I'm following this thread, and would like to make a couple of observations on something that has been hinted at. The video that Nick posted brought this to my notice.
The young pacu in the first half of that video is under significant stress. This is obvious from its behaviour. Being on its own without the shoal that it "expects" is the probable cause, though water parameters and other aspects of the environment can factor in and we don't know what those may be here. But the fish is clearly under stress, and this weakens a fish in several ways. First, it affects the immune system. Then it can affect the fish's development, both externally (size) and internally (organs). The aggressiveness (both increasing or decreasing from the norm) is almost always impacted. And the fish will not live to its normal expected lifespan. All this is a biological fact.
Since the discovery of DNA in the 1970's, ichthyologists have learned much about fish that was perhaps assumed but certainly not confirmed previously. The green citation from Dr. Paul Loiselle, a leading authority on cichlids, in my signature block is what I am referring to; fish "expect" many things because evolution has programmed these expectations into the DNA of each species. When any one of the "expectations" is not present, it causes stress. We now know that like all animals, stress impacts a fish significantly. I alluded to some of this above. What is now certain is that the homeostasis is impacted by all environmental factors when these are outside the norm for the species.
Homeostasis is defined as “the tendency of an organism or a cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions.” Physiological homeostasis, or physical equilibrium, is the internal process animals use to maintain their health and life: “the complex chain of internal chemical reactions that keep the pH of its blood steady, its tissues fed, and the immune system functioning” (Muha, 2006).
Here is how Biology Online defines stress: The sum of the biological reactions to any adverse stimulus—physical, mental or emotional, internal or external—that tends to disturb the organisms homeostasis; should these compensating reactions be inadequate or inappropriate, they may lead to disorders.
Few scientific studies have been done on all this in the past, but those that are now being undertaken have confirmed what many have always assumed but been unable to document. As one example, a study on the size of the group for shoaling species proved that with less than five in the group, fish like angelfish and black neon tetras (these were two species studied) became much more aggressive; naturally aggressive species (like the angelfish) showed significant increase in the aggressive behaviours, while normally peaceful species (like the tetras) became aggressive.
The point of all this is that we cannot change nature (you would think with all the environmental destruction and climate change today we would begin to understand this fact). If we want healthy fish, we must understand their needs and provide them, reasonably closely. It is, as Dr. Loiselle says, inhumane to do otherwise. The pacu from that video has obviously managed to "live," but at what cost? Nathan Hills' comment in blue in my signature block says it; a fish surviving is not the same as a fish thriving, and the latter should be our constant goal. The fish cannot tell us verbally how it feels about what we force upon it, so common sense dictates that we should take the advice of ichthyologists and biologists and provide what the fish "expects," learning this from its habitat.
