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Every year salmon embark on an exhausting and often treacherous journey from the ocean back to the streams where they were born. Once the salmon reach their destinations, the females dig pits in the stream gravel to deposit eggs, which are then fertilized by the males. This annual ritual, called spawning, happens in late fall. The timing ensures that the young emerge from the gravel in the spring, when the local environment is the most favorable for survival.
How does a salmon know it is time to spawn? Studies of salmon and other related fish indicate that the timing of spawning is determined by the change in photoperiod, or the daily amount of light, which synchronizes an internal biological clock. The strongest evidence for a biological clock comes from experiments using rainbow trout, a close cousin to salmon. Female rainbow trout maintained under a constant schedule of light and dark, constant temperature, and constant feeding rate for 4 to 5 years spawn on a cycle ranging from 11 to 15 months. This type of experiment shows that a circannual (meaning "about a year") rhythm of spawning exists even in the absence of external time cues, such as longer days or warmer weather. Such a rhythm must, therefore, be controlled by an internal timing system that tells the fish that "about a year" has passed since the last time it spawned. This internal clock needs to be synchronized (entrained) to the natural seasonal cycle to ensure that all salmon spawn in late fall. A series of experiments suggests that photoperiod entrains the clock. When salmon were exposed only to artificial light on a light-dark schedule that either expanded or compressed the natural seasonal cycle of daylight into periods greater than or less than 12 months, the spawning time was delayed or advanced, respectively. Experiments have also shown that the actual photoperiod is less significant than the direction of change of photoperiod—in other words, the same photoperiod can be interpreted as either a spring day or an autumn day, depending on the previous photoperiod experienced. Again, this suggests that an internal biological timing mechanism somehow keeps track of each photoperiod and compares it with the previous one. In mammals that breed only during certain seasons, photoperiod is perceived by the eyes and relayed via a complex neural pathway to the brain's pineal gland. The pineal gland then transmits this information into a daily rhythm of melatonin secretion that controls the timing of reproduction. Fish have a similar daily melatonin rhythm that is changed by photoperiod. Consequently, researchers believe that melatonin may also play a role in controlling the timing of reproduction in salmon; however, the evidence is less clear than in mammals. Aside from its scientific interest, an understanding of the mechanism underlying spawning in salmon has commercial value. Because of the circannual rhythm of spawning, the availability of fish of market size is restricted to certain times of year. Photoperiod techniques, which involve artificial manipulation of day length, are currently used by a number of fish farms to modify the spawning times of their fish stock and hence the supply of young fish to industry. If scientists can determine how the biological clock controls reproduction, better ways of raising fish commercially might be developed. And that means an ample supply of fresh salmon in grocery stores year round!
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