Abstract
The tilapia Oreochromis alcalicus grahami is a unique ureotelic teleost, the only fish that lives in the alkaline hotsprings of Lake Magadi, Kenya. Physical conditions and fish behavior were monitored in the Fish Springs Lagoon area, a site where the tilapia were particularly abundant. Water Po₂ and temperature fluctuated more or less in parallel in a diurnal cycle from less than 20 Torr and less than 25° C at night to greater than 40° Torr and 38° C during the day, whereas pH remained constant at approximately 9.8. Field laboratory tests demonstrated that routine $\dot{M}o_{}$ (under normoxia) increased greatly from 27° C to 36° C ($Q_{10}$ = 62) but then stabilized at a very high level (~34.5 μmol $g^{-1}$ $h^{-1}$) up to the lethal temperature (~ 42.5° C), a pattern that was adaptive to the natural diurnal regime. The Po₂ threshold for survival during acute exposure (≤ 1 h) was approximately 16 Torr. $\dot{M}o_{2}$ from water was well maintained down to a Po₂ of 60 Torr, below which it declined. Under ...
Highlights
The tilapia Oreochromisalcalicus grahami is the only fish species known to exist in the highly alkalinepools fed by geothermalspringsat LakeMagadi in the Rift Valley of Kenya (Coe 1966)
Oreochromisalcalicus grahami were present at all 14 sites, sometimes in such quantity that the water appeared dark in areas where the fish congregated in large masses
No other fishis able to live in this particularcombination of very high pH, very high alkalinity,and extreme diurnal fluctuationsin dissolved 02 (DO) and temperature
Summary
The tilapia Oreochromisalcalicus grahami is the only fish species known to exist in the highly alkalinepools fed by geothermalspringsat LakeMagadi in the Rift Valley of Kenya (Coe 1966). Small adult Oreochromisalcalicus grahami (1-4 g) used in most experiments were collected with a seine in Januaryand February,1992, from the Fish Springs Lagoonsite on LakeMagadi,Kenya,described by Coe (1966). Small adult fish (1-4 g) were loaded into Tusker chambers filled with continuously aerated lagoon water at approximately 270C and allowed to settle for 30 min. In the course of a separate study comparingurea excretion and Mo2 (Wood et al 1994), Mlo2measurements were made by closed-system respirometry in 530-mLTusker chambers on several hundred small adult fish (1-5 g) at 300-33'C Seven of these fish died because the water Po2 was accidentally permitted to fall too low. Small adult fish (2-5 g; N = 7) were allowed to settle for at least 1 h as the respirometers were flushed with water of Po2 greater than 120 Torr at 310-320C from the head tank (as described above). Radiometer pH (E5021) and CO2electrodes (E5036) were used for these determinations and displayed on the same RadiometerpHM71 meter as the 02 electrode
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