentrations. In essence, fewer
molecules are required to cause the specific toxic response than is the case for narcosis. If a chemical
has specific toxic activity, it is the mode of action that is expressed first. Chemicals that act on a specific
tissue, enzyme, or process without harming others are selective. They usually produce effects at lower
concentrations then chemicals that act through a nonselective mechanism. Rapid death is the most obvious
and overt response (effect) to chemical exposure and is typically preceded by other premonitory signs
of intoxication and malfunction (see Chapters 6 to 12). A major concern in fish toxicology, however, is
the extent to which fish survive low-level exposure concentrations (sublethal) but function less efficiently
because of less obvious signs of injury. In a sublethal toxicity test, the lowest-observed-effect concen-
tration (LOEC) is the one that produces at least one statistically significant observed effect compared
to control groups upon the most sensitive measured performance criteria (e.g., growth, reproduction). It
is reasonable to assume that in the field any adverse biological effect that reduces the organism’s ability
to cope in the environment has a probability of impacting some aspect of its overall lifespan; however,
sublethal effects may not reasonably be preliminary stages of death. Also of interest is the exposure
concentration at which fish can survive, grow, and reproduce and show no adverse symptomology. In a
toxicity test, the highest test concentration that produces no statistically significant adverse effect on the
performance criteria is the no-observed-effect concentration (NOEC). The LOEC and NOEC are statis-
tically defined concentrations in tests. The term safe concentration is a biologically defined concept.
The above implies that sublethal exposure to a chemical either exerts no effects if the exposure is
below a toxic threshold concentration or has an adverse effect, the severity of which increases with
increasing chemical concentrations. Note that, in toxicology, tests are used to study negative or adverse
affects as a result of chemical exposure, but effects of chemical exposure may be stimulatory or positive
deviations (e.g., increased growth) from background. There may also be an intermediate subinhibitory
exposure concentration that is actually beneficial to organisms. Southam and Erhlich (1943) termed this
phenomenon hormesis (after the Greek word for “to excite”). The organism, when challenged with low
levels of stress, overcompensates and not only eliminates any low-level induced damage but also reduces
background stress or damage more effectively than before the stress was applied (Calabrese and Baldwin,
2000). This response is independent of phyla, chemical class, and endpoint. The low-concentration (or
low-dose) hormetic responses are not easy to differentiate from typical background variations. It is not
known how frequently hormetic responses occur because toxicity tests are generally designed to deal
with high concentration responses to determine NOECs. Hormetic responses imply the existence of
significant biological activity below the traditional NOEC that typical toxicity testing cannot evaluate
unless multiple concentrations are used in the low concentration range to determine the consistency and
variability of stimulatory effects.

Concentration–Response Relationship
Toxicity tests are guided by the concentration–response relationship—that is, the relationship between
exposure concentration (in water) or dose (in sediment, food) of a chemical and the response of the
biological system (e.g., whole organism, cell) being exposed. The relationship has several assumptions:

• A cause-and-effect relationship exists, and there is a high degree of certainty that the response
or effect is a result of organism exposure to the chemical being investigated.