VII. DESERT FISHES
445
success of stocking salmonids at high elevations far south of their natural
ranges.
5. Activity Cycles
Study of normal, daily activities of some desert fish has received attention in recent years. Harker (1958) reviewed diurnal rhythms in animals,
and defined two types, as follows: (1) endogenous systems which continue
to operate when environmental conditions are held constant and (2) exogenous rhythms occurring as responses to regular environmental cycles.
Endogenous rhythms of metabolic rates in largemouth bass (Micropterus
salmoides) (Clausen, 1936), Fundulus paravipinnis (Wells, 1935), and
many other fish species (Harker, 1958) have not been demonstrated for
desert fishes, but doubtless exist. Numerous cyprinodontids and poeciliids
from southwestern United States and Mexico, for example, demonstrate
typical spring surges in reproduction (although breeding throughout the
year) under laboratory conditions of constant light and temperature.
Field studies cannot distinguish between the two types of rhythms, but
frequently do permit definition of environmental components affecting
rhythmicity. Exogenous rhythms based on light as the dominant factor
seem a general rule for fishes that are considered diurnal, nocturnal, crepuscular, or even arrhythmatic (Spencer, 1939; Spoor and Schloemer,
1939; Hasler and Villemonte, 1953; Hobson, 1965). Most desert fishes,
particularly those living in springs, seem visually oriented, and are therefore diurnal. The springfishes, Crenichthys baileyi and C. nevadae, have
activity cycles that are light-initiated and dark-inhibited (Deacon and Wilson, 1967; Hubbs et al., 1967), with some inferred modifying influences
by temperature and dissolved oxygen content. Populations of this genus
reduce normal activities in response to oxygen levels below 2 mg/liter,
temperatures above 35°C, or cloud cover. Changes in the normal activity
pattern (generally compression of activity into a relatively short period
of the day) also resulted from apparent stress induced by establishment
of exotic species and/or an increased incidence of parasitism. Similar
light-initiated, dark-inhibited activities have been documented for a number of species of pupfishes (Barlow, 1958a; Deacon, 1967; James, 1969;
Arnold, 1970) and for cichlids of the Cuatro Ciénegas area.
Midday depression of activity seems a rule in many desert fish species,
as has been observed for Crenichthys (Deacon and Wilson, 1967), Cyprinodon (Deacon, 1967; James, 1969), and Empetrichthys. Cyprinodon
diabolis, on one hand, seems to actively avoid direct sunlight (James,
1969). C. n. nevadensis in Zzyzx pond, California (LaBounty, 1968),
and a number of other species (Barlow, 1958a; Itzkowitz and Minckley,
445
success of stocking salmonids at high elevations far south of their natural
ranges.
5. Activity Cycles
Study of normal, daily activities of some desert fish has received attention in recent years. Harker (1958) reviewed diurnal rhythms in animals,
and defined two types, as follows: (1) endogenous systems which continue
to operate when environmental conditions are held constant and (2) exogenous rhythms occurring as responses to regular environmental cycles.
Endogenous rhythms of metabolic rates in largemouth bass (Micropterus
salmoides) (Clausen, 1936), Fundulus paravipinnis (Wells, 1935), and
many other fish species (Harker, 1958) have not been demonstrated for
desert fishes, but doubtless exist. Numerous cyprinodontids and poeciliids
from southwestern United States and Mexico, for example, demonstrate
typical spring surges in reproduction (although breeding throughout the
year) under laboratory conditions of constant light and temperature.
Field studies cannot distinguish between the two types of rhythms, but
frequently do permit definition of environmental components affecting
rhythmicity. Exogenous rhythms based on light as the dominant factor
seem a general rule for fishes that are considered diurnal, nocturnal, crepuscular, or even arrhythmatic (Spencer, 1939; Spoor and Schloemer,
1939; Hasler and Villemonte, 1953; Hobson, 1965). Most desert fishes,
particularly those living in springs, seem visually oriented, and are therefore diurnal. The springfishes, Crenichthys baileyi and C. nevadae, have
activity cycles that are light-initiated and dark-inhibited (Deacon and Wilson, 1967; Hubbs et al., 1967), with some inferred modifying influences
by temperature and dissolved oxygen content. Populations of this genus
reduce normal activities in response to oxygen levels below 2 mg/liter,
temperatures above 35°C, or cloud cover. Changes in the normal activity
pattern (generally compression of activity into a relatively short period
of the day) also resulted from apparent stress induced by establishment
of exotic species and/or an increased incidence of parasitism. Similar
light-initiated, dark-inhibited activities have been documented for a number of species of pupfishes (Barlow, 1958a; Deacon, 1967; James, 1969;
Arnold, 1970) and for cichlids of the Cuatro Ciénegas area.
Midday depression of activity seems a rule in many desert fish species,
as has been observed for Crenichthys (Deacon and Wilson, 1967), Cyprinodon (Deacon, 1967; James, 1969), and Empetrichthys. Cyprinodon
diabolis, on one hand, seems to actively avoid direct sunlight (James,
1969). C. n. nevadensis in Zzyzx pond, California (LaBounty, 1968),
and a number of other species (Barlow, 1958a; Itzkowitz and Minckley,