Responses of Fishes to a Low pH Environment Page: 42
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42
and summer in southern states (Carlander, 1977) and in Ferndale lake
during 1948 (Estes, 1949), when the pH was higher. Generally, early and
late spawns produce fewer surviving individuals than spawns in mid-
summer (Carlander, 1977). The shorter spawning period evident from my
data can be interpreted to be an example of "bet hedging," in which the
highest reproductive effort is put into the most optimal period for sur-
vival of hatched eggs and fry (Pianka, 1978). In a stressed population
it would be energetically conservative to reproduce at a time so as to
buffer offspring from seasonal trauma. The overall mean bluegill GSI
for the period sampled was lower in the 1979 study than in the 1948 study!
however, the mean GSI was significantly higher in my study during mid-
summer months (t test; P<0.001). Gunderson (1980) showed that a posi-
tive correlation exists between GSI and the natural rate of mortality (M).
Considering my data in light of such a correlation, it could be conclud-
ed that in months during which GSI is highest, M is also highest. For
small southwestern lakes, midsummer is accompanied by maximum water
temperatures and minimum dissolved oxygen levels. Additional stress
from an acid environment could push M and GSI higher during this period.
Fish in Ferndale Lake are chronically exposed to levels of pH
that cause mortalities in experimental situations. Only one minor fish
kill, following surface freezing of a large portion of the lake, was
seen or reported for the study period. Levels of pH in the lake were
depressed to levels experimentally determined to result in high mortality
rates. Bluegill collected from the lake when its pH level was 4.2 ex-
hibited 100% survival at exposure levels of pH 3.8 through 4.2 for 96
hr. In previous testing when lake pH was 4.6, exposure to pH 3.8 resulted
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Prete, Philip J. (Philip John). Responses of Fishes to a Low pH Environment, thesis, August 1981; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc663820/m1/48/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .