Hi all,
I'll remember what Scatz said, so this really will be my last post on this thread. JoePlec wrote: That is exactly what we are saying, an individual fish doesn't, & can't, adapt outside of very limited parameters.
All living organisms have a range of conditions in which they can survive. I'll use humans and altitude/oxygen content as my example, as it is an example that every-one can relate to.
Altitude sickness - Wikipedia, the free encyclopedia For most people they begin to suffer some effects of altitude at above about 8000ft. However people who have always lived at these altitudes (or most sportsmen doing "high altitude training") will grow more oxygen carrying red blood cells when living at high altitude allowing them to function more normally. Some people from will not have the genes available to produce more red blood cells, and they will eventually become stressed and may die if they don't descend to levels where the atmosphere is more oxygen rich. They don't have the capability to
adapt to lower oxygen levels.
"Normal" peoples red cell count will decline after a while at sea level, but people with genetic descent from high altitude tribes will retain elevated levels of red blood cells at sea level. There high red blood cell count is genetic and they won't grow less cells even at sea level. At high altitude over generations natural selection had winnowed out all those who don't have this capacity, they were less genetically "fit". If you wonder why all the successful long distance runners in the Olympics are Kenyan or Ethiopian (step forward Haile Gebrselassie), or Himalayan porters are Sherpas, it is because they come from people who live, and have lived for 100's of generations, at 9000ft.
You might wonder why everybody doesn't have more red blood cells in their blood if it offers advantages at altitude, the answer is there is a down-side at sea level, more red blood cells makes your blood "thicker" and raises the risk of thrombosis, strokes etc.
There is also an absolute limit to the height where humans can live, quote from Wikipedia "
Finally, the death zone, in mountaineering, refers to altitudes above a certain point where the amount of oxygen cannot sustain human life. The point is generally tagged as 8,000 m (26,000 ft) [less than 356 millibars of atmospheric pressure]. Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly (loss of vital functions) or indirectly (wrong decisions made under stress, physical weakening leading to accidents). In the "death zone", no human body can acclimatize. The body uses up its store of oxygen faster than it can be replenished. An extended stay in the zone without supplementary oxygen will result in deterioration of body functions, loss of consciousness, and ultimately, death"
Go on you saying why is this relevant to fish? It is exactly the same process, a fish that has evolved in stable conditions usually has next to no genetic variability, it can't adapt to anything. It is maximally genetically fit in the "extreme" conditions of a black-water river or Lake Tanganyika, and totally unfit for life anywhere else.
You can keep common Bristlenoses or Cardinal Tetras in "unsuitable" water because there was some
genetic variation to work with initially, and
natural/artificial selection over generations has selected those fish that survive to breed in water with higher pH and carbonate buffering. If you got a wild Cardinal tetra, it will still be as difficult to keep as the first ones imported.
cheers Darrel
