Behavior and circadian rhythm

The physiological state of an organism often has a cycle of approximately 24 hours. For example, the body temperature of a mammal has a constant cycle of temperature changes, the cycle being 24 hours. However, if the animal is placed in a permanently dark or permanently light environment with a constant temperature, the phase of the body temperature change can advance or delay. However, the cyclic temperature change is maintained. This endogenous rhythm generated by the biological clock, which is independent of the external environment and has a cycle of about 24 hours (close to 25 hours in the case of humans), is called a circadian rhythm.

The behavior of many marine organisms also has a periodicity in the amount of daily activity. In many cases, the periodicity is due to circadian rhythms, and when light is present in the living environment, the light-dark cycle is a tuning factor, as in many animals. Biological rhythms that are lost in constant dark or light are not endogenous and are distinguished as diurnal rhythms rather than circadian rhythms.

Fisheries have empirically made use of the horizontal and vertical migrations associated with the diurnal behavior of organisms. For example, these behaviors of target organisms play an important role in determining the location and depth of trawl nets, gill nets, longlines, traps and pots, etc. The cycle of feeding behavior is also linked to the periodicity of these behaviors. In angling fisheries, it is important to understand such time periods.