the past several years, there has been growing acknowledgement that
synthetic chemicals released into the environment have disrupted
teh development and/or functoin of the reproductive, endocrine,
immune, and nervous systems of vertebrates. Effects of this nature
have been documented for many wildlife species in the field and
have also been demonstrated in laboratory studies. In several instances,
exposure to endocrine-disrupting chemicals (EDCs) has led to instability
or declines in wildlife populations. To address the relevance of
these findings to freshwater and saltwater fishes, a group of scientists
gathered in retreat at a work session at Wingspread, in Racine,
Wisconsin, on 21-23 July 1995, to examine the scientific evidence
and to come to some conclusions about the magnitude and scope of
these effects. A multidisciplinary group of 22 scientists participated
in the work session, including experts in the field of aquatic sciences,
biochemistry, biometrics, chemistry, ecology, endocrinology, fisheries
biology, immunology, marine sciences, pathology, pharmacology, physiology,
toxicology, veterinary medicine, wildlife biology and zoology. Participants
were asked to address the following questions:
is the present state-of-knowledge concerning the effects of chemical
exposure on fish development and reproduction?
are the major gaps in this knowledge? What research is needed
to answer the major questions?
what extent are environmental contaminants contributing to fish
population declines or impeding recovery of depleted stocks?
are confident of the following points:
are many uncertainties in our predictions for the following reasons:
classes of synthetic chemicals present in the aquatic enviornment
disrupt reproduction, endocrine, immune and nervous system functions,
growth, and development in fishes (e.g., polychlorinated bipheynyls
[PCBs], dibenzo-p-dioxins [PCDDs], and dibenzofurans [PCDFs],dichlorodiphenyl-trichloroethate
[DDT]-group chemicals, alkylphenols, and their metabolites). Some
naturally occurring compounds used in industrial and agricultural
processes also have this capability (e.g., heavy metals). Laboratory
studies have shown that these chemicals can act directly as hormone
mimics or blockers. They can also act indirectly through effects
on neuroendocrine homeostasis, and they can affect immune system
wide range of effects may result from exposure to these chemicals,
including anatomical malformations, functional alterations, and
biochemical or molecular changes. Developing embryos and larvae
are particularly sensistive to chemical exposure; however, adult
fish are also affected. Depending upon the timing of exposure,
irreversible effects can occur in the offspring of exposed fish
which may not be immediately apparent. Because many of these chemicals
eventually reach aquatic ecosystems, fish may serve as reliable
sentinels of effects of chemicals in other vertebrates, including
fish populations in both saltwater and freshwater are presently
threatened by chemicals introduced into the environment through
human activities. There is a high degree of certainty that some
wild fish populations have already been affected.
well-characterized toxic effects of PCDDs, PCDFs, and PCBs on
lake trout sac larvae, the past exposure conditions, and the effects
found in Lake Ontario provide compelling evidence that these chemicals
probably caused reproductive or developmental damage to entire
stocks of fish. In most other cases, the specific agents responsible
for injury have not been definitively identified, nor has damage
to reproduction and development been adequately assessed.
great concern is the global distribution of many known reproductive
and developmental toxicants. These deleterious agents can move
through the environment, expose fishes distant from their points
of release, and accumulate to high levels in some locations.
need to be taken to safeguard fish stocks from the effects of
synthetic chemical exposures. The general public and many scientists
are not aware of the extent to which fish species, including economically
important fisheries, are presently at risk or have already been
improve our predictive capability, we must accomplish the following:
multigenerational studies of wild fish populations have looked
in detail at the sublethal effects caused by synthetic chemical
exposure which lead to long-term functional impairment (e.g.,
developomental, endocrine, reproductive, neurologic, immunologic)
or long-term consequences (e.g., endocrine disruption leading
to reproductive failure and population decline).
variables complicate field research linking exposure to chemicals
with effects on wild fish populations. Epidemiological studies
are complicated by the influence of changing climatic and geophysical
forces on fish population dynamics. Additional confounding factors
include impacts of human activities, such as overfishing, release
of hatchery-reared fishes, habitat alterations, and the introduction
of exotic species.
characteristics of fish biology further complicate field research.
For example, year class recruitment is affected by variables that
are difficult to quantify; reproductive strategies vary greatly
among different fish species, making interspecies comparisons
difficult; little is known about normal physiological parameters
for most fish species; and routine sampling and observation of
specific fish populations is logistically difficult.
knowledge is needed of fundamental physiological processes and
developmental mechanisms in fish at levels of organization from
molecular and cellular to the whole organism. In particular, basic
research is needed for a variety of species on sexual differentiation,
ontogeny of the neuroendocrine system, hormone receptor structure
and function, normal levels of circulating hormones at various
life stages, and normal developmental processes.
assessments of the present threat to fish stocks are needed, requiring
a) detailed information on the demographics and reproductive health
of fish populations where developmental or reproductive dysfunction
is occurring; b) knowledge of the extent and patterns of discharge
and accumulation of potential and known toxicants in aquatic ecosystems;
c) ecoepidemiological studies to identify chemical exposures to
demonstrate cause and effect and delineate the mechanisms of action;
and e) consideratoin of the effects of the other stressors, either
alone or in combination, on the toxicological action of chemicals
present in the environment.
and laboratory research strategies should include identification
of critical times within specific life stages when organisms are
most sensitive to the toxic actions of chemicals: this will require
long-term multigenerational studies. The effects of chemical exposure
during development must be linked to later life history characteristics
(e.g., behavioral, reproductive, nutritional, immunological, and
neurological), which subtly alter the organism's ability to survive
and reproduce. Field studies should aslo address the long-term
effects of chemical exposures on fish population and community
studies should quantify the tissue burdens of chemical associated
with injury in standardized units of measure to facilitate coordinated
laboratory research. This information will guide laboratory studies
by calibrating the exposure required to produce the effects observed
in the field. However, there is particular concern for non-bioaccumulative
agents which are not detectable in the organism when the damage
is expressed, making it difficult to establish causal relationships.
Toxicological studies of these chemicals must include exposures
of ova, embryos, and early life stages followed by detailed observations
for effects in the mature adults and their offspring (i.e. second
generation effects). It is very important that these studies incorporate
the impact of maternal transfer of contaminants to the ova before
spawning, mimicking the conditions in the environment.
basic research into the mechanisms and sites of action of known
toxicants is needed. This research should include both direct
and indirect effects of chemicals on specific reproductive and
developmental processes, and it should identify the most sensitive
endpoints and the most sensitive fish species. From this informatoin,
reliable biomarkers of the effects of specific chemicals should
vivo and in vitro screening methods are needed to assess
the potential reproductive and developmental toxicity of compounds
to fishes before these chemicals are released into the environment.
possibility of multiple impacts of several chemicals or chemical
classes should be considered, because the net effects of complex
mixtures of chemicals cannot always be explained on the basis
of the individual actions of known components of the mixture.
Models are needed for utilizing information a) about single chemical
effects to predict effects from exposures to mixtures of chemicals
with a common mode of action and b) to predict effects from exposures
to mixtures of chemicals with different modes of action.
difficulties in assessing the causes of fish population declines
need to be addressed through cooperative, multidisciplinary research.
Field observations of dysfunction at individual, populatoin, community
and ecosystem levels should direct the course of coordinated experimental
and analytical laboratory studies. The results from these coordinated
field and laboratory studies on chemical exposures, effects, and
dose-response can tehn be used to characterize risk and to address
the causes of fish population declines.
activities will require long-term multidisciplinary studies by
an international consortium of scientists from academia, the private
sector, governmental, and nongovernmental organizations. These
efforts should be supported by governments, the private sector,
and an informed citizenry.