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Marine Aquarium Fish Aggression

Understanding and dealing with marine fish aggression and conflicts.

By Scott W. Michael

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Aggression among fish, particularly marine species, is a problem that plagues both beginning and advanced aquarists. For example, you buy a new fish and add it to your apparently peaceful community tank and suddenly violence erupts. Long-term residents react aggressively toward a newly introduced fish, causing acclimation difficulties. This can lead to stress, disease and often the subsequent death of the new addition. The purpose of this article is to examine reef fish aggression and suggest techniques that will allow us to reduce conflict in our saltwater aquariums.

In order to understand, and ultimately curb, aggression in reef fish, it is helpful to examine the behavior of these animals in their natural environment. Aggression is "expensive." It requires energy, takes time and makes the aggressor more vulnerable to predators. Therefore, aggression must serve a purpose or natural selection would have done away with it long ago. In the highly diverse reef environment, there are limited resources that many fish must defend to ensure their survival. Reef fish defend food, shelter, mates and offspring from competitors and predators. Many damselfishes, for example, behave aggressively toward members of their own species or other species that feed on algae, the damsel's primary source of food. These damselfishes defend a specific area, known as a territory, from food competitors. One study conducted on the white-tail damselfish (Pomacentrus flavicauda) showed that it attacked 38 different species of fish representing 12 different families (Low, R. M., Ecology 52:648-654).

Anemonefishes defend their place of shelter — their anemone — against competitors, as well as anemone-eating fish. After millions of years of genetic programming, it should not be surprising that these fish do not abandon these behaviors when removed from the reef and placed in a home aquarium.

So, reef fish aggression does serve a purpose in the wild, but how do we squelch this undesirable behavior in the aquarium? There are numerous variables that will determine whether a fish will behave in an aggressive manner. The aquarist can control or manipulate some of these factors to facilitate the maintenance of peace in an aquarium. Let's look at some of these "aggressivity" factors in detail.

One variable, the species of an aggressor, is a factor that most aquarists are very familiar with. Certain species of fish are more prone to being aggressive than others. For example, members of the angelfish genus Holacanthus are often belligerent in a home aquarium. There are, however, always exceptions to the rule within a species, and making generalizations can be dangerous, especially if conclusions are based on the observation of only a few individuals.

Prior residence in probably the most important factor influencing aggression in the home aquarium. Even an apparently docile species can become extremely agitated when a new fish is introduced into a tank in which it has been a long-term resident. Because it is always unwise to introduce all of your marine aquarium inhabitants to a tank within a short period of time, problems of this sort of often inevitable.

The lack of space and shelter will increase aggression. As competition increases, so does aggression. Therefore, if space and shelter are limited, behavioral problems usually occur. Most reef fish use a large area of a reef for their daily activities. For example, the achille's tang (Acanthurus achilles) maintains a feeding territory that covers from 54 to 215 square feet (Barlow, G. W., Am Zool 14:9-34) and a male valentini pufferfish (Canthigaster valentini) may defend an area that is more than 1000 square feet or larger (Gladstone, W., Mar. Biol. 96:185-191). It should be apparent from these figures that space limitations can lead to problems in the confines of an aquarium. Shelter sites are also important. Most fish will defend their place of refuge vigorously, particularly if shelter is in short supply.

The physical characteristics of a potential aggressor's tankmates may determine how it behaves toward them. In their natural environment, reef fish do not indiscriminately attack every fish they encounter, but instead are selective about who they are aggressive toward. For example, the white-tail damselfish primarily attacks intruders that compete with it for food. The obvious question is, how does a damselfish, or any other fish, recognize those species that compete with it? Studies have shown that shape, color and behavior are characteristics fish use to recognize their own and other species.

In certain damselfish, it has been demonstrated that the shape of an intruding fish will determine if the damsel attacks it or not (Thresher, R. E., Anim Behav 24:562-569). Shape is often indicative of the feeding behavior of a fish. Herbivores tend to have a deeper, flatter shape than carnivores, which are built for speed and tend to be more elongate and streamlined. Therefore, instead of being able to recognize every species that enters its territory, the damsel is able to predict, with a great degree of accuracy, the feeding habits of the intruding species by its shape.

Color can also elicit aggression. In a study conducted on captive butterflyfish, it was shown that species with similar color patterns were more aggressive toward each other than toward species that differed in color and/or pattern (Zumpe, D. Z., Tierpsychol 22:226-236). Butterflyfish use color pattern to recognize members of their own species. Keeping in mind that individuals are more aggressive toward members of their own species (due to higher levels of competition), it makes sense that captive butterflyfish are more likely to attack fish with similar markings. For example, a thread-fin butterfly (Chaetodon auriga) is more likely to attack a vagabond butterfly (Chaetodon vagabundus) than a collaris butterfly (Chaetodon collaris) because a vagabond looks more like a thread-fin.

Some fish not only cue in on shape and color but also on behavioral characteristics. Thus, a damselfish may be able to tell if an intruder is a food competitor by how it feeds (Losey, G. S., Anim Behav 29:1271-1272). If a species feeds in a characteristic herbivore manner — it nips the substrate, backs off and then nips the substrate again — the damsel will attack it and drive the fish out of its territory.

The size of an intruder may also determine if it is attacked, avoided or ignored. One damselfish species is known to attack fish that are 2½ inches in length or longer, while fish smaller than this are ignored (Kohda, M. Z., Tierpsychol 56:205-216). It is probably not worth the damsel's effort to chase a fish that will consume only a meager amount of food. When a fish exceeds a particular length and has the appropriate shape (i.e., that of a predator), it will be avoided. Except in the case of certain species of damsels and tangs, larger fish are usually avoided by smaller residents in an aquarium.

Finally, in the case of aggression within a species, sex can determine whether fighting will occur. In the harem-forming pygmy angels (Centropyge spp.), males welcome females into their territories, but bachelor males are viciously expelled.

How, then, does all of this relate to the maintenance of reef fish in captivity? I have provided some guidelines below that demonstrate practical applications of the observations above. By using these techniques, an aquarist should be able to minimize aggression in his or her aquarium.

Before stocking your aquarium, read about the fish that are available to you and learn as much as you can about their dispositions. Also, talk to a reputable retailer about the behavioral traits of potential selections. Keep in mind that many people are hasty in forming conclusions about a species even though they have kept only one or two specimens.

Write down a list of the species you would like to maintain and then select and introduce them to the tank in a sequence in which the least aggressive fish is added first. It is also advisable to add the largest fish last, because, in most cases, tank residents are less likely to be aggressive to a larger intruder. As already noted with damselfishes, however, this is not always the case.

Provide as much aquarium space as possible for each tank inhabitant, especially if some of the species are known to be aggressive. Overcrowding your aquarium will lead to profuse amounts of fighting.

There should be a large number of hiding places in the tank. This doesn't simply mean to load the tank with coral or live rock. Make sure there are caves, crevices and holes suitable for all members of your aquarium community. Also, utilize topography to curb aggression. Using a number of smaller coral heads or groupings of live rock rather than a continuous wall of them can be advantageous in dispersing territory holders and reducing skirmishes among them. A territorial fish will often restrict its defense to one coral head, which is the core area of its territory, and the immediate area around it. If the aquascaping structure is contiguous, more battles over boundaries are likely to occur.

Avoid keeping members of the same species in the same aquarium unless they are known to school or group in captivity. Some species that school on the reef may not tolerate each other in the confines of a smaller tank. Even in schools, individuals usually maintain a specific distance between each other, and often in a home aquarium these spacing requirements cannot be met. The size of the tank is important in determining if you will be successful in keeping species members together.

If you intend to keep two fish of the same species together, it is often wise to introduce them into your aquarium at the same time. Your success rate may also improve if they are of the opposite sex. In many reef fish, it is difficult to differentiate the sexes, but in some species there are color and other physical differences. For example, in the orchid dottyback (Pseudochromis fridmani), the male is more colorful and has a more elongate tail, and in many of the dragonets (i.e., the mandarin fish, Synchiropus splendidus) the male has an elongate first dorsal spine.

Many of the species we keep will change sex depending on their social status. Often, a larger, more dominant individual will be one sex and the smaller, subordinate specimens in the group will be the opposite sex. To increase the likelihood of acquiring a pair in a sex-changing species, purchase two individuals of different sizes. If a subordinate individual is removed from its natural environment and isolated in a dealer's tank, it may begin to change sex. Once an individual has changed sex, it cannot be reversed. Therefore, if you place two individuals in the same tank and fighting occurs, it could be that the fish are both the dominant sex and will have to be separated. This technique is especially effective when trying to keep species such as clownfish or pygmy angelfish.

Avoid keeping similarly shaped or colored species together, particularly if one of them is known to be overly aggressive. If the two fish are in the same genus, it indicates that they are physically similar — this knowledge can help you choose fish. It would be wiser, for example, to keep a yellow tang (Zebrasoma flavescens) with a power-brown surgeonfish (Acanthurus japonicus) than with a scopas tang (Zebrasoma scopas), because the yellow tang and the powder-brown are in different genera (they are less similar in shape). This technique, and the one on pairing opposite sexes, are particularly valuable if you have a tank of less than 100 gallons in size.

If problems do occur when you introduce a new fish, there are several techniques you can use to eliminate or minimize the hostility. Turning off all lights will temporarily reduce aggression, unless, of course, the aggressor is a nocturnal species. Fish are often less aggressive on a full stomach, so feeding your fish can sometimes be effective for short-term behavioral modification.

If these simple procedures are ineffective, then more drastic steps will be necessary. Because many territorial fish remember the boundaries of their territory by the topographical features of their habitat, changing the aquascaping around immediately before and after introducing a new fish can reduce aggression. In effect, changing the topography is the same as moving the old residents of the tank to a new area.

Another method for introducing a new fish into a more aggressive community tank is to partition a portion of the aquarium off, creating an area where you can isolate the new resident. The partition should be clear, such as plexiglass, so that the older residents can see the new fish and habituate to its presence. Depending on the type of filtration used, it may be necessary for the partition to contain numerous holes and the top to be slotted to allow water exchange between the two compartments. This isolation period allows the new fish to become familiar with a part of the tank so that it can more easily find a place to hide, if necessary, when the partition is removed. After a week or so, the partition can be removed. With any luck, aggression will be minimal.

Another option is to pull the aggressor out of the tank, isolate it in a quarantine tank and then reintroduce it after the new arrival has settled in. If all of these techniques fail, you will probably have to permanently remove the aggressive individual(s) or give up the idea of introducing more fish into the tank.

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