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

The recent collapse of the Northwestern Atlantic cod fisheries has coi ncided with a cooling of water temperatures. During this time the cond ition factor of cod has been poor. The objective of the present study was to determine the effects of longterm temperature acclimation on gr owth reproduction and thyroid function in laboratory held Atlantic cod (Gadus morhua). One of the key parameters used to assess thyroid func tion is the peripheral metabolism of L-thyroxine (T-4) by microsomal d eiodinase enzymes. Deiodinase function has not been described for gadi d fish. T-4 outer-ring deiodinating activity (apparent K-m 1-2 nM) was confined primarily to liver. Its properties resembled those for hepat ic T4ORD activity of other teleosts and the mammalian type II deiodina se. The T4ORD activity of cod liver exceeded that of salmonids and cou ld explain the high plasma T-3 levels (10-18 ng/ml), which were 2-5 ti mes greater than T-4 levels. T-4 and T-3 inner-ring deiodination was c onfined mainly to brain. In order to determine the effects of long-ter m temperature acclimation on cod, somatic growth, reproduction, and th yroidal status were assessed monthly in 400-900-g satiation-fed male A tlantic cod captured in June from the St. Lawrence Estuary and then ac climated from August to the following June under a natural photoperiod at 2-4 degrees C (LT) or 6-10 degrees C (LIT). Reproductive status wa s determined from the gonadosomatic index (GSI), plasma testosterone ( T) and 11-keto-testosterone (11-KT) levels, and the appearance of milt ; thyroidal status was determined from plasma T-4 and 3,5,3'-triiodo-L -thyronine (T-3) levels and hepatic T4ORD activity to produce biologic ally active T-3 Testis maturation (high levels of T and 11-KT, and mil t release) occurred in April and May and was uninfluenced by acclimati on temperature. LT cod grew more slowly than HT cod. Differences in bo dy weight were particularly evident from December to February. In conc lusion, (i) cod possess outer-and inner-ring deiodinase activities, pr edominating respectively in liver and brain, and with properties resem bling those of other teleosts, (ii) T4ORD activity of liver is unusual ly high and may account for the high plasma T-3 levels in this species , (iii) T4ORD activity tends to increase during periods of increased s omatic growth, and (iv) chronic acclimation of male cod to 2-4 degrees C, as opposed to 6-10 degrees C, decreases somatic growth but does al ter circulating levels of thyroid hormones and androgens and it does n ot change the time of sexual maturation. (C) 1998 Academic Press.