When was the last time you had a stimulating discussion about a new aquatic discovery or read a recent article on aquarium keeping? If you didn't love the hobby of aquarium keeping and appreciate aquatic life, then you probably wouldn't be reading this right now. People get into this pastime as a means of enjoyment, a way to relax and for entertainment. In the process, they make friends with similar interests, learn about animal husbandry and just have fun. I think it is safe to say that all of us have an appreciation for the animals that we keep and want to see them in good health. We rely on one another, as well as other sources of information, to take care of our aquatic friends. Some hobbyists make reading and learning a large part of this hobby (and this is to be encouraged).

Despite our best efforts and intentions there still remain practices that are less than ideal. One of the most questionable practices in aquarium keeping is the use of nets. This all starts long before any fish reaches a private aquarium. For the most part, fish are captured using nets, or nets are used to remove them from a breeder's stock. Then, when they get to a wholesaler, they are generally netted a couple more times and so on, until they have reached their final destination in a home aquarium. To top it off, they are frequently captured with nets by hobbyists. It is easy to see that any particular fish may have been netted on a half-dozen occasions or more in a short period of time.

Most of us have realized that fish do not like to be netted. Just place a net in your aquarium and watch the fish duck for cover. Maybe their instinctual reaction to being captured and netted should tell us something. It is easy to see that nets scare fish, but is there more to it than that? Actually, there are a couple of good reasons not to use nets when catching live fish. First, netting and removing fish from the water induces an acute stress reaction that can take days for them to recover from (Bartelme 2000, Carragher & Sumpter 1990, Klontz 1995, Kreiberg 1994, Rottmann et al 1992, Spotte 1993, Wedemeyer 1976). Secondly, chasing and netting fish causes more injuries to aquarium fish than any other practice (Kreiberg 1994, Spotte 1993). Fish also tend to struggle less when held in a plastic bag or specimen container compared to being held out of the water in a net.

Nets can break fin rays, inflict abrasions and cuts, damage or remove scales, rip fins, and strip away their natural protective layer of mucus. The first question that I always ask about a fish with the condition known as popeye is "Has it been netted recently?" All of the previously mentioned types of injuries may be caused directly by nets. The animal may also injure itself in a frantic effort to avoid being netted. One of the common causes of injury to fish are those inflicted as they bump into objects in the aquarium while being chased with a net (Wedemeyer 1996).

"Death by net" can come in several forms, some of which may not be apparent to an unaware aquarist. The initial response of fish to being chased or netted is that it induces a "fight or flight" response. This stress response has a number of negative consequences that affect the blood chemistry, acid-base balance, immune system function, mucus production, respiration and digestion of fish. It also causes difficulties in osmoregulation (Bartelme 2000, Klontz 1995, Rottmann et al 1992, Wedemeyer 1972).

If what is written above is not reason enough to find another means to capture and transport fish, then consider the consequences of the injuries inflicted by netting. These injuries open fish up to various pathogens that would otherwise not attack them (Pro, 2003). The mucus/scale/skin barrier of fish is a primary form of protection from bacterial, viral, fungal and even parasitic pathogens. This barrier also serves to help fish maintain normal homeostasis and osmotic balance (balance between the internal fluids of fish and the surrounding ambient water)(Bartelme 2000, Klontz 1995). If you lose a fish to some "mysterious disease," consider whether that fish was recently netted. Netting may have been the beginning of troubles or may have simply contributed to the demise of an animal that was already in poor health.

The eyes of most fish do not have a protective lid, and in most species the eyes protrude from the body, making them one of the first things that come into contact with a net. For the same reason, the eyes are also frequently injured as the fish bumps into objects in the aquarium in an attempt to avoid capture. Is it any surprise when fish, that have been recently netted, suffer from a condition known as popeye, or have a bacterial eye infection? This is a serious injury that can cause fish to go blind. When a bacterial infection spreads throughout the body of the fish (this is called sepsis) it often leads to the death of the animal (Kreiberg 1994, Rottmann et al 1992, Spotte 1993).

The stress reaction of fish includes the release of stress hormones. The most well known of these hormones is adrenaline. This hormone is released as part of the acute stress reaction known as the "fight or flight" response. Adrenaline gives the fish a temporary energy boost, but it also affects other normal body functions such as digestion, respiration and maintaining normal homeostasis. 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 while helping fish absorb more oxygen from the water. As the surface areas of the gills expand saltwater fish experience increased ion influx and water loss. At the same time, the increased gill surface means that more energy must be consumed in process of normal osmoregulation. This is what is known as "osmorespiratory compromise" or "respiratory-osmoregulatory compromise" (Bartelme 2000, Brill et al 2001, Folmar & Dickhoff 1980, Mazeaud et al 1977, Nilsson 2000). Immune function is impaired and mucus production is effected; thus, there is less protection available from pathogens (Bartelme 2000, Barton et al 1987, Maule et al 1987, Pickering & Pottinger 1989, Rottmann et al 1992, Spotte 1993, Wendelaar 1997).

Osmorespiratory Compromise:

1. Stress hormones released into the bloodstream cause an increase in heart-rate and an increase in blood flow to the gills.
2. The increased blood flow to the gills results in the dilation of blood vessels in the gills.
3. As the blood vessels in the gills dilate the surface area of the gills is expanded.
4. The expanded gill surface causes an increased influx of ions and water loss in saltwater fish. Conversely, freshwater fish experience an increased influx of water and loss of ions.

"Adrenaline disturbs ion transport at the gill membrane, and both adrenaline and cortisol cause temporary changes in gill permeability which, in fresh water, results in dilution of the blood by excessive gain of water, and vice versa in normal seawater."(Folmar & Dickhoff 1980, Mazeaud et al 1977)

Whether the stress response causes an increase or decrease in mucus production is dependent on the type of stressor. Parasites or toxins in the water can cause an increase in mucus production. However, other stressors can cause a reduction in mucus production because of the stress hormones released into the blood. Keep in mind that the primary stress hormones released in reaction to acute stress are catecholamines (i.e. adrenaline) and cortisol in response to chronic stressors. However both of these types of hormones are released into the blood stream at higher than pre-stress levels in response to stressors that are categorized as either acute or chronic.

"Fish respond to handling as a stress. Their stress response is practically speaking, like our own; upon perception of the stress by the nervous system, adrenaline is released into the bloodstream. This hormone is followed closely by other steroids such as cortisol, which prepare the fish for its reaction, e.g., escape. The result is blood glucose levels, red blood cell counts, heart and ventilation rates all increase, and digestive processes may cease temporarily."(Mazeaud & Mazeaud 1981)

Examples: low pH, toxins in the water and high water temperature are all categorized as chronic stressors. Aggression by other fish, handling, etc., is considered to be acute stressors in fish. Thermal stress can cause as much as a 50% reduction in mucus production. This may be one reason that Cryptocaryon irritans (Ich) outbreaks are so common an occurrence following a drop in water temperature.

Try to avoid removing fish from the water whenever possible during capture and transport. Stress causes an increase in the blood glucose level (hyperglycemia). Exposing fish to air is sufficient to cause asphyxiation hyperglycemia (Spotte 1979). This condition is magnified by an elevated ammonia level, increase in temperature, or low dissolved oxygen concentration in the water and exciting fish during capture or transport (Kreiberg 1994).

Moving a fish quickly when they are netted or exposed to the air is advisable. However, experiments indicate that even when fish are moved in a matter of seconds to an aquarium that is identical in water chemistry and temperature, the blood glucose level may not return to normal for a week (Spotte 1993, 1979).

So how can using a net and the pitfalls associated with this practice be avoided? The first thing to do is to plan ahead. In this way, situations can be avoided that would require catching the fish in a net or with some other method. One of the reasons that people most frequently need to catch fish is because the fish have a disease in the display aquarium. If proper quarantine methodology is practiced, then many of the reasons that people need to catch or transfer a fish will be avoided. Another reason that people often have to catch their fish is because they have made some error in the maintenance of the aquarium. If reading and acquiring knowledge about your hobby are made a priority, then many of the most common mistakes that an aquarist can make will be prevented. I suggest that, for the first six months in the hobby, hobbyists reserve a large portion of their "aquarium budget" for learning all they can about caring for their aquatic friends. This would include reading several books and magazine articles that can be used later as references. Even the well-seasoned hobbyist will benefit from regularly reserving a portion of their hobby budget for learning more about their pastime. It has been said that "knowledge is power" and I feel this certainly pertains to our hobby. Take the initiative to study the dietary needs, natural habitant and compatibility of each species that you are interested in keeping prior to purchasing them. This will help avoid placing incompatible species together that may injure or cause undue stress to tankmates. Know how to provide an adequate environment for them rather than guessing, and plan to place animals together that have similar eating styles.

Despite best efforts, situations will arise that require capturing and moving fish. How can this be accomplished in the least intrusive manner? It may require that some of the rocks or decorations from one end of the aquarium be removed. Additionally, taking out some of the water from the aquarium will reduce the swimming area for the fish. Entice the fish to the end of the aquarium where the rockwork has been removed by tempting them with food. Then, place a partition in the aquarium separating the fish from their hiding places. In the least threatening manner, try to corral the fish into a clear polyethylene bag or specimen container. These bags and containers are difficult for the fish to see and avoid (Spotte 1993). This means they will not become as easily excited or stressed in the process of catching them. A net could be used to encourage them to move in the direction that you want them to go without actually catching them in the net. Fish captured in this manner never have to be removed from the water during transport. These fish have a much greater chance of making a move unscathed, and the odds of them suffering from hypoxia, stress or injury has been greatly reduced (Bartelme 2000, Flagg & Harrell 1990, Iwama et al 1997, Murai et al 1979, Spotte 1993 & 1979). It may take some time, effort and a little practice to perfect the technique of using a polyethylene bag or specimen container for fish capture, but the fish will benefit. They may live longer, healthier lives because of these efforts (Ferguson & Tufts 1992, Iwama et al 1997, Kreiberg 1994, Wood et al 1983).

If you have any questions about this article, please visit my author forum on Reef Central.

For further reading:

"A Survey of Animal Care Considerations for Fish Handling" by Henrik Kreiberg can be found at: http://www.psyeta.org/hia/vol8/kreiberg.html.


Bartelme, Terry. "Understanding and Controlling Stress in Fish: Part One," Freshwater and Marine Aquarium Magazine, February 2000.

Barton, B.A., Schreck, C.B., & Barton, L.D. "Effects of Chronic Cortisol Administration and Daily Acute Stress on Growth, Physicological Conditions, and Stress Responses in Juvenile Rainbow Trout." Diseases of Aquatic Organisms, 2, 173-185, 1987.

Brill, R., Swimmer, Y., Taxboel, C., Cousins, K., Lowe, T. Marine Biology, 2001, 138:935-944. http://www.soest.hawaii.edu/PFRP/pdf/osmoreg1.pdf

Carragher, J.F. & Sumpter, J.P. "Corticosteroid Physiology in Fish," Progress in Comparative Endocrinology. Epple, A., Scanes, C.G. & Stetson, M.H. eds., pp. 487-492. Wiley-Liss, New York, 1990.

Ferguson, R.A. & Tufts, B.L. "Psychological Effects of Brief Exposure in Exhaustively Exercised Rainbow Trout: Implications for "Catch and Release" Fisheries," Canadian Journal of Fisheries and Aquatic Sciences. 49, 1157-62, 1992.

Flagg, T.A., & Harrell, L.W. "Use of Water-to-Water Transfers to Maximize Survival of Salmonids Stocked Directly into Seawater," Progressive Fish Culturist, 52, 127-129, 1990.

Folmar, L.C., & Dickhoff, W.W. "The Parr-Smolt Transformation and Seawater Adaptation in Salmonids (review)," Aquaculture, 21, 1-37, 1980.

Iwama, G.K., Pickering, A.D., Sumpter, J.P., Schreck, C.B.eds. Fish Stress and Health in Aquaculture. Cambridge University Press, New York, NY, 1997.

Klontz, G.W. "Care of Fish in Biological Research," Journal of Animal Science 73: 3485- 3492, February 1995.

Kreiberg, Henrik. A Survey of Animal Care Considerations for Fish Handling, Psychologists for the Ethical Treatment of Animals, Washington Grove, MD, 1994.

Maule, A.G. Schreck, C.B.& Kaattari, S.L. "Changes in the Immune System of Coho Salmon During the Parr-to-Smolt Transformation and After Implantation of Cortisol," Canadian Journal of Fisheries and Aquatic Sciences, 44,161-6 , 1987.

Mazeaud, M.M. Mazeaud, F. & Donaldson, E.M. "Primary and Secondary Effects of Stress in Fish: Some New Data with a General Review," Transactions of the American Fisheries Society, 106, 201-12, 1977.

Mazeaud, M.M. & Mazeaud, F. "Adrenergic Responses to Stress in Fish," Stress and Fish, Pickering, A.D. Ed. Pp.49-75. Academic Press, London, 1981.

Murai, T., Andrews, J.W., & Muller, J.W."Fingerling American Shad: Effect of Valium, MS-222 and Sodium Chloride on Handling Mortality," Progressive Fish Culturist, 41(1), 27-29, 1979.

Nilsson, Stefan. "Cardiovascular Control Systems in Fishes: An Overview," The Journal of Physiology, 523P,pp. 86S, 2000.

Pickering, A.D. & Pottinger, T.G. "Stress Responses and Disease Resistance in Salmonid Fish: Effects of Chronic Elevation of Plasma Cortisol," Fish Physiology and Biochemistry, 7, 253-8, 1989.

Pro, Steven. "Marine Ich/Cryptocaryon irritans: A Discussion of this Parasite and the Treatment Options Available: Part One," Reefkeeping, August 2003, Volume 2, Issue 7.

Rottmann, R.W., Francis-Floyd, R., Durborow, R. The Role of Stress in Fish Disease, Southern Regional Aquaculture Center, 1992. http://www.siu.edu/~readi/aqua/sdisease474fs.pdf

Spotte, Stephen. Marine Aquarium Keeping, John Wiley & Sons, Inc. New York, NY, 2nd edition, 1993.

Spotte, Stephen. Seawater Aquariums - The Captive Environment, John Wiley & Sons, Inc. New York, NY, 1979.

Wedemeyer, G.A. "Handling and Transportation of Salmoniods," Principals of Salmoniod Aquaculture. Pennel, W. & Barton, B., eds., Elsevier Publishing, Netherlands, 1996.

Wedemeyer, G.A., Meyer, F. P. & Smith, L. Environmental Stress and Fish Diseases. TFH Publications, Neptune City, New Jersey, 1976.

Wedemeyer, G."Some Physiological Consequences of Handling Stress in the Juvenile Coho Salmon and Steelhead Trout," Journal of the Fisheries Research Board of Canada, 29(12), 178-1783, 1972.

Wendelaar, Bonga SE, "The Stress Response in Fish," Physiological Reviews 77(3):591- 625 July 1997.

Wood, C.M., Turner, J.D. & Graham, M.S. "Why Do Fish Die After Severe Exercise?" Journal of Fish Biology, 22,189-201, 1983.

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