Medaka Rain and Typhoon Care Guide | Preventing Overflow from Heavy Rain and Water Quality Changes

Behind the joy of outdoor medaka breeding on sunny days lies constant risk from rain and typhoons. Weather-related troubles are not uncommon—medaka can be swept away by overflow from heavy rain, or water quality can suddenly change due to acid rain. This article explains concrete measures to protect medaka from rain and typhoons.

Effects of Rain on Medaka

There are three main problems that occur when large amounts of rainwater enter a medaka container. First is a sudden drop in water temperature. Rainwater is often cooler than the cultivation water, and when a large amount enters, the water temperature can drop several degrees. Medaka are sensitive to sudden temperature changes, and even a drop of 2–3°C causes stress. Second is sudden water quality change. Rainwater is often weakly acidic (pH 5–6 or so) and suddenly lowers the pH of the cultivation water (normally around pH 7–8). pH changes cause osmotic stress, and fry with weak constitution can suffer fatal damage. Third is overflow (water spilling over). When water spills from the container, medaka are swept away. Fry in particular cannot resist the force of the current, so there is a risk of total loss.

Overflow Prevention Measures

Overflow prevention is one of the most critical tasks in outdoor breeding. The simplest method is to create a drain hole by inserting a sponge or wool mat into a portion of the container's rim. When the water level rises, excess water drains through the sponge, but medaka cannot pass through the sponge, so they don't flow out. Use fine-mesh sponge (such as car-washing sponge) for drainage, and be careful that fry don't pass through. A more reliable method is to drill a hole in the side of the container and attach stainless mesh over it to create an overflow opening. Position the hole at the expected water level (about 5 cm below the container's rim). You can also attach a commercially available overflow pipe; specialized parts for NV boxes and trough containers are sold.

Preparations and Measures Before a Typhoon

Advance preparation is essential before a typhoon approaches. First, lower the water level in the container by 5–10 cm from the usual level. This provides margin before overflow even if heavy rain enters. Light containers that might be blown away by wind (such as plastic planters) should be moved to a heavy spot or fixed with blocks or similar. Secure the lid and net firmly so they aren't blown away, and clear the surrounding area so that flying debris doesn't fall into the container. If possible, temporarily moving the container to a place with a roof, such as under the eaves or in a garage, is most secure. For large containers that are difficult to move, cover with a blue sheet to prevent rainwater from entering while keeping ventilation gaps. Be careful not to seal it completely, as it will cause oxygen depletion. If heavy rain or a typhoon is forecast, completing these measures by the previous day means you can avoid panic.