Gilles Boeuf

Salmonid smolting: a pre-adaptation to the oceanic environment

Salmonids are a small family of fishes characterized by high morphological and behavioural homogeneity. These characteristics make it possible, for example, to produce many interspecific hybrids, some of them fertile. However, there is one specific aspect where a clear difference appears, allowing separation of the family into two groups, one remaining in fresh water, the other migrating to the sea. Although at present, no salmonid species is totally marine, and some species remain in fresh water for their entire life. others achieve an active migration from fresh water (FW) to the estuary, and finally to the sea. Later they return to fresh water for spawning.

Melatonin effects on the hypothalamo–pituitary axis in fish

Melatonin, a hormonal output signal of vertebrate circadian clocks, contributes to synchronizing behaviors and neuroendocrine regulations with the daily and annual variations of the photoperiod. Conservation and diversity characterize the melatonin system: conservation because its pattern of production and synchronizing properties are a constant among vertebrates; and diversity because regulation of both its synthesis and modes of action have been profoundly modified during vertebrate evolution. Studies of the targets and modes of action of melatonin in fish, and their parallels in mammals, are of interest to our understanding of time-related neuroendocrine regulation and its evolution from fish to mammals, as well as for aquacultural purposes.

Smoltification and seawater adaptation in Atlantic salmon (Salmo salar): plasma prolactin, growth hormone, and thyroid hormones.

To obtain more information on the role of prolactin and growth hormone during the parr-smolt transformation of Atlantic salmon, a population of fish in fresh water was sampled from January to June during two consecutive years. Gill Na+,K+-ATPase activity increased steadily during smoltification and a plasma thyroxine peak was observed 2-3 weeks before the gill Na+,K+-ATPase peak. On the basis of these two parameters, smoltification was considered complete in our populations in April 1985 and May 1986. Two peaks in plasma growth hormone levels occurred in 1986, one in mid-April and the second in mid-May. In both cases, these peaks coincided with a peak in plasma triiodothyronine and preceded the thyroxine peak by 1-2 weeks. Moreover, the second peak which lasted for 1 month coincided with maximal gill Na+,K+-ATPase activity. A decrease in plasma prolactin levels was observed during smoltification of Atlantic salmon in 2 consecutive years. During this period of decreasing and low plasma prolactin levels, gill Na+,K+-ATPase activity increased to its highest values. Atlantic salmon smolts were also directly transferred into seawater. After 2 days or more in seawater, plasma prolactin levels were not significantly different from those on Day 0, whereas in fresh water they showed large fluctuations. All these data indicate that growth hormone may play an important role in the development of hypoosmoregulatory activity. Increased hypoosmoregulatory ability also appears to be associated with low prolactin levels.

Potential of plant-protein sources as fish meal substitutes in diets for turbot (Psetta maxima): growth, nutrient utilisation and thyroid status.

Abstract An experiment was conducted in order to assess the incorporation in diets for juvenile turbot of extruded lupin ( Lupinus albus ) and heat-treated (RM1) or untreated (RM2) rapeseed meals ( Brassica napus ) (26 and 40 μmol glucosinolate/g DM, respectively). The level of incorporation of 30% for each plant-protein, as well as 46% for RM1 and 50% for lupin was tested and compared with a fish meal based control diet. Triplicate groups of turbot (initial body weight of 66 g) were fed by hand with isonitrogenous and isoenergetic experimental diets, twice daily and to visual satiety, during 63 days. Extruded lupin can be incorporated in diets of turbot up to a level of 50% without adverse effects on growth performance and body composition. Rapeseed meal can only be incorporated at levels up to 30%, but a preliminary heat treatment of RM is necessary in order to improve its nutritional quality. In turbot-fed the RM-based diets, plasma T 4 levels were reduced with low dietary content in glucosinolate breakdown products (3.6 μmol/g), but no decrease in plasma T 3 levels was observed with the higher level of toxic compounds (4.4 μmol/g). A significant deiodinase type II compensatory effect, leading to an increase of the conversion of T 4 to T 3 , was observed in vitro in the liver of turbot fed RM1-based diets. The intake of lupin-based diets also had an effect on thyroid status with an increase of plasma T 3 levels and of deiodinase type I activity in liver and kidney, suggesting an increase in the degradation of rT 3 and in the conversion of T 4 to T 3 .

Effects of hypoxia on growth and metabolism of juvenile turbot.

The effects of hypoxia on growth, feed efficiency, nitrogen excretion, oxygen consumption and metabolism of juvenile turbot (120 g) were studied in a 45-day experiment carried out in sea water at 17.0±0.5°C and 34.5 ppt salinity. Fish were fed to satiation at O2-concentrations of 3.5±0.3, 5.0±0.3 mg l−1 (hypoxia) and 7.2±0.3 mg l−1 (normoxia). Both feed intake (FI) and growth were significantly lower under hypoxia than under normoxia, with no significant differences being observed between 3.5 and 5.0 mg O2 l−1. During the first 2 weeks of the experiment, FI was halved under hypoxic conditions, and there were large differences among treatments in feed conversion ratio (FCR), i.e., it was 3.2, 1.5, and 0.9 in turbot exposed to 3.5, 5.0, and 7.2 mg O2 l−1, respectively. Thereafter, FCR was not significantly affected by O2-concentration. Nitrogen excretion and oxygen consumption of feeding fish were significantly higher under normoxia than under hypoxia, but following 7 days of feed deprivation oxygen consumption was similar under normoxia and hypoxia. Plasma osmolarity, ionic balance, and acid-base status were not affected by the two hypoxic conditions tested. Overall, our results indicate that turbot have some capacity to adapt to relatively low ambient O2-concentrations.

Structural and Functional Evolution of the Pineal Melatonin System in Vertebrates

In most species daily rhythms are synchronized by the photoperiodic cycle. They are generated by the circadian system, which is made of a pacemaker, an entrainment pathway to this clock, and one or more output signals. In vertebrates, melatonin produced by the pineal organ is one of these outputs. The production of this time‐keeping hormone is high at night and low during the day. Despite the fact that this is a well‐preserved pattern, the pathways through which the photoperiodic information controls the rhythm have been profoundly modified from early vertebrates to mammals. The photoperiodic control is direct in fish and frogs and indirect in mammals. In the former, full circadian systems are found in photoreceptor cells of the pineal organ, retina, and possibly brain, thus forming a network where melatonin could be a hormonal synchronizer. In the latter, the three elements of a circadian system are scattered: the photoreceptive units are in the eyes, the clocks are in the suprachiasmatic nuclei of the hypothalamus, and the melatonin‐producing units are in the pineal cells. Intermediate situations are observed in sauropsids. Differences are also seen at the level of the arylalkylamine N‐acetyltransferase (AANAT), the enzyme responsible for the daily variations in melatonin production. In contrast to tetrapods, teleost fish AANATs are duplicated and display tissue‐specific expression; also, pineal AANAT is special—it responds to temperature in a species‐specific manner, which reflects the fish ecophysiological preferences. This review summarizes anatomical, structural, and molecular aspects of the evolution of the melatonin‐producing system in vertebrates.

Incorporation of high levels of extruded lupin in diets for rainbow trout (Oncorhynchus mykiss): nutritional value and effect on thyroid status

Three experiments and a digestibility trial were conducted in order to assess the incorporation of extruded lupin (Lupinus albus) in diets for juvenile rainbow trout. Digestibility of protein and phosphorus were higher in lupin than in fish meal, but digestibility of dry matter and energy were lower. The first trial was designed to determine the maximum level of incorporation of lupin in the diet of trout. Levels of 30, 50 and 70% were tested and compared with a fish meal-based control diet. The diets were formulated to be isonitrogenous and isoenergetic. Triplicate groups of trout were fed twice daily to visual satiety by hand during 64 days. Two subsequent trials, using another yearly crop of lupin, were performed to analyze the effects of very high levels of incorporation of lupin. Growth performances, feed intake and nitrogen balance of fish fed diets with 50% of lupin incorporation were comparable to those of fish fed the control diet. However, higher fat deposition was observed. Incorporation of lupin led to higher phosphorus retention and lower phosphorus excretion, but only in two of the three trials. In trout fed the diets containing 70% lupin, growth was reduced by 41%, feed intake by 15% and nitrogen retention by 12% when the first crop of lupin was used. Feed intake was not reduced and growth performance was higher when the second crop of lupin was used, i.e., a decrease of only 16% when fish were fed by hand to satiety (decrease of feed efficiency) or null when fish were fed on demand using self-feeders. The incorporation of lupin can lead to a decrease in plasma thyroxine levels, but this effect was not clear and not recurring. However, when this effect was observed, a deiodinase compensatory effect adjusted the plasma triiodothyronine levels. In general, the plasma triiodothyronine levels were related to the growth performance of the trout.

Growth hormone and thyroid hormones during Atlantic salmon, Salmo salar L., smolting, and after transfer to seawater

Abstract Gill (Na + K + )-ATPase activity of juvenile salmon, Salmo salar L., increased “classically” from February to the end of April in experiments in two different years, and its level before transfer to seawater was correlated directly with seawater performance (survival, plsma osmotic pressure and plasma chloride levels). A thyroxine (T4) surge occurred 2–3 weeks before the peak of gill enzyme levels. After the T4 peak the fish were clearly euryhaline, but T4 levels were not correlated with seawater adaptability. They remained able to adapt rapidly to seawater for at least 1 month after the surge. Two significant increases of tri-iodothyronine (T3) occurred while the fish were in freshwater: transfer to seawater either decreased T3 and T4 or had no effect. In freshwater, plasma growth hormone (GH) levels rose sharply concomitant with the T3 peak, 2 weeks before peak gill ATPase activity. After transfer to seawater GH increased significantly, remaining high for 7–10 days, and returning to base levels after 14 days. GH appeared to play a major role in smolting and seawater adaptation.

Genetic, Temporal and Developmental Differences Between Melatonin Rhythm Generating Systems in the Teleost Fish Pineal Organ and Retina

Complete melatonin rhythm generating systems, including photodetector, circadian clock and melatonin synthesis machinery, are located within individual photoreceptor cells in two sites in Teleost fish: the pineal organ and retina. In both, light regulates daily variations in melatonin secretion by controlling the activity of arylalkylamine N‐acetyltransferase (AANAT). However, in each species examined to date, marked differences exist between the two organs which may involve the genes encoding the photopigments, genes encoding AANAT, the times of day at which AANAT activity and melatonin production peak and the developmental schedule. We review the fish pineal and retinal melatonin rhythm generating systems and consider the evolutional pressures and other factors which led to these differences.

Dietary low-glucosinolate rapeseed meal affects thyroid status and nutrient utilization in rainbow trout (Oncorhynchus mykiss).

Two rapeseed (Brassica napus) meals, RM1 and RM2, with two levels of glucosinolates (GLS; 5 and 41 mumol/g DM respectively) were incorporated at the levels of 300 and 500 g/kg of the diets of juvenile rainbow trout (Oncorhynchus mykiss) in replacement of fish meal, and compared with a fish-meal-based diet. A decrease in the digestibility of the DM, protein, gross energy and P was observed with high-rapeseed meal (RM) incorporation. In trout fed on RM-based diets, growth performance was reduced even after only 3 weeks of feeding. Feed efficiency was adversely affected by RM and GLS intake. Protein and energy retention coefficients were significantly lower in fish fed on the diet containing the higher level of GLS. P retention was significantly lower with all the RM-based diets than with the fish-meal diet. Irrespective of the degree of growth inhibition, fish fed on RM-based diets exhibited similar typical features of hypothyroid condition due to GLS intake, expressed by lower plasma levels of triiodothyronine and especially thyroxine and a hyperactivity of the thyroid follicles. This hypothyroidal condition led to a strong adjustment of the deiodinase activities in the liver, the kidney and the brain. A significant increase of the outer ring deiodinase activities (deiodinases type I and II respectively) and a decrease of the inner ring deiodinase activity (deiodinase type III) were observed. It is concluded that the observed growth depression could be attributed to the concomitant presence of GLS, depressing the thyroid function, and of other antinutritional factors affecting digestibility and the metabolic utilization of dietary nutrients and energy.