VII. DESERT FISHES
469
In summary, foods used by desert fishes provide a pattern of increasing
restriction in variety in the diet as ichthyofaunal diversity increases. Tolerant forms living alone usually are ubiquitous in feeding. Kinds living in
faunistically more saturated habitats exhibit increased specialization of
feeding niches.
One marked difference in foods available in low desert streams is the
general absence of allochthonous detritus. Input of vegetative debris is
obviously far more important in more mesic zones, and may provide a
major food base in aquatic habitats of such regions (Hynes, 1961;
Minckley, 1963; Darnell, 1967; Minshall, 1968). Allochthonous animals,
however, may provide a major part of the food base.
High water temperatures in deserts may create exceptional nutritional
problems for fishes. Metabolic demands at high ambient temperatures must
be intense, and a number of authors have noted a maximum in feeding
activities at higher environmental temperatures (Barlow, 1961b; Deacon,
1968a; Lowe and Heath, 1969), so long as the temperatures do not rise
high enough to suppress activity (Deacon, 1968a) or to initiate an avoidance reaction (Barlow, 1958a).
Barlow (1961b) suggested that high temperatures demand almost continuous feeding because of the elevated metabolic activity, and Kinne
(1960) documented more than a 24-fold increase in food intake of desert
pupfish as temperature was increased from 15° to 30°C. However, conversion efficiency declined markedly as temperatures were increased above
20°C. Foods passed through the digestive tract in a nearly undigested state.
Leser and Deacon (1968) obtained similar results at even cooler temperatures and also under conditions when fish were supplied an excess of food.
The situation when desert fish cannot digest and assimilate adequate food
may be aggravated in a territorial male pupfish, for example, to such a
degree that its incipient lethal tolerance may be lowered. This may explain
the deaths of only larger, mature, brightly colored males, under certain
conditions of hyperthermia.
Lowe and Heath (1969) suggested that high food intake at high midday
temperatures might be adaptive, in that lower temperatures at evening
would allow conversion to proceed at a more efficient level. Studies of
C. n. nevadensis (Leser and Deacon, 1968, and subsequent information)
have shown complete assimilation of food to occur in about 16 hours at
22°C. Thus, even at higher thermal levels, food would likely remain in
the gut long enough for the normal diel temperature cycles to permit its
efficient use by the fish. It seems probable that, especially for young, maximum conversion efficiency is less important than maximum rate of growth.
The latter is achieved in C. macularius at 30°C with unrestricted food
intake. Maximum conversion efficiency in that species was at 20°C and
469
In summary, foods used by desert fishes provide a pattern of increasing
restriction in variety in the diet as ichthyofaunal diversity increases. Tolerant forms living alone usually are ubiquitous in feeding. Kinds living in
faunistically more saturated habitats exhibit increased specialization of
feeding niches.
One marked difference in foods available in low desert streams is the
general absence of allochthonous detritus. Input of vegetative debris is
obviously far more important in more mesic zones, and may provide a
major food base in aquatic habitats of such regions (Hynes, 1961;
Minckley, 1963; Darnell, 1967; Minshall, 1968). Allochthonous animals,
however, may provide a major part of the food base.
High water temperatures in deserts may create exceptional nutritional
problems for fishes. Metabolic demands at high ambient temperatures must
be intense, and a number of authors have noted a maximum in feeding
activities at higher environmental temperatures (Barlow, 1961b; Deacon,
1968a; Lowe and Heath, 1969), so long as the temperatures do not rise
high enough to suppress activity (Deacon, 1968a) or to initiate an avoidance reaction (Barlow, 1958a).
Barlow (1961b) suggested that high temperatures demand almost continuous feeding because of the elevated metabolic activity, and Kinne
(1960) documented more than a 24-fold increase in food intake of desert
pupfish as temperature was increased from 15° to 30°C. However, conversion efficiency declined markedly as temperatures were increased above
20°C. Foods passed through the digestive tract in a nearly undigested state.
Leser and Deacon (1968) obtained similar results at even cooler temperatures and also under conditions when fish were supplied an excess of food.
The situation when desert fish cannot digest and assimilate adequate food
may be aggravated in a territorial male pupfish, for example, to such a
degree that its incipient lethal tolerance may be lowered. This may explain
the deaths of only larger, mature, brightly colored males, under certain
conditions of hyperthermia.
Lowe and Heath (1969) suggested that high food intake at high midday
temperatures might be adaptive, in that lower temperatures at evening
would allow conversion to proceed at a more efficient level. Studies of
C. n. nevadensis (Leser and Deacon, 1968, and subsequent information)
have shown complete assimilation of food to occur in about 16 hours at
22°C. Thus, even at higher thermal levels, food would likely remain in
the gut long enough for the normal diel temperature cycles to permit its
efficient use by the fish. It seems probable that, especially for young, maximum conversion efficiency is less important than maximum rate of growth.
The latter is achieved in C. macularius at 30°C with unrestricted food
intake. Maximum conversion efficiency in that species was at 20°C and