Dry and Rainy Seasons in the aquarium
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Dry and Rainy Seasons in the aquarium

A lot of popular aquarium fish hails from environments subjected to seasonal droughts and rainy seasons. It is common for such species to spawn only at the onset of the rainy season, since this will provide their offspring with ideal conditions and ensure a higher survival rate. If you have tried in vain to spawn a certain species in captivity, you may wish to take a closer look at the suggestions below. In many cases, you will not need to use all the triggers – sticking to a few ones will be plenty enough. It is always important to research your particular species and its natural habitat before you start, since this will provide you with a wealth of clues as to which triggers that will work best and which ones that should be refrained from.

Increased and/or altered food supply
Fish are not the only creatures that spawn at the onset of the rainy season, and as the water fills with a myriad of different animals and their offspring (including fish eggs and fish larvae) the amount of food as well as the type of available food will change drastically for the fishes. During the dry season, it is common for fish to survive by eating decaying plants, detritus and bottom dwelling red mosquito larvae. When the rainy season starts, the aquatic creatures will start to reproduce rapidly, and they will be accompanied by their terrestrial counterparts, e.g. insects that fall onto the water’s surface or lay their eggs in the water. Trees and flowers will also live up again and the surface can be covered in pollen. An increased or altered food supply cannot only be used for omnivore and carnivore species, herbivore fish can also fatten up by the onset of the rainy season.

If you have healthy fish in your aquarium, you can let them starve, or feed them only minimal amounts of “boring” food, for at least 10 days to mimic the dry season. In the wild, many fish species actually look like swimming skeletons by the end of the dry season. When you start to carry out the other changes that mimic the onset of the rainy season, combine them with feeding your fish large amounts of many different types of live food. Many South American species is for instance known to start spawning when the water becomes filled with mosquito larvae.

Last but not least, the rainy season will often cause an increased plankton level. In some species this will trigger the adults to spawn since it tells them that there will be plenty of food for their tiny offspring. Increasing the plankton level in your aquarium is of course a bit tricky, but you can try to create a similar effect by adding infusoria.

Increased water depth
In many parts of the world, even major water ways will become shallow during the dry season, and a lot of fish species will start spawning when the rains has continued long enough to make the water deep again. This will be amplified further by water from higher terrains reaching the place where the fish live. Even fish that spend their entire life at the bottom can notice a change in water depth, because as the dept increases, so will the water pressure at the bottom. Gradually lower the water level in your aquarium during the dry season until it is down to 25% of its normal depth. When you start the rainy season, gradually refill the aquarium over the course of a few days.

Decreased amounts of dissolved substances
As water vanish during the dry season, the remaining body of water acquires higher and higher concentrations of salts, organic waste products, humic substances and similar. When the rain starts, these concentrations will be dramatically lowered and this can trigger many fish to spawn. You have probably already noticed how a major water change can induce breeding in the aquarium. Rainwater has virtually zero hardness and is often slightly acidic, and the heavy rains will therefore cause a sharp drop in both water hardness and pH-value. During your “dry season”, you can decrease your normal water changes and add humic substances, e.g. peat or alder cones. Fertilizers that include salts can also be used. Be careful not to overdo it, you don’t want to kill your fish. When you kick-start the rainy season, use soft and slightly acidic water, e.g. RO water or real rain water. 

Decreased water temperature
The cloudy sky and the vast amounts of chilly rainwater can cause the water temperature to drop dramatically. If you keep high terrain species, they may actually be used to a 10ºC change. For lowland species, the change is normally no more than a few degrees. When you mimic the dry season you should therefore keep the water temperature fairly high (it is easier for the sun to heat up a small, shallow body of water) and then drop it as you start to simulate the onset of the rainy season.

It is however important to keep in mind that some species do not handle rapid changes well and dramatic alterations in water temperature must be avoided in such cases. Some species will even need an increased water temperature to spawn since they seek out flooded plains to spawn in shallow waters as the rainy season commences. Always keep temperature changes within the recommended temperature range for your particular species.   

Increased water flow  
As rivers and streams start to refill, the flow of water can increase dramatically and the fish will have to become more active to avoid being flushed away. Some species are even adapted to migrating upstream to reach calmer breeding grounds. Other species prefer to lay their eggs where they will be exposed to the strongest water flow possible (and will often spawn in front of the filter outflow in aquariums). In the aquarium, an increased water flow can be simulated by a water pump that creates a strong current. If your species need to get to calm breeding grounds to spawn, you must naturally decrease the water flow after a while.

Increased oxygen levels
As the rain crushes down onto the surface the turbulence will boost gas exchange and increase the oxygen levels while simultaneously decreasing the levels of carbon dioxide in the water. Increased water flow from higher terrains will also have a beneficial effect. This elevation of the oxygen levels is necessary to keep eggs and fry of many fish species alive and the adult fish will therefore refrain from spawning until they notice a significant increase in the amount of dissolved oxygen. In the aquarium, you can use air stones or an air driven filter to produce the same effect. You can even create an arrangement where water from a filter will splash down onto the surface just like rain.

This might sound far fetched, by there are actually some indications that certain fish species might use the sound of splashing rain and thunderstorms to determine the right time to spawn. If you have tried in vain to make your fish spawn in captivity, why not give it a shot and produce some thunder and splashing rainstorms? You can for instance let water trickle down from a Plexiglas with a lot of small holes in it and shake a metal sheet to produce a thunder effect. 

The biological clock
Some fish species have a very strong biological clock that will only allow them to breed during a certain time of year, i.e. when the rainy season would commence in their native habitat. This type of behavior is especially noticeable in wild caught specimens that have already experienced a rainy season before being caught. Captive bred specimens and young fish caught before their first rainy season are normally easier to coax into spawning regardless of time of year. If your fish refuse to spawn, check when the rainy season normally starts in their native habitat and coordinate the switch from dry to rainy season in the aquarium accordingly. If your species is found in a large region you may have to research from which area your particular specimens hail.   

Tinkering with the biological clock
In some cases, the biological clock of fish is determined by the sun. This means that you can fiddle with the biological clock by changing the amounts of daylight in your aquarium. The further away from the equator your fish lives, the more used it will be to short winter days and long summer days, and may use this change to determine which part of the year it currently is. Even close to the equator changes in light can however tell fish that the rainy season has started since clouds and intense rain will decrease the amount of light. Under a cloudy sky, the dawn will be noticeable later and the dusk will be perceptible earlier. Getting a timer is the easiest way of fiddling with the light in the aquarium. You can also decrease the intensity of the light. In some parts of the world, switching the lights off altogether and relaying on natural light will be the best course of action.

When talking about light, it should be noted that some fish species prefer really dark conditions when spawning. This is normally because their natural habitat is shadowed by dense tree canopies and surface plants, and/or is tea colored due to a high degree of humic compounds.

Changes in pressure
The dry season is a long period of high pressure and right before the rainy season starts the barometric pressure will drop considerably. Changing the air pressure is of course not really something that hobby aquarists can hope to accomplish, but you can keep an eye on the barometer and coordinate the onset of the rainy season in your aquarium with the onset of low air pressure. 

Spawning sites and new surroundings
As the water vanishes during the dry season, it is common for only the middle of the river or stream to be water filled. When the water comes back, the fish will once again be able to reach suitable spawning spots among tree roots, fallen tree branches and similar. In some areas they will even be able to swim into flooded plains and spawn there. You can simulate this change of scenery by redecorating the aquarium while simultaneously adding more suitable spawning sites, e.g. flower pots, caves, roots, bushy plants and spawning mops – all depending on which species you keep.  

The hormones released by fish species that spawn early might trigger other fish to spawn in the wild. A big fish might for instance prefer to spawn when it knows that the water will be filled with a myriad of tiny fish for its offspring to devour. Place an easily bred species in the same aquarium as the fish you are trying to coax and hope for the hormones to rub off on them. Sometimes adding water from an aquarium where other fish are spawning will be enough to trigger breeding.


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