From Green to Serene: Investigating the Effects of Algal and Light Conditions on Stress Levels and Welfare in Fish

Abstract

The increasing use of fish in laboratory research and aquaculture threatens fish welfare. Thus, environmental enrichment strategies have gained increasing attention to promote the well-being of captive fish. This study investigated the preferences and stress responses of Japanese medaka (Oryzias latipes) to different environmental enrichments and conditions. In the behavioral experiments for specific light colors and intensities, male medaka exhibited a preference for colored LED lights, especially in purple over white lights when assessed individually and in groups. Females did not show clear color preferences but tended to prefer colors over white when tested in groups, suggesting social influences. The medaka favored a lower light intensity in purple lights but tolerated a wider intensity range in blue, green and white lights. Additionally, separate experiments found that medaka preferred environments with reflective tanks walls and algae-free conditions over excessive algal growth present. Medaka may have potentially perceived mirror reflections in the tank walls as conspecifics. The open field test revealed a higher exploratory, lower anxiety-like behaviors in medaka from the clean tanks compared to medaka from algae tanks. It was however, unexpectedly found higher physiological stress in medaka from the same clean tanks compared to algae tanks when measuring cortisol levels. The findings highlight the medaka’s responsiveness and preference to environmental enrichments and factors like lighting, habitat complexity/cleanliness, social settings and reflective surfaces. By implementing enrichment tailored to their preferences it could significantly improve their welfare for this research model.

The increasing use of fish in laboratory research and aquaculture threatens fish welfare. Thus, environmental enrichment strategies have gained increasing attention to promote the well-being of captive fish. This study investigated the preferences and stress responses of Japanese medaka (Oryzias latipes) to different environmental enrichments and conditions. In the behavioral experiments for specific light colors and intensities, male medaka exhibited a preference for colored LED lights, especially in purple over white lights when assessed individually and in groups. Females did not show clear color preferences but tended to prefer colors over white when tested in groups, suggesting social influences. The medaka favored a lower light intensity in purple lights but tolerated a wider intensity range in blue, green and white lights. Additionally, separate experiments found that medaka preferred environments with reflective tanks walls and algae-free conditions over excessive algal growth present. Medaka may have potentially perceived mirror reflections in the tank walls as conspecifics. The open field test revealed a higher exploratory, lower anxiety-like behaviors in medaka from the clean tanks compared to medaka from algae tanks. It was however, unexpectedly found higher physiological stress in medaka from the same clean tanks compared to algae tanks when measuring cortisol levels. The findings highlight the medaka’s responsiveness and preference to environmental enrichments and factors like lighting, habitat complexity/cleanliness, social settings and reflective surfaces. By implementing enrichment tailored to their preferences it could significantly improve their welfare for this research model.