428
JAMES E. DEACON AND W. L. MINCKLEY
those issuing from springs, generally have high to moderate amounts of
gases (unless polluted) due to mixing by turbulence, and they also are
generally ignored in this section. Natural desert waters are typically rather
clear of pollutants, but interfering substances may nonetheless be high and
earlier dissolved oxygen analyses, for example, may be subject to considerable doubt. Also, the high total dissolved solids in many desert pools may
influence analysis of gases to a greater or lesser degree; this aspect often
has been neglected as a variable by field investigators. As with salinity
then, some data may be valid only in a general way, and the reader should
refer to the original work prior to generalizing from this review.
a. Oxygen. The oxygen supply of natural waters is probably the most
frequent single factor influencing the life of desert fishes, or for that matter,
of freshwater fishes in general. Adequate supplies of this gas may be defined by survival of fishes at a given level, but quite often low levels that
may be tolerated in nature are, in fact, detrimental, causing losses of
weight, impaired swimming abilities, malformed or weakened larvae, and
so on (Doudoroff and Warren, 1965). The voluminous literature on relations of fishes and dissolved oxygen has been recently reviewed by Fry
(1957, 1960), Jones (1964), Bemish and Dickie (1967), and Phillips
(1969 ). Trout, salmon, and goldfish have been most intensively investigated.
Metabolic demands for oxygen increase as activity increases. However,
below a certain level in the environment, activity and therefore oxygen
utilization become oxygen dependent (and oxygen limited) (Fry, 1957).
This incipient limiting point marks the level at which additional uptake
by the fish is limited by its ability for absorbing oxygen from the environment, rather than by the ambient availability of the gas. The level
that forces a fish to reduce maximum activity is distinctive for each
species, and in many oxyphilic forms corresponds to 100% saturation,
or at high temperatures, to a supersaturation (Fry, 1957; Macan, 1963).
As Macan has pointed out, theoretically, the most important parameter
from the ecological standpoint is not the amount of oxygen consumed by
an organism, but rather the concentration at which consumption is reduced
by oxygen availability (i.e., the incipient limiting point). This information is
specifically available for only a few fishes (Jones, 1964), none of which
live naturally under desert conditions.
In general, fishes respond to oxygen depletion by increasing the frequency, regularity, and amplitude of respiratory movements (Jones,
1964). Different species demonstrate various characteristic avoidance reactions to water of low dissolved oxygen. Nervous, erratic responses that
increase activity (and therefore oxygen demand) may well allow species,
living in habitats where escape from local oxygen depletion is possible,
to do so. However, in confined desert pools, a response that reduces activ-
JAMES E. DEACON AND W. L. MINCKLEY
those issuing from springs, generally have high to moderate amounts of
gases (unless polluted) due to mixing by turbulence, and they also are
generally ignored in this section. Natural desert waters are typically rather
clear of pollutants, but interfering substances may nonetheless be high and
earlier dissolved oxygen analyses, for example, may be subject to considerable doubt. Also, the high total dissolved solids in many desert pools may
influence analysis of gases to a greater or lesser degree; this aspect often
has been neglected as a variable by field investigators. As with salinity
then, some data may be valid only in a general way, and the reader should
refer to the original work prior to generalizing from this review.
a. Oxygen. The oxygen supply of natural waters is probably the most
frequent single factor influencing the life of desert fishes, or for that matter,
of freshwater fishes in general. Adequate supplies of this gas may be defined by survival of fishes at a given level, but quite often low levels that
may be tolerated in nature are, in fact, detrimental, causing losses of
weight, impaired swimming abilities, malformed or weakened larvae, and
so on (Doudoroff and Warren, 1965). The voluminous literature on relations of fishes and dissolved oxygen has been recently reviewed by Fry
(1957, 1960), Jones (1964), Bemish and Dickie (1967), and Phillips
(1969 ). Trout, salmon, and goldfish have been most intensively investigated.
Metabolic demands for oxygen increase as activity increases. However,
below a certain level in the environment, activity and therefore oxygen
utilization become oxygen dependent (and oxygen limited) (Fry, 1957).
This incipient limiting point marks the level at which additional uptake
by the fish is limited by its ability for absorbing oxygen from the environment, rather than by the ambient availability of the gas. The level
that forces a fish to reduce maximum activity is distinctive for each
species, and in many oxyphilic forms corresponds to 100% saturation,
or at high temperatures, to a supersaturation (Fry, 1957; Macan, 1963).
As Macan has pointed out, theoretically, the most important parameter
from the ecological standpoint is not the amount of oxygen consumed by
an organism, but rather the concentration at which consumption is reduced
by oxygen availability (i.e., the incipient limiting point). This information is
specifically available for only a few fishes (Jones, 1964), none of which
live naturally under desert conditions.
In general, fishes respond to oxygen depletion by increasing the frequency, regularity, and amplitude of respiratory movements (Jones,
1964). Different species demonstrate various characteristic avoidance reactions to water of low dissolved oxygen. Nervous, erratic responses that
increase activity (and therefore oxygen demand) may well allow species,
living in habitats where escape from local oxygen depletion is possible,
to do so. However, in confined desert pools, a response that reduces activ-