Monday, May 4, 2015


   Reef fishes, like this dwarf hawkfish, comes into the tank with behavioral baggage that can cause problems in a captive fish community. Photo by Scott W. Michael. 

One of the biggest problems encountered by marine fish keepers is aggression. It can cause disease and death, especially in newly introduced fishes. So how do we deal with it in our aquariums? There are several phenomenon related to animal ethology that we need to “unpack” before we try and solve our problems with “tank rage.”

The first of these is displacement behavior. Animal behaviorists have found that when the level of anxiety rises in an animal (including fishes) they may engage in out-of-context or irrelevant response as a result. For example, place a clear acrylic divider in a tank that contains a highly territorial species, like a Fiji blue devil (Chrysiptera taupou) – a particularly pugnacious damselfish. Now place a conspecific on the other side of the acrylic. The resident fish will display and attempt to get at the intruding blue devil. It will display and nip at the glass anytime the conspecific moves into view. As aggression and frustration escalates, the damsel may then engage in displacement behavior. This may include attacking a shrimp that shares its side of the tank or biting at the Caulerpa that grows throughout the aquarium. These behaviors do not help it to achieve its ultimate goal (that is, to get the intruder out of its territory), but by attacking the crustacean neighbor or “punishing” a plant, it releases some of the accumulating anxiety.

As time passes, the Fiji blue devil’s attempts to push its way through the clear acrylic divider will become more half-hearted. Agonistic displays will decrease in number and intensity and the frequency of displacement behaviors will subside. After a certain period of time (which is usually a function of the species in question and possibly the target of its aggression), it will get use to the releasing stimulus (that is, the intruder). This is known as habituation.   

We can use habituation to our advantage when adding a new fish to a established fish community. You have quarantined your new fish and are ready to add it to your tank. But you are worried that upon entering its new environment, the newcomer will be accosted by the resident bullies. You can habituate aggressive residents to a newcomer by allowing the former to see, but not have access to the fish to be added. In the example above, I mentioned an acrylic divider. If your tank is small enough and you can section off a portion of the tank for the “newbie,” a divider like this will work well. Make sure the divider is perforated so that oxygenated seawater readily passes between the sectioned-off portion of the tank and the main aquarium. Of course, it is also important that the divider is see through. While reef fish can detect each other via other sensory modalities, visual contact is important to ensure habituation.

Another way to facilitate habituation is to place the new fish in a plastic basket or food storage container (e.g., Tupperware) that float at the surface of the tank. In the case of the basket, the holes should not be large enough for the acclimating fish to escape from but must be large enough so visual contact can occur between the “greenhorn” and the “veterans.” If you use the plastic storage container, it should be clear and must have holes drilled in it so water can freely circulate between it and the tank. If you can place it in a corner of the tank that is buffeted by pump output, there is more likelihood good water exchange will occur between the container and the tank. You can also use clear breeding traps that hang on the inside of the tank. These are typically designed to house freshwater livebearers, which are small. Therefore, they will only work for more diminutive marine species (e.g., dottybacks, damsels, gobies, blennies). Whatever type of “habituation pen” you decide to use, keep the new fish in the container for several days or until the aquarium residents consistently ignore it.

Since a territorial species will recognize its home by certain physical features, it is not a bad idea to rearrange the aquarium décor when adding a new fish to a tank that contains a potentially pugnacious resident. By confusing the “old-timer” the newly added fish may have a better chance of getting its own chunk of ground before the established resident gets reacquainted with its surroundings. Of course, many of us are not willing to rearrange our reef tank every time we add a new fish. We will have to rely on other techniques.

Remember that if important resources, like food and shelter, are in short supply, levels of aggression are likely to escalate. So, frequent feeding and plenty of shelter sites can be helpful in curbing some fish aggression. Space is also the aquarist’s best friend. The larger the tank, the less likelihood that aggression is going to cause you persistent headaches. One final thought, you should have a good fish trap at your disposal because sooner or later you are likely to have to remove a piscine sociopath.

©  Scott W. Michael- Reef Tectonics

 Reef Tectonics Aquarium Maintenance and Design - Lincoln, Omaha, Des Moines, Kansas City 

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