Understanding territories

The territorial nature of some fish species can be the bane of the uninformed fishkeeper. The wrong mix of fish can escalate into all-out war, leaving the losers huddled in a corner or worse yet, dead. While this behaviour is woven into the inner workings of many species, it is a challenge that is possible to overcome once one understands the concepts behind territoriality, and why it is so important to accommodate this behaviour properly.

We don’t have to stretch our minds too far to think like a fish. Humans understand territory too well — it’s generally frowned upon to hop a fence or cross a border without express permission. People value what’s on their side of the wall, and will go to great lengths to keep others out.

Many fish are wired a similar way. Much like food supplies and breeding partners, territories are just one of many resources available to a fish in nature. Within the bounds of a territory are safety from predation, a home for a mate, a substrate to make a nest or lay and guard eggs, and an environment conducive to the survival and rearing of vulnerable young.

The aggression associated with territoriality often shapes our perceptions about the overall behaviour of a species. It is our lack of understanding of the nature of territories that leads to rather unfortunate incidents between our pets - and this is largely avoidable.

What is a territory?

Territories are a physical space, usually defined by an area near the substrate but can extend upward well into the water column — we are working in a three-dimensional environment, after all. Some territories are held year-round, while others are set up seasonally to accommodate breeding activity.

When viewed by the opposite sex, a territory is more than space. It takes a keen eye to find a nice location, and a robust and healthy body to chase off competitors. A nice territory is therefore a proxy for the fitness of the territory holder. Any fish that is unable to protects a clear boundary and keep out the undesirables will likely make a poor choice of mate.

In a good many cases, territories are highly visual in nature. The extremities of the space need to be seen, and quite importantly, be identified and remembered. The area of a territory may be defined by proximity to food, shelter, or mates, but a fish cannot be too greedy since larger territories become increasingly difficult to defend. The limits therefore are often defined by objects and underwater topography. These landmarks help the fish identify and remember what to guard, and the extent to which their territory lies.

Most animals don’t want to expend energy unnecessarily. It’s important to conserve resources, especially if you’re spending your whole day defending a territory rather than going out and foraging. The other important aspect of being conservative is that fighting is costly, and not just in terms of energy. When it comes to blows, either party becomes  vulnerable to incurring injury, which puts them at risk of disease, weakens them against future competitors, and may make them less attractive to would-be partners. 

To avoid a clash, most territorial species have a laundry list of actions to take before pouncing upon an intruder. Visual measures, such as dancing, flashing, or extending their gill plates and fins, can usually be perceived from a fair distance and most weaker fish can determine from this point that they’d likely lose a fight.

Many species of fish are quite vocal about their spot and may utilize sound to announce their presence in a particular area without bringing themselves out into the open, and without expending energy chasing potential rivals. How often these calls are heard depends on the species. Some will only vocalize when confronted with a foe, while others will ‘sing’ periodically to announce occupation.

Closer encounters follow for braver bachelors. Contending fishes will size each other up, display more enthusiastically, and even waft water at each other. Although seemingly non-threatening, these are indirect ways for fishes to determine just how strong their opponent may be without finding out first-hand.

Should all else fail, physicality ensues. Clashes don’t have to last long to determine the victor, and are usually resolved after a brief sparring session. Real damage occurs when neither party is willing to yield. In nature, fights usually resolve without mortal outcomes. Things may differ in aquaria, as we will soon find out.

Who’s allowed in?

Social structures vary between fishes, and not everyone and everything will be chased from a territory. Part of this ‘social selection’ is dependent on what is being defended, and what the intruder may offer or threaten to take. For the most part members of the same species are viewed as transgressors.

Males will readily defend against incoming males, but may also chase off transgressing females that are not in season, or if the male himself is not ready. This makes sense, as your own species will demand the same kinds of food, or the same sexual partners. In other species the territorial instinct is a lot stronger. Anything wandering into the defended zone is fair game, from fish to invertebrates to the aquarist’s tending hand. 

These rules aren’t universal. For example, the cichlids, the diverse lot that they are, may have territories and family duties that can be shared inter-generationally as in some Lamprologus; or in some such as Pelvicachromis, females pick out the best spots and lure the males in for spawning, playing an important role in territory formation.

How do we accommodate them?

Researching your territorial species is half the battle, the other half being practice and observation. As with any aquarium pet, plenty of research should be conducted before going ahead with a purchase. In this way, behaviours can be expected, predicted, and hopefully, managed. It is important to realise that you will never really ‘tame’ a territorial species, but rather that the situation can be best manipulated to ensure minimal destruction and chaos.

It can be difficult to mimic the kinds of space that some species are used to manning in nature. Some lake-dwelling cichlids can have homesteads a few metres across if they play their cards right. Fortunately, many aquarium species will settle for far less.  In many cases it is best to plan a minimally stocked tank from the outset. This enables the few individuals to maximize the space available to them, rather than having strong overlap and constant battles. There are exceptions to this, as with Rift Valley cichlids that can not only benefit from being crowded, but become more hostile once they have managed to define a territory of their own.

In species where territories are established and maintained in the presence of females or while in breeding season, playing around with the sex ratio of that particular species usually yields good results. In such cases, a single male or indeed a single-sex (often female) group may result a lot less violence.

Identifying the sex of your fish early on can help prevent unnecessary bloodshed, although some fish may start developing territorial behaviours at a surprisingly young age. This in itself can be a way in which you can sex an individual — see who is trying to dominate other fish early on, and you’re likely looking at a male.

For other species, territory is strongly defined by space and borders regardless of whether there’s someone to show off to. For these fish, you’ll need to manipulate the environment.

Out of sight, out of mind

Within a given space, the effective area can be increased by increasing the complexity of the habitat. Plants, rocks, and branches help define boundaries, for sure, but more importantly they also break lines of sight. This is an incredibly useful tool in maintaining territorial species, because a fish won’t take umbrage with another fish that it cannot see.

As explained above, many animals don’t want to expend more energy than necessary chasing out undesirables. Should a chase occur, the offender being able to disappear into the environment effectively ends the bout. As far as the defending fish is concerned, the troublemaker is out of sight and out of mind, and he can return to his post.

On the contrary, an open-planned tank with crystal-clear water offers no reprieve for non-dominant fish. The defending individual will nearly always have visual contact with a potential intruder, and unfortunately, might never stop harassing the outsider because the competitor has few ways to bow out of a territory gracefully. Regardless of the warnings and displays of the territory-holder, the other fish cannot leave, short of leaping from the aquarium itself.

Particular objects are hot property for some species. Cave spawners will demand caves and crevices, for example, so be sure to provide more opportunities for a territory rather than too few. Often a single site will be popular between several fish, but the less dominant ones will eventually settle for less. Creating effective borders between such spaces is an extra step that can help calm tensions further.

Introducing new specimens into your aquarium, now a part of an established territory, carries risks that can turn out deadly. The established fish will heartily defend the homestead, while the new, confused fish will be far too panicked and lost to realize that it has trespassed where it shouldn’t.

Rearranging the furniture (and I mean really turning things upside down!) puts a hard reset on territorial spaces. The edges of the boundary that once defined the territory are either no longer there or are in unfamiliar locations. A fish presented with this situation will start working on creating a new territory with these new parameters, but now on in the same playing field as the new fish, who are also ‘new’ to the area. 

It is also important to avoid mixing similar-looking species with territorial types. It is unlikely that a protective individual will tolerate this lookalike competitor, and may display threatening behaviours and signals that the other species may not understand. This can have injurious consequences.