Mar. 21, 2001 COLLEGE STATION -- Biologists have discovered that longer days make redfish grow faster. This discovery could have far-reaching consequences on fish culture techniques aimed at increasing fish growth and improving their quality.
Duncan S. MacKenzie, a Texas A&M University professor of biology, and Kevin Leiner, a zoology graduate student, made this discovery while studying the workings of the thyroid gland in redfish and how it affects their behavior and metabolism.
The thyroid gland releases a chemical, thyroxine, which regulates fish growth and metabolism - the way fish receive and use energy.
"Although we still do not know exactly how the thyroid regulates fish growth, the work we have done indicates that animals that are happy and healthy and are growing very actively have very high thyroid hormone levels," says MacKenzie.
Although thyroid activity in mammals is quite high, the first scientists who measured thyroid hormones in fish found very little. So it initially was assumed that thyroid hormones play a minor role in fish metabolism.
"We discovered that, in fact, thyroid hormone is very dynamic in some fish species like the redfish," says MacKenzie. "The hormone is very precisely regulated, its level increasing and decreasing regularly at different times of the day."
Since 1995, MacKenzie has been investigating the mechanisms of thyroid hormone regulation in redfish. He has been mainly interested in the effects of food intake and light exposure on thyroid hormone release.
MacKenzie has conducted experiments involving large fish tanks sampled at different times. In each experiment, lasting between two to eight weeks, the amount of light, food, and the times of day that fish are fed have been precisely controlled.
For each experiment, Leiner measured the daily redfish thyroid hormone levels by taking blood samples every two hours. At the end of each experiment, the fish are examined in order to determine the amount of fat and muscle.
"Different daily feeding times have a minimal effect on the hormone levels of the fish," MacKenzie says. "Instead, dawn appears to activate an increase in the thyroid hormone 5 to 10 times greater than night-time levels. We had not anticipated that light was going to be as important as it was in driving thyroid hormone cycles"
"This has an important implication in aquaculture. If you run a fish farm and keep your fish indoors, how long do you leave the lights on? Keeping them on a long time may cost you more on electricity, but if the fish grow faster, you would be able to get them to the market sooner."
In future studies, MacKenzie would be interested to know if there is also an optimal period of time for light above which fish do not grow faster.
MacKenzie is also interested in the internal mechanisms of the thyroid gland. Leiner's studies on redfish suggest that the biological clock mechanism in the brain may be the primary driver of thyroid hormone production.
"Though we know that thyroid hormone helps animals to stay healthy and grow actively, the question remains: 'How does the thyroid hormone work?'" says MacKenzie. "We think that the two primary controls of the activity of thyroid gland are the day-night cycle of light, which, through a clock in the brain, sets up periodic patterns of the production of the thyroid hormone, and a nutritive control on food intake, which can amplify the production of the hormone."
While MacKenzie needs more research work to better understand how the thyroid gland operates, his research has at least led to the following conclusion: if you want the fish in your fish tank to grow, you should leave the light on longer.
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