Outdoor Medaka Breeding: Sunlight and Shade Management | Summer High-Temperature Prevention and Winter Sunlight Securing

Outdoor Medaka Breeding and the Importance of Sunlight

Medaka naturally inhabit sunny rice paddies and small streams. Sunlight is essential for outdoor breeding for the following reasons:

• Green water (phytoplankton) maintenance: Serves as a natural nutrient source, contributing to medaka health and coloration
• Vitamin D synthesis: Ultraviolet rays support medaka bone formation
• Breeding cycle stimulation: Extended daylight hours trigger spawning behavior
• Water temperature elevation: Naturally creates appropriate water temperature (20–28°C)

However, in summer when sunlight is strong, water temperature can exceed 35–40°C, which is lethal for medaka. Balancing sunlight and shade is key to successful outdoor breeding.

Summer High-Temperature and Direct Sunlight Prevention

Shade cloth (cold cloth): Black or white shade cloth placed over the container blocks 50–70% of sunlight. A 50% light transmission shade cloth is generally most practical. Placing it 5–10cm above the container rather than directly on it improves air circulation.

Sudare (bamboo/reed blind): Bamboo or reed blinds offer good appearance and ensure good ventilation. Leaning them against the container's sides also prevents sunlight from the sides.

Location selection: An ideal location receives morning sun and afternoon shade. Particularly, western sun hits at low angles for extended periods, so avoid west-facing placement in summer or add additional west-side shade.

Increase water volume: Greater water volume moderates temperature changes. Containers deeper than 30cm maintain lower bottom-water temperatures, allowing medaka to escape high heat.

Shade Rate and Water Temperature Guide

Water temperature monitoring is fundamental to shade management.