"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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Contact: Keith Randall
kr@univrel.tamu.edu
979-845-4644
Texas A&M University
18-Mar-2001