Evaluation of commercial catfish, tilapia and salmonid diets for growth promotion of white suckers

In an attempt to identify appropriate diets for use in intensive baitfish culture, the efficacy of three commercial diet formulations, a high-protein, high-lipid, low-carbohydrate diet [a semi-moist salmonid diet (SD)] and two low-protein, low-lipid, high-carbohydrate diets [a catfish diet (CD) and a tilapia diet (TD)] on growth performance of juvenile white suckers, Catostomus commersoni, was examined. All three diets permitted growth during the 16 week trial. The SD promoted highest growth and somatic protein and lipid deposition, whereas the CD and TD formulations promoted markedly higher accumulations of glycogen in the carcass and liver. The high hepatic glycogen content was associated with an elevated hepatosomatic index in the CD- and TD-fed groups. Plasma T3 levels were lower in the CD- and TD-fed groups, possibly a response to the low dietary protein or indicative of dietary insufficiency in these fish. Of the three diets evaluated, the SD was considered to be the best in terms of the growth performance parameters measured, and offers an economical and appropriate diet for intensive culture of juvenile white suckers.     

Temporal changes in plasma thyroid hormone,growth hormone and free fatty acid concentrations,and hepatic 5′-monodeiodinase activity,lipid and protein content during chronic fasting and re-feeding in rainbow trout (Oncorhynchus mykiss)

Temporal changes in growth, plasma thyroid hormone, cortisol, growth hormone (GH) and non-esterified fatty acid (NEFA) concentrations, hepatic T₃ content and hepatic 5-monodeiodinase activity were measured in rainbow trout (Oncorhynchus mykiss) subjected to a sustained fast for up to eight weeks, and during a four-week re-feeding period. The purpose of the study was to examine aspects of the endocrine control of energy partitioning processes characteristic of short-term (acute; fasting) and long-term (chronic; starvation) food-deprivation states in fish, and to explore the role of the thyroid hormones, cortisol and GH in the energy repartitioning that takes place during an acute anabolic (re-feeding) state following chronic food deprivation.Differences in growth rate between fed and fasted groups were evident after two weeks, but significant weight loss by the fasted groups was not evident until between four and six weeks into the fast. Hepatosomatic indices (HSIs) were significantly reduced in the fasted fish within seven days, and as early as two days in one study; recovery of the HSI in fasted fish was evident within three days of re-feeding. Liver protein content (expressed as % wet weight) was consistently depressed in the fasted fish in only one of the three studies. Liver total lipid content (expressed as % wet weight) was depressed in the fasted fish within two days of food deprivation. Because of the rapid and sustained decrease in the HSI of fasted fish, the hepatic total protein and lipid reserves, when considered on a body weight basis, were markedly lowered within the first few days of the fast. Plasma GH concentrations exhibited a bi-modal pattern of change, with a transient fall in levels, followed by a sustained increase in fasted fish. The indicators of interrenal activity were suggestive of a depressed pituitary-interrenal axis in fasted animals; plasma cortisol levels were elevated to levels of fed animals within one day of re-feeding. The indicators of thyroid hormone economy (plasma thyroid hormone levels, liver triiodothyronine content, hepatic 5-monodeiodinase (MD) activity, thyroid epithelial cell height) were similarly indicative of a depressed pituitary-thyroid axis in fasted animals, with recovery to levels of the fed animals within one week. Despite the compensatory changes in accumulation of reserves (as indicated by a compensatory increase in HSI), there were no apparent compensatory changes in any of the endocrine parameters evident during the re-feeding period.     

Transportation and handling stress of white suckers raised in cages

Changes in plasma cortisol concentrations were measured in juvenile white suckers, Catostomus commersoni (Lacepede), as a measure of their response to transportation (8 h) and chase challenge stressors. The study attempted to evaluate the ability of this species to withstand standard aquacultural husbandry practices. Plasma cortisol concentrations were elevated during and following transportation. Recovery from transportation (as plasma cortisol levels reached baseline values) was prolonged in fish transported under winter conditions compared with fish transported during the summer months. The plasma cortisol response to a 5 min chase challenge in well-fed juvenile white suckers was typically of 2 to 3 h periodicity with the peak values between 15 and 30 min. This pattern was similar in fish fed three commercial diets (although rates of recovery differed), and was unaffected by fasting. The return of plasma cortisol concentrations to baseline values following the chase-challenge stressor was prolonged in fasted groups. The results of this study showed that juvenile white suckers require several days to recover from transportation and under the acclimation conditions applied, the recovery was faster in the fish transported during the summer.     

Smoltification induced by a ‘skeleton’ photoperiod in underyearling coho salmon (Oncorhynchus kisutch)

Underyearling coho salmon fry were subjected to three initial photoperiod treatments (6L18D, 10L14D, 14L10D) for two months and subsequently to three final treatments (16L8D, 9L6D1L8D, 10L14D) in a factorial design. Growth rates and seawater adaptability were monitored regularly. The groups that were exposed initially to 6L18D or 10L14D and then to 16L8D grew faster and had lower plasma sodium ion levels after seawater challenge tests than any of the other groups. Fish which were initially exposed to 6 L or 10 L daylength and then to a 9L6D1L8D skeleton photoperiod, showed a slightly lower growth rate and seawater adaptability than those given the corresponding complete 16L8D photoperiod. However fish maintained on skeleton photoperiods had significantly greater growth rates and seawater adaptability than those kept on the 10L14D photoperiod. This indicates that it is not the accumulated number of hours of exposure to light that initiates smolting, but rather the time during the day when light is experienced. Fish exposed initially to 14L10D showed little or no response to subsequent changes in photoperiod, suggesting that responsiveness to inductive photoperiods depends on the initial photoperiod treatment.     

Vitamin requirements in great scallop larvae

Vitamins were analysed in food (microalgae) and larvae of great scallop, Pecten maximus, during larval development. Microalgae used to feed larvae in hatcheries show great variability in their vitamin composition depending on both the species and culture condition (phase of growth). The microalgae used to feed Pecten maximus larvae were rich in vitamins; their content compared with diets used in fish culture appeared sufficient, with the possible exceptions of pyridoxine, biotin and pantothenic acid. Vitamins in bacteria, isolated from the larval rearing tank were also analysed, as they can also contribute to the diet. Vitamin B₁₂, -tocopherol and -carotene were detected in very low concentration in bacteria; however, some bacterial strains were rich in pantothenic acid, and the pattern of other vitamins was similar to that from microalgae. The presence of bacteria can complement the diet in panthothenic acid, as it has been demonstrated that bacteria are ingested by larvae. The vitamin content of Pecten maximus larvae was analysed from the second day after hatching to just before metamorphosis. The content of some vitamins, ascorbic acid (C), -tocopherol and -carotene, increased during larval development, suggesting that their requirement was satisfied. However, thiamin and riboflavin decreased during larval development and further studies, possibly using microencapsulated vitamins supplements, are needed to determine whether these vitamins are limiting during larval development.     

Gonadotrophin-releasing hormone agonist raises plasma concentrations of progestogens and enhances milt fluidity in male Atlantic halibut (Hippoglossus hippoglossus)

In two separate spawning seasons, spermiating male Atlantic halibut were implanted with pellets containing gonadotrophin-releasing hormone agonist (GnRHa). Males were bled repeatedly, and milt samples were collected. Blood samples were assayed for free and conjugated steroids: testosterone, 11-ketotestosterone, 17,20-dihydroxy-4-pregnen-3-one (17,20-P), 17,20-dihydroxy-4-pregnen-3-one (17,20-P), 17,20,21-trihydroxy-4-pregnen-3-one and steroids with a 17,20 configuration. Towards the end of the first season, pellets were implanted into three wild-caught and three hatchery-reared males. No control fish were available. The major progestogen in plasma was identified as sulphated 5-pregnane-3,17,20-triol (3,17,20-P-5-S). Concentrations of this steroid were stimulated by the GnRHa. Sulphated 17,20-P was also identified in the plasma, but at 10-fold lower concentrations than 3,17,20-P-5-S. In the middle of the second season, pellets were implanted into five hatchery-reared males; five unimplanted males were used as controls. Levels of androgens fell following GnRHa treatment, levels of progestogens rose briefly, and there was a significant increase in the fluidity of the milt. Of all the measured steroids, free and sulphated 17,20-P showed the best correlation with milt fluidity.     

The influence of the dietary carbohydrate: lipid ratio on the chronic toxicity of sodium pentachlorophenate to rainbow trout (Salmo gairdneri Richardson)

The influence of diets varying in carbohydrate: lipid ratio on the chronic toxicity of waterborne sodium pentachlorophenate (0 or 50 g NaPCP.l^–1) to rainbow trout (Salmo gairdneri) was examined over a 12 week period. The three diets used were practical salmonid formulations, equivalent in energy and protein content, with cerelose: fish oil ratios of 18.24.7% (high carbohydrate, HC), 9.39.1% (intermediate, INT), and 0.613.8% (low carbohydrate, LC). With increasing levels of dietary carbohydrate trout exhibited enlarged livers, elevated liver glycogen content, reduced liver protein content, reduced body lipid content, reduced weight gain, and elevated feed: gain ratio. For all diet groups, exposure of trout to NaPCP resulted in reduced liver glycogen content, higher liver protein content, reduced levels of plasma total triiodo-L-thyronine and L-thyroxine, increased thyroid epithelial cell height, and increased feed: gain ratios relative to their respective dietary control group. NaPCP exposure led to nominal reductions in weight gain and body lipid content of fish reared on the HC and INT diets. NaPCP exposed fish on the LC diet gained significantly less weight but had a higher body lipid content than control fish. The increasing NaPCP bioconcentration factor (BCF) associated with increasing lipid content of fish (log BCF=2.75 [log % lipid] +0.30; r=0.95) may explain the differences in response. Correction of bioconcentration data for the lipid content of fish reduced the range in BCF from 5- to 2.8-fold.     

Turnover of α-, γ, and δtocopherol and distribution insubcellular and lipoprotein fractions indicate presence of an hepatic tocopherolbinding protein in Atlantic salmon (Salmo salar L.)

The purpose of this work was to study the turnover of a, andtocopherol (TOH) in Atlantic salmon (Salmo salar, L.). Fish induplicate tanks were fed a diet containing 150 mg kg^-1-TOH and 100 mg kg^-1 each of and TOHadded as tocopheryl acetates. After fillet TOH concentrations had adjustedto the dietary supplementation levels, samples were taken from fish that hadbeen deprived of feed for 100 h, and from fish that had been fed regularlyuntil sampling. The retained levels of tocopherols in plasma correspondedgrossly with their biological activities, as found in experiments withmammals (::100: 20:3). The plasma concentrationsof -, and TOH amounted to 65, 44 and 15%,respectively, in unfed compared to fed fish. Very low density lipoprotein(VLDL), appeared to contain a greater fraction of plasma -TOH than ofplasma TOH. The mitochondrial fraction of liver, but not that of darkmuscle, was highly enriched in -TOH, and less in andTOH. The concentration ratios in liver and bile indicate that, and to some extent, TOH are excreted in the bile at a higherrate than -TOH. The data fit the hypothesis that Atlantic salmonliver contains a tocopherol binding protein with higher affinity for-TOH than for the other tocopherol homologues. This appears toprevent excretion of -TOH in the bile, and stimulate incorporation of-TOH in VLDL for subsequent secretion into the blood stream. As aconsequence, -TOH is retained in the body to a greater extent than and -TOH.     

Variation in content of Ω3 fatty acids in farmed Atlantic salmon,with special emphasis on effects of non-dietary factors

The main aim of the present study was to examine the impact of some biological and environmental factors on the lipid and fatty acid compositions of farmed Atlantic salmon (Salmo salar), with special emphasis on 3 fatty acids. Two year groups of salmon at nine fish farms distributed along the Norwegian coast were fed the same diet and were sampled every second month. The data are believed to give a representative characterization of lipid and fatty acid content of salmon farmed in Norway.Multiple regression analysis revealed that variation in lipid content and body weight explained 80% of the variation found in 3 fatty acids in farmed salmon, and 22:6 3 showed greater variation than other 3 fatty acids. Further analysis of lipid-corrected values revealed only minor effects of latitude on the per cent content of highly unsaturated 3 fatty acids, and hardly any effect of seawater temperature, with the exception of 22:6 3, which decreased slightly with increasing temperature.The per cent 22:6 3 in the fillet became gradually reduced with increasing fish age and body weight, whereas the content of 20:5 3 and other 3 fatty acids remained relatively constant. The per cent content of 22:6 3 of young salmon was higher than in the feed, but approached the feed value gradually as body weight increased. The lipid content of the salmon increased with fish age, and the absolute quantitative contents of both 22:6 3 and 20:5 3 increased meanwhile, even though the per cent content of 22:6 3 decreased quite pronouncedly.The per cent 22:6 3 and other 3 fatty acids was higher in wild than in farmed salmon, but the absolute quantitative content was higher throughout in farmed salmon, which had higher lipid contents. The 3/6 ratio, which is important in human health evaluation, was lower in farmed than in wild salmon. The large flexibility of 3 fatty acids and lipid content of farmed salmon leave us with the option of producing a wide variety of salmon qualities requested by the market. Both per cent and absolute quantitative 3 contents, as well as the 3/6 ratio, may readily be manipulated.     

Effects of dietary lipids on body composition and liver function in juvenile red drum, Sciaenops ocellatus

This study was conducted to evaluate the effects of different concentrations of dietary lipids on body composition and liver function in juvenile red drum, Sciaenops ocellatus. Diets were formulated to contain 40% crude protein from solvent-extracted menhaden fish meal and 0, 7, 14 or 21% lipid from menhaden fish oil. The basal diet, without supplemental fish oil, contained lipid at 0.4% of dry weight. The diets were fed to groups of 25 juvenile red drum initially averaging 7.3 ± 0.18 g fish^–1 in a recirculating culture system for 8 weeks and weight gain was recorded. After an additional 8 weeks, 16 fish from each treatment were sacrificed and the following measurements were recorded: hepatosomatic index (HSI), intraperitoneal fat (IPF) ratio, and liver -tocopherol, malondialdehyde (MDA) formation, and cytochrome P-4501A activity (measured as 7-ethoxyresorufin O-deethylase (EROD) activity). The activity of alanine and aspartate aminotransferases and concentrations of -tocopherol also were measured in plasma.Weight gain was significantly (p<0.05) affected by dietary lipid concentration, with values ranging from 361% of initial weight for fish fed the basal diet to 527% of initial weight for fish fed the diet containing 7% lipid. The HSI and IPF ratio values also were significantly affected by lipid with the lowest values recorded for fish fed the basal diet and the highest values observed in fish fed the diet containing 21% lipid. Increasing dietary lipid significantly increased oxidative stress as reflected in reduced -tocopherol in liver and plasma and increased MDA formation in the liver, although no overt pathological signs were observed. These findings suggest that lipid concentrations between 7 and 14%, when the diet contains 60 IU vitamin E kg^–1, are likely to limit oxidative stress and result in normal physiological responses of red drum.     

Growth of rainbow trout fed a diet supplemented with earthworms,after chemical treatment

Whole frozen earthworms (Eisenia foetida) were evaluated as a partial replacement for commercial pellets for rainbow trout, Oncorhynchus mykiss. Earthworms were blanched and treated with concentrated sodium chloride (10%). Fish were submitted to four treatments: control fish were fed with a commercial diet and three groups of fish were fed with diets partially supplemented with earthworms. All fish showed the same appetite for the earthworm-supplemented diets as for the control diet during the course of the experiment (8 weeks). No significant differences (p > 0.05) were detected in the mean final body weights of all groups of fish. A significant decrease was found in the whole-carcass lipid content of fish fed diets containing 25%, 50% and 75% frozen earthworms. The results on growth rate and feed utilization efficiency of fish fed diets containing high levels of whole frozen worms suggested an adverse effect of worm incorporation, probably due to dietary energy/protein imbalance.     

The effects of prolonged exercise training on growth performance and production parameters in fish

When juvenile salmonids are reared in water flowing with currents equivalent to swimming speeds of about 0.75–1.5 BL s^–1, the fish tend to grow faster and make more efficient use of the food provided than do conspecifics held in standing water. There may be a more even distribution of food within a group of exercised fish leading to uniformity of growth rates and a reduced size range of the fish at harvest. In addition, frequencies of aggressive interactions may be lower in exercised fish than in those reared in standing water, leading to fewer exercised fish having fin damage. Thus, several benefits may be expected to accrue from growing salmonids in flowing water. The training of salmonids by exposing them to moderate water currents for prolonged periods leads to improvements in physiological performance, so rearing such fish in flowing, rather than standing, water may be particularly beneficial when they are destined for release in connection with restocking or ranching programmes.     

Brackishwater carp culture in potentially waterlogged areas using animal wastes as pond fertilizers

In order to assess the possibilities of utilizing drainage effluents (salinity range 5.0–12.5), fish culture experiments were carried out. Experiments on polyculture using cow dung (24 000 kg ha^–1 y^–1) as pond fertilizer were conducted at five different salinity levels (0.3–8.5). Studies have revealed that carp perform well in salinities up to 7.5 and reasonably high fish production has been obtained. Even though the ponds had a high trophic status, higher salinities ( > 7.5) appear to repress fish growth probably due to low dissolved oxygen (DO), high BOD and high NH₄-N. Experiments on monoculture of common carp (Cyprinus carpio) conducted at two different salinity levels (0.3–0.9 and 6.0–7.0) using four different organic fertilizers (cow dung at 24 000 kg and 20 000 kg, poultry at 1500 kg, duck at 6000 kg and sheep/goat at 1500 kg ha^–1 y^–1) have revealed the highest fish growth to be in poultry-treated ponds, followed in decreasing order by duck and sheep/goat wastes. Similar trends in fish production were observed both in fresh- and salt-water ponds. However, fish production was lower in ponds having higher salinities ( > 7.5). Nevertheless, these studies indicated that inland saline waters can be utilized for fish culture. With minor modifications in the existing technology of fish culture in stagnant freshwater fish ponds, animal wastes could be used to fertilize brackishwater fish ponds.     

The nature of hematological response in fish

Hematological status was examined in rainbow trout,Oncorhynchus mykiss, held for 3–4 weeks under temperature, photoperiod and P_{O 2} conditions approximating those of their winter, spring and summer habitats. The most striking change observed was in red cell population composition. In winter fish mature cells were predominant; juvenile and developing erythrocytes characterized spring and summer animals. Hemoglobin, hematocrit and both mean erythrocytic volume and hemoglobin were modestly lower in spring and summer than in winter fish. Red cell numbers were not significantly affected. These observations suggest that avoidance of viscosity-based increases in circulatory work cost is more advantageous than elevation of blood O₂-carrying capacity. Although hemoglobin isomorph profiles were significantly altered, there is little evidence that such changes are of critical adaptive importance. Given presumed age-based reduction in gas transport effectiveness, the replacement of mature and senescent cells by more metabolically-competent juvenile cells appears to be the pivotal event in hematological response. Leucocyte counts were significantly elevated in spring and summer as compared to winter fish. Lymphocyte/heterophil ratios declined from 8.27 in winter fish to 3.13 in summer trout. Thrombocyte, monocyte, eosinophil and basophil abundances were little changed.     

Presence of maturation-promoting factor in 17α,20β-dihydroxy-4-pregnen-3-one-induced oocytes of catfish,Clarias batrachus

Full-grown immature Clarias batrachus oocytes respond in vitro to exogenous 17,20-dihydroxy-4-preg-nen-3-one ( 17,20-DP) by undergoing germinal vesicle breakdown (GVBD). Cytosolic extract (CE) prepared from 17,20-DP-induced oocytes has been shown to produce similar effect when microinjected into unstimulated immature oocytes of the same fish. A dose of 50 nl is enough to cause 100% GVBD after 4 h. Maturation-promoting factor was investigated from 17,20-DP-induced, immature and cycloheximide treated oocytes incubated in presence of [³⁵S] methionine. When the proteins were extracted and analyzed on SDS-PAGE, two prominent bands corresponding to molecular weight 34- and 46-kDa were detected in the CE of mature oocytes. However, labelling of [³⁵S] methionine was observed mainly in the region of 46 kDa protein band indicating de novo synthesis of this particular protein during l7,20-DP-induction. Further, immunoblotting study by using rabbit anti-cyclin B₁ antibody has clearly demonstrated that the protein which is newly synthesized is highly homologous to Xenopus cyclin B₁ and goldfish cyclin B.     

Effects of the aromatase inhibitor Fadrozole on plasma sex steroid secretion and oocyte maturation in female coho salmon (Oncorhynchus kisutch) during vitellogenesis

17-estradiol, 17-20-dihydroxy-4-pregnen-3-one (17-20-P), and testosterone levels were measured in plasma samples obtained from vitellogenic coho salmon (Oncorhynchus kisutch) before and 32 days after injection of the aromatase inhibitor Fadrozole (AI). Plasma 17-estradiol levels decreased significantly 6 h after injection in all AI treated fish. The higher the dose the longer the maintenance of low plasma 17-estradiol levels. Inversely, plasma 17-20-P increased significantly 6 h after injection in all AI treated fish, and the higher the dose the longer the maintenance of high plasma 17-20-P levels. At 48 h after injection plasma testosterone levels were significantly higher in the AI treated groups. The oocyte maturation index showed that multiple injections with AI retarded oocyte development. Besides, oocyte diameter and GSI were lower in the same group, which presented high incidence of atresia of vitellogenic oocytes. The ovarian follicles and brain of the fish which received multiple injections secreted less 17-estradiol, in vitro. These findings suggest that aromatase inhibitors such as Fadrozole may have a potential as a tool to regulate sexual development in salmon.     

Modulation of glycoprotein hormone α- and gonadotropin IIβ-subunit mRNA levels in the pituitary gland of mature male African catfish, Clarias gariepinus

The cDNAs encoding the glycoprotein hormone -subunit (GP) and the gonadotropin II-subunit (GTH II) were cloned from the pituitary gland of the African catfish, Clarias gariepinus. Using RNase protection analysis, we studied the steady-state mRNA levels of GP as well as GTH II in the pituitary gland of adult male catfish. Castration of adult male catfish resulted in a significant decrease of GTH II mRNA levels, whereas there was no change in the GP mRNA levels. Treatment of intact males with a single dose of 11-ketotestosterone (11-KT) resulted in dose-dependent increases in mRNA levels of both GP and GTH II. We conclude that 11-KT, a prominent, non-aromatizable teleost androgen, has a stimulatory effect on the pituitary mRNA levels of GP and GTH II of adult male fish.     

Feed-growth relationships of Arctic charr transferred from freshwater to saltwater at different seasons

Groups of Arctic charr, Salvelinus alpinus (L.), originating from an anadromous stock, were transferred directly from freshwater to seawater (35 S), from freshwater to brackish water (20 S), or were gradually adapted to seawater over a period of 10 days. A control group was kept in freshwater. Feed intake and growth were then monitored during the following 59 days. Two experiments were carried out, one during winter (December–January) and the other during summer (June–July). Water temperature was maintained at 8 C. All fish had been held under natural photoperiod and temperature conditions from the end of the first feeding period (mid-April) until the experiments were carried out. Rates of growth did not differ between the groups of charr held in fresh- and brackish water, but growth of these groups was better than that of fish reared in seawater. Relationships between individual feed intake (FI) and individual specific growth rates (SGR), within groups were expressed as: ln (SGR+1) = a + ln (FI+1), and regression lines were compared using analysis of covariance. In winter, the regression line for fish transferred directly to seawater had a significantly steeper slope, and a lower elevation, than the lines for fish transferred to brackish water or held in freshwater. Fish transferred gradually to seawater had an intermediate position. In summer, the regressions for all groups were parallel with intercepts in the seawater groups being significantly lower than in the freshwater group. This indicates that the gross feed conversion efficiency (FCE) in Arctic charr was lower in seawater than in freshwater. The results are discussed in relation to digestion, gut function and osmoregulation. yc]Keywords xc]Arctic charr (Salvelinus alpinus (L.)) xc]Feed utilization xc]Salinity xc]Season     

Effects of cortisol and triiodo-L-thyronine on the steroidogenic capacity of rainbow trout ovarian follicles at two stages of oocyte maturation

The in vitro basal and salmon gonadotropin (sGTH)-stimulated steroidogenic capacity of rainbow trout follicles was examined at four stages [early (EV)-, mid (MV)- and peak-vitellogenic (PV), and pre-ovulatory, post-vitellogenic (PO)] of gonadal recrudescence using radioimmunoassays (RIAs) to measure 17-estradiol (E₂) and testosterone (T) production. In addition, follicles were incubated in the presence of [³H]pregnenolone ([³H]P⁵) and the radiolabelled steroid metabolites produced were separated using high performance liquid chromatography (HPLC). Peak basal and sGTH-stimulated E₂ and T production was found in PV stage follicles and lowest in PO stage follicles, and there were marked differences in the HPLC profiles of steroid metabolites. For EV stage follicles the major metabolite eluted as a peak that co-eluted with the androstenedione (A₄) and 17-hydroxyprogesterone (17-OHP) standards. A smaller peak that co-eluted with 11-hydroxyandrostenedione (11-OHA₄) and very small peaks co-eluting with 20-dihydroprogesterone (20-DHP) and E₂ were also seen. MV and PV stage follicles produced predominantly E₂, together with a small combined A₄ + 17-DHP peak, traces of 11-OHA₄ and two peaks that did not co-elute with any of the reference standards. The PO stage follicles produced only 17, 20-dihydroxy-4-pregnene-3-one (17,20-P).In addition, the effects of cortisol and triiodothyronine (T₃) on steroidogeneis were investigated in PV and PO stage ovarian follicles. For PV stage follicles, cortisol at 100 ng ml^–1 in the incubation medium significantly suppressed both basal and sGtH- stimulated T and E₂ production relative to control treatments. T₃ at 10 ng ml^–1 in the medium had no significant effect on either basal or sGtH-stimulated T or E₂ production compared to the controls, nor did it have any beneficial effect over the suppressive effect of cortisol. PO phase follicles taken 1 to 2 weeks prior to anticipated spawning had very low E₂ and T production, and there was no effect of cortisol or T₃, alone or in combination, on E₂ or T production. For PV stage follicles incubated in the presence of [³H]P⁵, cortisol suppressed T and E₂ production, but did not block the steroid pathway at any specific level; T₃ had no apparent affect on the metabolism of [³H]P⁵. The PO stage follicles produced little or no E₂; the major metabolite was 17,20-P. Cortisol and T₃ had no apparent effect on either basal or sGtH-stimulated 17,20-P production by the follicles at this stage of maturation.     

Dexamethasone receptors and their distribution in the brain of the red tilapia

Cytosolic extracts of liver and brain showed specific, saturable binding with [³H]dexamethasone: Scatchard plots were linear. Cortisol and 11-deoxycortisol acted as competitors, whilst 17,20-dihydroxy-4-pregnen-3-one was relatively much less effective. There was a four-fold range of differences in specific binding capacity between dissected brain regions, the order being telencephalon > hypothalamus > rhombencephalon > optic tectum (thalamus + tegmentum + torus semicircularis) when N_max was calculated in terms of DNA content. There was no evidence for any binding of [³;H]dexamethasone with membrane fractions.