URI oceanographers test method to control metamorphosis in summer flounder

May 28, 2002

Flounders start life as larvae that live and drift in the coastal ocean. During metamorphosis, their body changes shape, and they move into estuaries and settle to the bottom as juveniles.

Adult flounder represent important commercial and recreational fisheries in Rhode Island waters. A common problem when culturing summer flounder is that faster growing fish metamorphose and settle first and often eat their slower growing siblings. In a recent issue of Aquaculture, URI biological oceanographers Steven Gavlik, Melissa Albino, and Jennifer L. Specker report on a method that addressed this problem through synchronization of growth and settling behavior of metamorphosing summer flounder.

Like the metamorphosis of a tadpole into a frog, flounder metamorphosis is controlled by thyroid hormones. The scientists devised and tested a treatment using thiourea, a naturally occurring thyroid inhibitor found in cabbage, and the thyroid hormone thyroxine. When added to the aquarium water, thiourea and thyroxine are absorbed by the flounder. Thiourea was added when the larvae were one month old. This prevented completion of metamorphosis and inhibited settlement, both of which depend on thyroid hormone.

About two weeks later, the thiourea was removed and thyroxine added. Giving the fish thyroxine in this way is comparable to a person taking a medication, such as Synthroid, to maintain their bodily levels of thyroid hormone. Thyroxine was removed after one week. By the time the flounder were two months old, the growth variability had been reduced, so that fish of the same age were closer in size. The thyroxine addition caused most fish to settle within 3 days, much less than the 2 weeks it normally takes in the culture facility.

The scientists concluded that this sequential thiourea - thyroxine treatment was an effective way to synchronize the settlement and growth of summer flounder. The treatments did not affect the survival of the flounder. Normally, flounder culturists devote a lot of time and effort grouping fish by size to reduce cannibalism. The URI researchers expect that this new method of synchronization will serve to reduce this costly labor and enhance the profitability of flounder aquaculture.
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The research and research training is supported by Rhode Island Sea Grant and the URI Partnership for the Coastal Environment. The Partnership is a collaborative group of university faculty, staff, graduate and undergraduate students, and external public and private sector partners.

University of Rhode Island

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