Thyroidal response of Atlantic salmon to seawater challenge: Predictor of growth in seawater

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The aim of these investigations was to apply current ideas about the relationships among salmon development, thyroid endocrinology, and seawater adaptation to the problems of delayed growth and stunting that occur unpredictably when commercial growers move salmon 'smolts' to sea cages. A test was designed that is analogous in many respects to the seawater challenge test. Whereas the seawater challenge test predicts survival in seawater, the seawater challenge-thyroxine (T4) response test is being developed to predict growth in seawater. Atlantic salmon at North Attleboro National Fish Hatchery (NAFH) were tested at two developmental stages: as parr in March and as smolts in May. Two groups of Atlantic salmon at St. Andrews Biological Station (STABS) were also tested in May. At this time, one group had been reared under simulated natural photoperiod (SNP) while the other had been exposed to constant light (L24) since August. Those under SNP are well-characterized as smolts, whereas those under L24 served as 'smolt controls' in that they did not survive or grow as well in seawater. In these tests, the salmon were injected with either 0.2 IU bovine thyrotropin (TSH) or the saline vehicle and transferred to seawater or returned to fresh water. Changes in plasma concentrations of thyroid hormones were measured at 8, 24 and 48 or 8 and 27 h after injection, depending on location. The responses to seawater and to bovine TSH occurred within 8 h. At this time, seawater suppressed thyroid activity in parr (NAFH) and in the 'smolt controls' (STABS). In contrast, seawater did not suppress, but tended to stimulate thyroid activity in smolts. These findings suggest the potential usefulness of the seawater challenge-T4 response test for predicting growth of Atlantic salmon in sea cages. © 1989.

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