I researched the issue of stress for an article some time ago, and it may be instructive to post it here. I think the evidence shows that we can never assume the fish are "thriving" or even healthy from outward observation because of the complex life processes. The article is lengthy, but here it is.
Stress and Freshwater Aquarium Fish
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. In a very real sense, we are directly responsible for any and all stress inflicted upon the fish. Later we’ll consider how this occurs, but before that we must understand what stress is and how it harms our fish. 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.
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).
How Stress Affects Fish
Stress is caused by placing a fish in a situation which is beyond its normal level of tolerance (Francis-Floyd, 1990). Stress makes it more difficult for the fish to regulate the normal day-to-day physiological functions—the homeostasis—that are essential to its life. Dr. Cliff Swanson, associate professor at North Carolina’s College of Veterinary Medicine, says that stress creates “a fundamental physiological shift in fish, from energy storage to energy usage—the fight or flight response” (Muha, 2005). The survival of any organism depends upon its ability to keep its internal chemical balance from fluctuating too much. When critical energy is being used to fight stress, it is diverted away from other functions. The fish must then work much harder just to “keep going.” Laura Muha (Muha, 2006) likens this to driving a car up a steep hill: it takes more gas (energy) and effort to maintain the same speed as on level ground (level being the norm for the fish).
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 slime coat or mucus layer is a physical barrier that inhibits entry of disease organisms from the environment into the fish. But it is also a chemical barrier because it contains enzymes (lysozymes) and antibodies (immunoglobulin) which can kill invading organisms. Mucus also lubricates the fish which aids movement through the water, and it is also important for osmoregulation (Francis-Floyd, 1990).
This deteriorated slime coat, along with a lowered immune response, is what allows parasites, bacteria, pathogens, protozoans and fungi to infect the fish and cause disease. More water enters the cells, further impacting osmoregulation and increasing work for the kidneys. The effects of stress on the immune function can linger for some time after other physiological changes have returned to pre-stress levels (Maule et al., 1989).
There are really only two requirements for a disease to occur in a fish: first, the pathogen must be present; and second, the fish must be under sufficient stress to weaken the immune system to enable that pathogen to actually cause an infection. The very few pathogens that can cause an infection without any stress in the fish are quite rare. Clearly, the best disease prevention is to eliminate stress as much as possible
The Causes of Stress
[There is more to this section, but for brevity I will just include the summation.]
Each species of fish has evolved over thousands of years to live in a particular environment. The water chemistry along with the environmental factors of the habitat are crucial not only to the life of the fish but to the state of its health during that life. As we learned above, the fish’s homeostasis only functions well within the species’ natural environment. A normal lifespan is virtually impossible if the fish’s environmental needs are not met to some extent. For instance, if one intends to house tropical forest fish, “the chemical and physical properties of aquatic environments associated with rainforests must be duplicated, or at least approximated, in order to keep these fishes in the best of health” (Weitzman et al., 1996).
References:
Bartelme, Terry D. (2004), “Short Take: Stress In Fish, Part II: Why You Should Care About Stress In Fish,” Advanced Aquarist online:
http://www.advancedaquarist.com/2004/9/fish2
Drs. Foster & Smith LiveAquaria, Stress and Fish Health:
http://www.liveaquaria.com/PIC/article.cfm?aid=88
Evans, Mark E. (2004), “The Ins & Outs of Osmosis,”
Tropical Fish Hobbyist, February 2004.
Francis-Floyd, Ruth (1990), “Stress—Its Role in Fish Disease,” CIR919 (December 1990), Fisheries and Aquatic Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Maule, A.G., R.A. Tripp, S.L. Kaattari, & C.B. Shreck (1989), "Stress Alters Immune Function and Disease Resistance in Chinook Salmon (
Oncorrhynchus tshawytscha),"
Journal of Endocrinology, 120, 135-142, 1989.
Muha, Laura (2005), “Stress” in “The Skeptical Fishkeeper” column,
Tropical Fish Hobbyist, December 2005.
Muha, Laura (2006), “Fish Growth vs. Tank Size” in “The Skeptical Fishkeeper” column,
Tropical Fish Hobbyist, December 2006.
Narten, Thomas, “Fish Stress and Healthy Fishkeeping,” The Aquaria FAQ at
http://fins.actwin.com/mirror/
Wedemeyer, G.A. (1996), "Transportation and Handling," in
Principles of Salmonid Culture, W. Pennell and B.A. Barton (eds.), pp. 727-758. Cited in Bartelme (2004).
Weitzman, Stanley H., Lisa Palmer, Naercio A. Menezes & John R. Burns (1996), “Maintaining Tropical and Subtropical Forest-Adapted Fishes,”
Tropical Fish Hobbyist, June 1996.
