JD's post is about as good as it gets for a summation of this issue, and I am in the same group of aquarists. I am not a professional biologist/ichthyologist; I've been in this hobby for 20+ years, but since taking early retirement in 2007 due to cancer (nasty, but fortunately in my case not life-threatening so far) I have had the time to delve into this more than previously. Over the past six years I have carried out a lot of research into fish species and habitats, and I have some observations to support much of what JD surmised.
The question of fish managing (JD's "surviving") as opposed to thriving is crucial. A German study by Rolf Geisler and Sergio R. Annibal on Paracheirodon axelrodi (cardinal tetra) back in 1984 was translated into English and appeared in August 1987 issue of TFH under the title "Ecology of the Cardinal Tetra, Paracheirodon axelrodi (Pisces, Characoidea), in the River Basin of the Rio Negro, Brazil, as well as Breeding-related Factors." Summarizing the habitat waters of this widely-distributed species they gave a pH range from 3.97 to 5.1, a general hardness range from 0 to 0.03 dH, and a conductivity from 3.4 to 41 siemens. Fish from these habitats when placed in water with higher and varying hardness had lifespans that basically related to the increase in GH. Dissection indicated that death resulted from calcium blockage of the kidney tubes, and this became more severe with a shorter lifespan the higher the GH. The study surmised that after gradual acclimation over more than five generations, the species was better able to manage up to a point, though spawning did not occur. Dr. Stanley Weitzman and a team of biologists carried out studies on different forest fish species and a major portion of their two-part article (TFH, June-July 1996) deals with providing suitable water for these fish.
But there is more to this, and here we come to stress. Stress is the root cause of almost all disease and health problems of aquarium fish. Today we recognize that the health of any living organism is directly related to the level of stress inflicted upon it; for fish this is a major problem because the fish cannot do anything to reduce or eliminate it—they can only fight it or succumb to it. Our fish are confined to the small space of their aquarium, and only the aquarist can control their environment. 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.
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).
Four important body functions of homeostasis are closely associated with processes in the gills: gas exchange, hydromineral (osmoregulation) control, acid-base balance [pH] and nitrogenous waste excretion [ammonia]. These processes are possible because of the close proximity of the blood flowing through the gills to the surrounding water, as well as the differences in the chemical composition of these two fluids (Bartelme, 2004). Each species of fish has evolved within a specific environment—and by “environment” in this context we mean everything associated with the water in which the fish lives—and the physiological homeostasis only functions well within that environment. This greater dependence upon their surrounding environment is why fish are more susceptible to stress than many other animals (Wedemeyer, 1996).
The effects of stress on fish are very complicated physiologically, and are often subtle. There may or may not be external signs discernible to us—it can continue for weeks and even months, sometimes up to the point when the fish just suddenly dies. The reasons for this are involved.
Adrenaline released during the stress response increases blood flow to the gills to provide for the increased oxygen demands of stress. The release of adrenaline into the blood stream elevates the heart rate, blood flow and blood pressure. This increases the volume of blood in vessels contained within the gills, increasing the surface area of the gills to help the fish absorb more oxygen from the water. The elevated blood flow allows increased oxygen uptake for respiration but also increases the permeability of the gills to water and ions. This is what is known as the osmorespiratory compromise (Folmar & Dickhoff, 1980; Mazeaud et al., 1977). In freshwater fish, this increases water influx and ion losses. This is more critical in small fish than larger due to the gill surface to body mass ratio (Bartelme, 2004).
Short-term stress will cause an increase in heart rate, blood pressure, and respiration as described in the preceding paragraph. The fish can only maintain these altered states for a short and finite period of time before they will either adapt or (more often) the stress will become chronic. During this initial stage the fish may look and act relatively normal, but it is depleting energy reserves because of the extra physiological requirements placed upon it. At the chronic stage the hormone cortisol is released, which is responsible for many of the negative health effects associated with stress.
One of the most characteristic aspects of stress in fish is osmoregulatory disturbance, which is related to the effects of both catecholamine and cortisol hormones. The extent of the disturbance following stress depends upon the ionic and osmotic gradients (difference) between the internal fluids of the fish and its surrounding environment (water)—something we will explore in more detail later. If the stress is persistent and of sufficient intensity, changes in the cellular structure of the gills may occur under the influence of cortisol. In this situation, increased death and turnover rates of branchial epithelial cells leads to accelerated aging of the gills. These degenerating and newly-formed gill cells do not function normally, which further limits the fish's ability to maintain water and ion homeostasis under stressful conditions. Thus, acute stress limits the fish's capacity to osmoregulate, and prolonged periods of extreme stress may result in osmotic shock and death (Bartelme, 2004).
Chronic stress impacts negatively on fish growth, digestion, and reproduction. It is the main cause of deterioration in the slime coat. It significantly lowers the ability of the immune system to respond effectively and fully. And in all cases—stress reduces the fish’s lifespan.
The above is mainly copied from an article I wrote a couple years back, and there is much more but I won't go into that unless questions arise pertinent thereto.
Byron.
/www.mbari.org/staff/vrijen/PDFS/Vrijen_1998JFB.pdf
