Effects of short-day treatment on long-term growth performance and maturation of farmed Arctic charr Salvelinus alpinus reared in brackish water.

The effects of a 6 week short-day photoperiod followed by continuous light, applied during the juvenile phase of Arctic charr Salvelinus alpinus in fresh water on smoltification and on the long-term growth and maturity following transfer to brackish water (BW) (constant salinity of either 17 and 27 or increasing salinity in steps from 17 to 27) were investigated. Prior to salinity transfer, the juveniles were either reared at continuous light (C group) or reared for 6 weeks on a short day (8L:16D, S group) followed by continuous light (24L:0D). Increased salinity had negative effect on growth, with female fish reared at 17 salinity weighing 19 and 27% more than the salinity-step group (17-27) and the 27 salinity group, respectively. The stepwise acclimation to salinity had limited advantage in terms of growth rate. Short photoperiod for 6 weeks (November to January) followed by continuous light improved growth, but not seawater (SW) tolerance. Gill Na(+) , K(+) -ATPase activity and plasma Na(+) levels changed with time, indicating some variation in osmoregulatory capacity during the experimental period. Overall, there appear to be interactive effects on maturation from applying short-day photoperiod followed by rearing at higher salinities. Plasma leptin varied with time and may be linked to stress caused by the observed variations in osmoregulatory ability. It is concluded that changes in growth rates observed in this study are mainly related to rearing salinity with higher growth rates at lower salinities. Short-day photoperiod has some growth-inducing effects but did not improve SW tolerance. Farmers of S. alpinus using BW for land-based rearing should keep salinity at moderate and stable levels according to these results to obtain best growth.

Long-term rearing of Arctic charr Salvelinus alpinus under different salinity regimes at constant temperature.

Arctic charr Salvelinus alpinus of the Hólar strain (mean ± s.e. body mass = 152·1 ± 3·1 g) were reared at four different salinity regimes at a constant temperature of 7·4° C. Two groups were given a three-month acclimation in salinity 18 before the salinity was increased to either 25 or 29 (groups called A25 and A29), and two groups were reared in salinities 25 or 29 over the full experimental period of 409 days (groups called F25 and F29). In the first 3 months, the A25 and A29 groups had the highest growth rates. By October 2011, there were no significant differences (two-way nested ANOVA, P > 0·05) in the mean body masses among A25, F25 and F29 (c. 1450 g), whereas A29 had a lower mean mass (1282 g). The growth in the last period from October 2011 to January 2012 was reduced by sexual maturation in the highest salinity regimes (A29 and F29), whereas fish in groups A25 and F25 showed high growth throughout the study. Males in all salinity groups had higher growth rates than females for the most part of the study, but the divergence between the sexes was most pronounced in the highest salinity regimes. All salinity groups showed distinct changes in Na(+) , K(+) -ATPase activity, with high activity in spring and summer, and lower activity in the autumn. Plasma sodium (Na(+) ) levels were stable indicating that none of the experimental groups had problems in maintaining hydromineral balance during the study. While plasma leptin levels were not affected by salinity regimes, it was noted that these levels were 13-30% higher in fish with empty guts compared with those having food in their gut at the time of sampling. This suggests a link between leptin levels and food intake, indicating that this hormone may play a role in food intake and energy allocation in fishes.

Changes in arterial oxygen tension and physiological status in resting, unrestrained Arctic charr Salvelinus alpinus (L.) exposed to mild hypoxia and hyperoxia.

In arctic charr Salvelinus alpinus, arterial blood partial pressures of oxygen (PaO2) and carbon dioxide increased with increasing water oxygen tension (PwO2), while the water to arterial PO2 difference (PwO2-PaO2) did not change in relation to PwO2.

Effects of high intensity exercise training on cardiovascular function, oxygen uptake, internal oxygen transport and osmotic balance in chinook salmon (Oncorhynchus tshawytscha) during critical speed swimming.

To examine cardiorespiratory plasticity, cardiovascular function, oxygen consumption, oxygen delivery and osmotic balance were measured at velocities up to critical swimming speed (Ucrit) in seawater-adapted chinook salmon. We used two groups of fish. The control group had swum continuously for 4 months at a low intensity (0.5 BLs(-1)) and the other was given a high-intensity training regimen (a Ucrit swim test on alternate days) over the same period of time. Compared with available data for other salmonid species, the control group had a higher maximum oxygen consumption (MO2max; 244 micromol O2 min(-1) kg(-1)), cardiac output (Qmax; 65 ml min(-1) kg(-1)) and blood oxygen content (CaO2; 15 ml O2 dl(-1)). Exercise training caused a 50% increase in MO2max without changing either Ucrit or CaO2, even though there were small but significant increases in hematocrit, hemoglobin concentration and relative ventricular mass. During swimming, however, exercise-trained fish experienced a smaller decrease in body mass and muscle moisture, a smaller increase in plasma osmolality, and reduced venous oxygen stores compared with control fish. Consequently, exercise training apparently diminished the osmo-respiratory compromise, but improved oxygen extraction at the tissues. We conclude that the training-induced increase in MO2max provided benefits to systems other than the locomotory system, such as osmoregulation, enabling trained fish to better multitask physiological functions while swimming. Furthermore, because a good interspecific correlation exists between MO2max and arterial oxygen supply (TO2max; r2=0.99) among temperate fish species, it is likely that CaO2 and Qmax are principal loci for cardiorespiratory evolutionary adaptation but not for intraspecific cardiorepiratory plasticity as revealed by high intensity exercise training.

Gut blood flow in fish during exercise and severe hypercapnia.

This paper reviews the effects of exercise and hypercapnia on blood flow to the splanchnic circulation. Brief struggling behaviours are known to decrease blood flow to the gut (GBF). Likewise, prolonged swimming in unfed fish has been shown to reduce GBF in proportion to the increased oxygen uptake. Therefore, the normal postprandial increase in GBF theoretically should be impaired whenever fish are active. However, indirect evidence suggests that GBF is spared to some degree when fed fish swim continuously but at a cost (10-15%) to their critical swimming speed. Severe respiratory acidosis can be created by the new intensive aquaculture settings that use oxygen injection into re-circulated water. The only study so far to examine the effects of severe hypercapnia on GBF and its regulation showed that routine GBF and alpha-adrenergic control of GBF remained normal in unfed white sturgeon (Acipenser transmontanus). However, severe hypercapnia produced a hyperactive state and increased sensitivity of GBF to struggling. As a result, routine GBF was maintained for a short period of time. Thus, environmental changes such as severe hypercapnia can indirectly impact GBF through altered struggling behaviour, but the implications of the overall reduction in GBF to food assimilation have yet to be established.

Gastrointestinal blood flow in the red Irish lord, Hemilepidotus hemilepidotus: long-term effects of feeding and adrenergic control.

Cardiac output, blood flow to the coeliac and mesenteric arteries, dorsal aortic blood pressure and heart rate were recorded simultaneously at rest and postprandial for 6 days in a teleost, the red Irish lord (Hemilepidotus hemilepidotus). We anticipated that gastrointestinal blood flow would increase postprandially, supported by an increase in cardiac output. However, we had no predictions for either the exact time-course of this response, or for the regional distribution of blood flow between to the two major arteries comprising the splanchnic circulation. In resting, unfed animals, blood flow to the coeliac artery and mesenteric artery was 4.1 +/- 0.6 ml min(-1) kg(-1) and 4.9 +/-1.3 ml min(-1) kg(-1), respectively (mean +/- SEM, n = 7), which together represented 34% of cardiac output. Feeding increased blood flow to the coeliac and mesenteric arteries in a time-dependent manner. The increase in coeliac artery blood flow preceded that in the mesenteric artery, a finding that is consistent with the coeliac artery supplying blood to the liver and stomach, while the mesenteric artery supplies blood to the stomach and intestine. Coeliac blood flow had increased by 84 +/- 18% after 1 day and had a peak increase of 112 +/- 40% at day 4 postprandial. Mesenteric blood flow was not significantly elevated at day 1, but had increased by 94 +/- 19% at day 4 postprandial. Cardiac output also increased progressively, increasing by a maximum of 90 +/- 30% at day 4. Because the increase in cardiac output was adequate to meet the postprandial increase in gut blood flow, the postprandial decreases in vascular resistance for the coeliac and mesenteric circulations mirrored the increases in blood flow. Intra-arterial injections of adrenaline and noradrenaline into resting fish more than doubled coeliac and mesenteric vascular resistances, and blood flow decreased proportionately. This adrenergic vasoconstriction was totally abolished by pretreatment with the alpha-adrenoceptor antagonist phentolamine, which in itself approximately halved coeliac and mesenteric vascular resistances. These observations indicate a significant alpha-adrenergic tone in the gastrointestinal circulation of the red Irish lord, the loss of which could not entirely account for the post-prandial increase in gastrointestinal blood flow. Other control mechanisms are suggested.

CO2 transport and excretion in rainbow trout (Oncorhynchus mykiss) during graded sustained exercise.

A quantitative analysis of CO2 transport and excretion was conducted in seawater acclimated rainbow trout (Oncorhynchus mykiss) swimming at different sustained swimming velocities. CO2 excretion increased linearly with cardiac output during exercise but arterial P(CO2) (Pa(CO2)) and total CO2 levels also increased indicating a diffusion limitation to CO2 excretion. The elevated Pa(CO2) was not accompanied by a decrease in pH, indicating that the acid-base compensation was rapid. Mixed-venous P(CO2) increased to a greater extent than Pa(CO2) resulting in a large increase in the venous arterial difference in P(CO2) (Pv(CO2) - Pa(CO2)). The Pv(CO2) - Pa(CO2) difference was used to calculate the proportion of total CO2 excreted comprised of dissolved CO2 which accounted for less than 1% of total CO2 excreted in fish swimming at 11 cm sec(-1) but increased to about 9% at the greatest swimming velocity indicating that the pattern of CO2 excretion changes during exercise. There was no effect of exercise on the proportion of CO2 excreted which was dependent upon HCO3-/Cl- exchange (54%) or that which was dependent upon the dehydration of HCO3- that resided within the red cell prior to gill blood entry (42%). The large proportion of total CO2 excreted that was dependent upon HCO3-/Cl- exchange is significant because this is thought to be the rate limiting step in CO2 excretion.

The interaction between O2 and CO2 exchange in rainbow trout during graded sustained exercise.

A quantitative analysis of O2 and CO2 transport was conducted in resting and exercising rainbow trout, and these data were used to quantify the magnitude of coupling between O2 and CO2 exchange, in vivo. The release of Bohr protons during haemoglobin-oxygenation was non-linear over the Hb-O2 equilibrium curve used in trout subjected to different levels of sustained exercise. At low swimming speeds, when venous blood O2 content (CvO2) was high, there was a small acidosis as blood passed through the gills, indicating more protons were released during oxygenation of Hb than were consumed during HCO3- dehydration. At higher swimming speeds, when CvO2 was low, there was a significant alkalosis in arterial relative to venous blood, indicating that fewer protons were released upon oxygenation than HCO3- ions were dehydrated to CO2. Haldane coefficients (moles of protons released per mole of O2 which binds to Hb), calculated from steady state arterial and mixed-venous parameters, revealed that under resting conditions all blood CO2 removed from the blood during gill transit was stoichiometrically related to O2 uptake through the release of Bohr protons during Hb oxygenation. The magnitude of coupling between CO2 excretion and O2 uptake decreased from 100% to less than 40% at the maximal swimming velocity when the largest region of the Hb-O2 equilibrium curve was used for gas exchange. The non-linear release of Bohr protons over the range of Hb-O2 saturation in the blood reduces HCO3- dehydration at the gills during greater work loads elevating arterial P(CO2) levels, leading to an increase in HCO3- buffer capacity of the blood and tissues.

Energy turnover in the normoxic and anoxic turtle heart.

We examined the possibility that the heart of the tuttle Chrysemys scripta is an exceptional anaerobic performer, by measuring myocardial power output, lactate output, and estimated ATP turnover in perfused heart preparations. Over a range of myocardial power outputs at 5 and 15 degrees C we find that turtle hearts perfused with anoxic saline do not show a particularly outstanding ability to produce ATP anaerobically. Furthermore, at 15 degrees C anoxia reduced the ATP turnover rate to 50% of the normoxic rate. At 5 degrees C the anoxia-induced depression of ATP turnover was even more pronounced, being 4-fold lower than the normoxic rate. In addition, anoxia at 5 degrees C reduced the basal metabolic rate of the tuttle heart. We conclude that long-term cardiac tolerance of hypoxia in this species is more likely related to metabolic depression rather than to an exceptional anaerobic performance.

11-Ketotestosterone suppresses interrenal activity in rainbow trout (Oncorhynchus mykiss).

Interrenal activity in relation to sexual maturity in male rainbow trout, and the effects of in vivo treatment with 11-ketotestosterone (11-KT) were investigated. Silastic pellets containing either 11-KT or no steroid were implanted in maturing male and immature male and female rainbow trout for a total of 11 weeks. Two weeks before termination of the experiment, animals were subjected to crowding, and blood samples were taken 15-90 min after initial disturbance. At the termination of the experiment, head kidneys containing interrenal cells were removed and incubated in vitro with either human adrenocorticotropin 1-24 (ACTH) or with pregnenolone. 11-KT reduced the elevation of cortisol in response to confinement stress. Head kidney tissue of control and 11-KT-treated maturing males was significantly less responsive to ACTH and pregnenolone than tissue from immature males. Treatment of immature females with 11-KT similarly led to reductions in interrenal responsiveness. No difference in response was seen between tissue from control immature males and females. These data suggest that the attenuated stress response in maturing males is due, in part, to elevated 11-KT levels acting at one or more levels of the hypothalamus-pituitary-interrenal axis.

Hematocrit in oxygen transport and swimming in rainbow trout (Oncorhynchus mykiss).

The optimal hematocrit (Hctopt) hypothesis was tested by altering Hct (and arterial blood oxygen content, CaO2) between extreme states of anemia and polycythemia (Hct = 8-55%) in the rainbow trout. Since blood viscosity (eta) effects on cardiac output (Q) and O2 transport (TO2) are likely to be greatest when O2 demand and Q are maximal, we challenged fish to swim to their critical swimming velocity (Ucrit) in a swim-tunnel respirometer at 13 degrees C and measured maximal oxygen uptake (VO2max), maximum Q(Qmax), and other cardiovascular variables. In addition, experimental temperature was lowered to 5 degrees C to increase eta. Consistent with the Hctopt hypothesis, the decreased CaO2 in anemic (Hct < 22%) fish caused significant reductions in Ucrit and VO2max. In contradiction to the Hctopt hypothesis, and despite an exponential relationship between eta and Hct, maximal TO2 (TO2max) and Ucrit increased with polycythemia up to Hct 55%. Although there was a peak for VO2max, it occurred at an Hct (42%) well above the normocythemic range (23-33%). These results clearly demonstrate that eta is not significant in setting normocythemia in rainbow trout. The novel finding of an Hct-dependent relationship for exercise-induced arterial hypoxemia may be indicative of a diffusion limitation to normocythemia. We suggest that factors involved in setting normocythemia in vertebrates should include diffusion limitations to oxygen transfer in addition to blood viscosity and oxygen transport constraints.

MECHANICAL PERFORMANCE OF AN IN SITU PERFUSED HEART FROM THE TURTLE CHRYSEMYS SCRIPTA DURING NORMOXIA AND ANOXIA AT 5 C AND 15 C

We developed an in situ perfused turtle (Chrysemys scripta) heart preparation to study its intrinsic mechanical properties at 5°C and 15°C using normoxic and anoxic perfusion conditions. The in situ preparation proved durable and stable. At 15°C and a spontaneous heart rate of 23.4 beats min-1, maximum stroke volume was 2.54 ml kg-1 body mass, maximum cardiac output was 62.5 ml min-1 kg-1 and maximum cardiac myocardial power output was 1.50 mW g-1 ventricular mass. There was good agreement between these values and those previously obtained in vivo. Furthermore, since the maximum stroke volume observed here was numerically equivalent to that observed in ventilating C. scripta in vivo, it seems likely that C. scripta has little scope to increase stroke volume to a level much beyond that observed in the resting animal through intrinsic mechanisms alone. The ability of the perfused turtle heart to maintain stroke volume when diastolic afterload was raised (homeometric regulation) was relatively poor. At 5°C, the spontaneous heart rate (8.1 beats min-1) was threefold lower and homeometric regulation was impaired, but maximum stroke volume (2.25 ml kg-1) was not significantly reduced compared with the value at 15°C. The significantly lower maximum values for cardiac output (18.9 ml min-1 kg-1) and power output (0.39 mW g-1 ventricular mass) at 5°C were largely related to pronounced negative chronotropy with only a relatively small negative inotropy. Anoxia had weak negative chronotropic effects and marked negative inotropic effects at both temperatures. Negative inotropy affected pressure development to a greater degree than maximum flow and this difference was more pronounced at 5°C than at 15°C. The maximum anoxic cardiac power output value at 15°C (0.77 mW g-1 ventricular mass) was not that different from values previously obtained for the performance of anoxic rainbow trout and hagfish hearts. In view of this, we conclude that the ability of turtles to overwinter under anoxic conditions depends more on their ability to reduce cardiac work to a level that can be supported through glycolysis than on their cardiac glycolytic potential being exceptional.

Smoltification induced by a 'skeleton' photoperiod in underyearling coho salmon (Oncorhynchus kisutch).

Underyearling coho salmon fry were subjected to three initial photoperiod treatments (6L∶18D, 10L∶14D, 14L∶10D) for two months and subsequently to three final treatments (16L∶8D, 9L∶6D∶1L∶8D, 10L∶14D) in a factorial design. Growth rates and seawater adaptability were monitored regularly. The groups that were exposed initially to 6L∶18D or 10L∶14D and then to 16L∶8D 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 9L∶6D∶1L∶8D skeleton photoperiod, showed a slightly lower growth rate and seawater adaptability than those given the corresponding complete 16L∶8D photoperiod. However fish maintained on skeleton photoperiods had significantly greater growth rates and seawater adaptability than those kept on the 10L∶14D 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 14L∶10D showed little or no response to subsequent changes in photoperiod, suggesting that responsiveness to inductive photoperiods depends on the initial photoperiod treatment.