VII. DESERT FISHES
417
and Heath, 1969). However, in the more severe habitats we have studied,
thermal (or thermal-related) deaths are not uncommon, especially at lower
ends of streams such as Salt Creek and the Amargosa River, channels in
Cottonball Marsh, all in Death Valley, and in cut-off pools of lagunas and
streams in Cuatro Ciénegas, Mexico. Fishes of intermediate elevations in
Arizona often move downstream in spring or in years of abundant runoff
and die there as low discharges of summer ensue, presumably as a result
of high temperatures. Numerous loach minnows (Tiaroga cobitis) have
been found dead and dying at 30.5°C in lower Aravaipa Creek, Arizona.
Colorado chubs (Gila robusta) in side pools of the Salt River also
succumbed at 30.5°C.
John (1964) studied temperature tolerances of Rhinichthys osculus, a
species with a longitudinal distribution in streams that often resembled that
of Gila and Tiaroga. Rhinichthys ranges to higher elevation, however.
In the Chiricahua Mountains, where his study was performed, the highest
summer water temperature recorded in a flowing stream was 31°C. In
standing pools the maxima were 36°C at the surface and 33°C at the bottom (8 cm deep). Minimum in the study periods was 16°C. In laboratory
tests the ultimate incipient upper lethal temperatures for young fish
(20 mm standard length) was about 33°C, and for older fish (more than
35 mm standard length) it was ~ 3 2 ° C . In addition, older fish died after
less exposure to increasing temperature. Incipient upper lethal temperatures of fish subjected to abrupt changes from 31°C (temperature of acclimatization) to 33°-35°C showed no correlation with size, but time to
death for 50% of the population decreased rapidly and total mortality
increased as temperature was increased.
In view of environmental temperatures, John (1964) attributed little
significance to a 2°C greater tolerance to high temperature by young of
speckled dace when compared to that of adults. However, he noted that
in cut-off pools, adult fish would realize lethal temperatures while juveniles
might not, a fact of considerable potential importance to a species. Many
other fish species show the same type of response to extremes (Gunter,
1945).
Deaths from cold have been frequently reported (Norris, 1963). In
most instances it appears that rapidity of onset of cold temperatures,
and the fact that cold water mixes rapidly to the bottom of aquatic habitats, linked with an inherently slow rate of gain of cold tolerance, results
in mortality. This is despite the fact that low temperatures often remain
within the biokinetic range. By contrast, layering of warmed water at the
surface provides opportunity for behavioral avoidance of change during
thermal adjustment to rapid warming (discussed later). In the very shallow desert waters, however, such layering may be minimal.
417
and Heath, 1969). However, in the more severe habitats we have studied,
thermal (or thermal-related) deaths are not uncommon, especially at lower
ends of streams such as Salt Creek and the Amargosa River, channels in
Cottonball Marsh, all in Death Valley, and in cut-off pools of lagunas and
streams in Cuatro Ciénegas, Mexico. Fishes of intermediate elevations in
Arizona often move downstream in spring or in years of abundant runoff
and die there as low discharges of summer ensue, presumably as a result
of high temperatures. Numerous loach minnows (Tiaroga cobitis) have
been found dead and dying at 30.5°C in lower Aravaipa Creek, Arizona.
Colorado chubs (Gila robusta) in side pools of the Salt River also
succumbed at 30.5°C.
John (1964) studied temperature tolerances of Rhinichthys osculus, a
species with a longitudinal distribution in streams that often resembled that
of Gila and Tiaroga. Rhinichthys ranges to higher elevation, however.
In the Chiricahua Mountains, where his study was performed, the highest
summer water temperature recorded in a flowing stream was 31°C. In
standing pools the maxima were 36°C at the surface and 33°C at the bottom (8 cm deep). Minimum in the study periods was 16°C. In laboratory
tests the ultimate incipient upper lethal temperatures for young fish
(20 mm standard length) was about 33°C, and for older fish (more than
35 mm standard length) it was ~ 3 2 ° C . In addition, older fish died after
less exposure to increasing temperature. Incipient upper lethal temperatures of fish subjected to abrupt changes from 31°C (temperature of acclimatization) to 33°-35°C showed no correlation with size, but time to
death for 50% of the population decreased rapidly and total mortality
increased as temperature was increased.
In view of environmental temperatures, John (1964) attributed little
significance to a 2°C greater tolerance to high temperature by young of
speckled dace when compared to that of adults. However, he noted that
in cut-off pools, adult fish would realize lethal temperatures while juveniles
might not, a fact of considerable potential importance to a species. Many
other fish species show the same type of response to extremes (Gunter,
1945).
Deaths from cold have been frequently reported (Norris, 1963). In
most instances it appears that rapidity of onset of cold temperatures,
and the fact that cold water mixes rapidly to the bottom of aquatic habitats, linked with an inherently slow rate of gain of cold tolerance, results
in mortality. This is despite the fact that low temperatures often remain
within the biokinetic range. By contrast, layering of warmed water at the
surface provides opportunity for behavioral avoidance of change during
thermal adjustment to rapid warming (discussed later). In the very shallow desert waters, however, such layering may be minimal.