Abstract
A histological assessment allows aquatic scientists to assess fish health in
polluted aquatic ecosystems at tissue and cellular level. However, a firm
knowledge and understanding of normal histological structure is essential to
ensure accurate and objective results. Hence, the histologist must be able to
distinguish between toxicant induced lesions and the range of histological
characteristics considered to be normal for that specific species. However,
limited histological reference material and data describing normal conditions are
available for southern African fish species.
The aim of this baseline study was to establish reference material, both
qualitative and quantitative, for two southern African freshwater fish species used
as indicator species in toxicity studies at the University of Johannesburg, Clarias
gariepinus and Oreochromis mossambicus. The reference material includes
descriptive histology of selected target organs, and the associated values and
intervals for related quantitative health aspects including somatic indices,
condition factor, blood parameters (haematocrit and plasma proteins) and
quantitative histological results. Essentially, this project originated from a need to
better understand the normal histology of C. gariepinus and O. mossambicus, as
a range of histological characteristics were identified in control groups in previous
toxicity studies which could not be confirmed to be associated with normal
conditions.
To be able to conduct a baseline study, with the purpose of establishing
reference material, it was essential that the history of the specimens be known
and documented including age, sexual maturity, nutritional status and physical
and chemical water quality data. The project therefore involved the breeding of
fish specimens under controlled conditions. A subsequent necropsy and
qualitative and quantitative histological assessments were executed on five
target organs (usually included in toxicity studies within the department): liver,
gills, gonads, heart and kidney.
Fish of both species were bred in an environmental room in reconstituted,
reverse osmosis water and reared until sexually mature. Twenty specimens with
an approximately 50:50% sex ratio were collected for both species. A necropsy
was performed on each specimen and tissue samples of the selected target
organs were processed using standard techniques, and prepared for the
qualitative and quantitative histological assessments using light microscopy.
The results The results showed that no macroscopic abnormalities were identified during the
necropsy regarding external features or internal organs of the specimens within
the sample groups, and all fish appeared to be in good health. The target organs
were then examined microscopically, followed by a description of the normal
histological structure (qualitative assessment). In addition, the target organs were
assessed by means of a specific quantitative histological assessment protocol,
which provides a standardised methodology of criteria to objectively assess fish
health (quantitative assessment). Histological alterations identified during this
assessment were quantified and subsequent reference index values could be
calculated for each organ.
Dr. G.M. Pieterse