Effects of Long-Term Temperature Acclimation on Thyroid Hormone Deiodinase Function, Plasma Thyroid Hormone Levels, Growth, and Reproductive Status of Male Atlantic Cod,Gadus morhua

Abstract

The recent collapse of the Northwestern Atlantic cod fisheries has coincided with a cooling of water temperatures. During this time the condition factor of cod has been poor. The objective of the present study was to determine the effects of long-term temperature acclimation on growth reproduction and thyroid function in laboratory held Atlantic cod (Gadus morhua). One of the key parameters used to assess thyroid function is the peripheral metabolism ofl-thyroxine (T4) by microsomal deiodinase enzymes. Deiodinase function has not been described for gadid fish. T4outer-ring deiodinating activity (apparentKm1–2 nM) was confined primarily to liver. Its properties resembled those for hepatic T4ORD activity of other teleosts and the mammalian type II deiodinase. The T4ORD activity of cod liver exceeded that of salmonids and could explain the high plasma T3levels (10–18 ng/ml), which were 2–5 times greater than T4levels. T4and T3inner-ring deiodination was confined mainly to brain. In order to determine the effects of long-term temperature acclimation on cod, somatic growth, reproduction, and thyroidal status were assessed monthly in 400–900-g satiation-fed male Atlantic cod captured in June from the St. Lawrence Estuary and then acclimated from August to the following June under a natural photoperiod at 2–4°C (LT) or 6–10°C (HT). Reproductive status was determined from the gonadosomatic index (GSI), plasma testosterone (T) and 11-ketotestosterone (11-KT) levels, and the appearance of milt; thyroidal status was determined from plasma T4and 3,5,3′-triiodo-l-thyronine (T3) levels and hepatic T4ORD activity to produce biologically active T3. Testis maturation (high levels of T and 11-KT, and milt release) occurred in April and May and was uninfluenced by acclimation temperature. LT cod grew more slowly than HT cod. Differences in body weight were particularly evident from December to February. In conclusion, (i) cod possess outer- and inner-ring deiodinase activities, predominating respectively in liver and brain, and with properties resembling those of other teleosts, (ii) T4ORD activity of liver is unusually high and may account for the high plasma T3levels in this species, (iii) T4ORD activity tends to increase during periods of increased somatic growth, and (iv) chronic acclimation of male cod to 2–4°C, as opposed to 6–10°C, decreases somatic growth but does alter circulating levels of thyroid hormones and androgens and it does not change the time of sexual maturation.