How do light, plankton movements, temperature, and breeding condition affect the depth of clupeiform schools?

The depth at which clupeiform schools swim is primarily determined by a combination of environmental and biological factors, including the movements of plankton, light intensity, water temperature, and the fish's breeding condition (maturation cycle of gonads).1

• Plankton and Light: Clupeiforms exhibit diurnal vertical movements that are closely linked to the corresponding movements of plankton. Generally, these schools remain in deep water or near the bottom during the day and migrate toward the surface at night.1
• Temperature: These vertical migrations often involve moving between different thermal layers. For example, herring may migrate from deep, cold waters (approximately 3 °C) during the day to warmer surface waters (5 °C to 7 °C) at night.1
• Breeding Condition: The maturation cycle of the gonads is a key factor influencing the depth at which these schools are found.1
• Light Sensitivity: On moonless nights, clupeid schools can be attracted to strong light sources, causing them to congregate near the surface, a behavior frequently utilized by the fishing industry.1