Effects of photoperiod on the reproductive success of fathead minnows (Pimephales promelas)

Rhythms in fish have long been recognized by man; there are several ancient writings describing the rhythms of seasonal and daily migrations as well as seasonal reproduction. These seasonal rhythms allow organisms to coordinate their biological processes with favorable environmental conditions. For maximum reproductive success to occur, it is necessary that environmental conditions, such as available food supply and proper water quality, are present at the time of spawning. By measuring day length (photoperiod), organisms are able to measure the passage of time and coordinate their biological processes with favorable environmental conditions. Recent experimental study has shown that photoperiod is one of the most influential environmental factors effecting rhythms.

The length and sequence of day and night cycles was manipulated to determine the effects of photoperiod on reproductive success of fathead minnows, Pimephales promelas (Rafinesque). Spawning success of fathead minnows was evaluated under four photoperiod treatments consisting of—(1) complete photoperiods, (2) skeleton photoperiods, and (3) 24 hours of light and (4) 24 hours of darkness. Reproductive success was significantly different within each experiment. Fathead minnows appear to require a long photoperiod of at least 14 hours of light, placing them into the category of long-day animals. Exposures to skeleton photoperiods show the possibility of a "daily" rhythm of sensitivity of light. Through this sensitivity rhythm, fathead minnows are most likely able to synchronize their reproduction with favorable environmental conditions. As interpreted day length grew longer, overall egg production increased. Results indicate that the longer the "day" the higher the reproductive success, especially if there is some period of darkness associated with it.