Like tourists going on holiday to the Dead Sea, endangered naked carp(Gymnocypris przewalskii) migrate annually between freshwater rivers where they spawn, and salty Lake Qinghai, where they take a break to feed and grow. Lake Qinghai, the largest lake in China, is located over 3000 m above sea level, but may soon become a toxic destination for the carp. The lake is continuously drying up and becoming ever more saline. Diversion of water for agriculture as well as climate change have led to a decline of the lake's water levels by about 10–12 cm per year during the past 50 years. With these problems in mind, Chris Wood from McMaster University, Ontario, and his eight collaborators from Canada and China were interested in how naked carp, a very sensitive, slow growing species which feeds on plankton, adapts to living in the saline lake's harsh environment.
To have a closer look how the fish cope with living in such salty waters,the international team measured the fishes' metabolism, water balance and excretion, both on-site in China and in the lab in Canada. To find out how the carps' metabolism was affected, they transferred fish from fresh river water to salty Lake Qinghai water and observed that naked carp also take a metabolic holiday in the salty lake water. In the first 36–48 h after the transfer, Wood and his team saw that naked carps' oxygen consumption was reduced to 60% of the oxygen uptake seen in carp kept in river water. This change reduces the carps' energy demands by 40% while they are in lake water.
Switching their attention to the effect of the transfer between river and lake water on water balance and excretion, they measured the function of the gill and kidneys, by looking at the activity levels of the sodium-potassium pump, an enzyme called the Na+,K+-ATPase. This enzyme regulates ion transport into and out of the cell and therefore plays an important role in water balance. They found that enzyme activity in salty fish was reduced by 50–70%, which would also lower the carp's metabolic costs further since this enzyme uses a lot of ATP to function. This reduction in enzyme activity affected the level of electrolytes, such as salts, in the blood: the researchers found that lake-water-acclimated fish had blood concentration levels of electrolytes very close to the ion concentrations of the saline lake. This suggests that naked carp equilibrate their blood plasma ionic concentration to match the salty waters in Lake Qinghai, by reducing the activity of ion pumps in the gills and kidneys.
But how would such salty blood affect the fishes' kidneys? Wood and his colleagues looked at the work done by the kidneys in response to the ionic challenge caused by the lake water. They found that in river water the kidney excreted water more efficiently than salt. However the kidneys of fish kept in the salty Lake Qinghai water conserved water better, excreting more salt. This observation indicates that carp adapt their metabolism as well as gill and kidney function to cope with the dangers of excessive water loss and a surplus of ions in their cells caused by salty water. Wood and his team show that naked carp can survive in Lake Qinghai, despite its saltiness. However, they warn that as the lake is getting ever more saline, all the carps' adaptations might not be enough to help them cope with the salty water, with lethal results.
