| Summary: | Studies In other parts of the world have proved that
turbidity affects aquatic life and work In Australia and
North America has shown that the distribution of some fish
species may be determined by the level of turbidity present.
This, coupled with the fact that: (i) Natal estuaries are
important as nursery areas for the juveniles of many marine
fish species, (ii) the estuaries exhibit a wide range of
turbidities and (iii) little was known of the effects of
turbidity on the fish populations in estuaries, led to this
study being undertaken.
Turbidity and its effects on fish distribution In
Natal estuaries was investigated from January 1980 to June
1983. Lake St. Lucia, which is predominantly turbid but also
has clear water areas, was the main study area. Six other
estuarine systems sampled were chosen to cover a wide range
of estuarine types and turbidities. Field sampling was
undertaken to determine which species were present under
different turbidities, simultaneously physical factors which
were potentially affecting fish distribution were also
monitored. In addition to this, laboratory equipment which
enabled a turbidity gradient to be established In a choice
chamber tank was used to test the turbidity preferences of 10
common estuarine species for which field data were available.
These tests allowed the elimination of all physical factors
except turbidity.
Of the physical parameters monitored In the field,
turbidity, temperature and food availability In the benthos,
were determined as being important In affecting fish
distribution within estauries. However, comparison of fish
distribution data for twenty species, with these factors
showed that turbidity was exerting the major influence. It
was also found that fish species occurred In one of five
groups, inhabiting either clear, 'clear to partially turbid',
intermediate or turbid waters or they were indifferrent to
turbidity. Laboratory results for eight of ten species tested
showed significant aoreement with the field data. The results of this study have shown that turbidity is
the most important factor determining the distribution of
juvenile marine fish In estuaries and that the greatest
number of species are present in , waters which are not clear.
The attraction to and presence in such systems appears to be
related to the fact that turbid estuaries 'provide protection
from fish and bird predators while also acting to reduce
intraspecific predation. Of factors attracting juvenile fish
into estuaries, turbidity is probably the single most
important acting in this respect. === Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1984.
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