Effects of graded hypoxia on Atlantic salmon infected with amoebic gill disease

Atlantic salmon Salmo salar with amoebic gill disease (AGD) were exposed to a graded hypoxia (135–40 mmHg water P O₂) and blood samples analysed for respiratory gases and pH at 119, 79·5 and 40 mmHg water P O₂. There were no differences in the rate of oxygen uptake between infected and control fish. However, arterial P O₂, and pH were significantly lower in the infected fish whereas P CO₂ was significantly higher in infected fish compared with controls prior to hypoxia and at 119 mmHg water P O₂. At 79·5 and 40 mmHg water P O₂ saturation, there were no significant differences in blood P O₂ or pH although blood P CO₂ was elevated in AGD affected fish at 50% hypoxia (79·5 mmHg water P O₂). The elevated levels of P CO₂ in fish affected by AGD resulted in a persistent respiratory acidosis even during hypoxic challenge. These data suggest that even though the fish were severely affected by AGD, the presence of AGD while impairing gas transfer under normoxic conditions, did not contribute to respiratory failure during hypoxia.     

Effects of thermal stress on respiratory responses to hypoxia of a South American Prochilodontid fish, Prochilodus scrofa

Oxygen consumption (o₂) and respiratory variables were measured in the Prochilodontid fish, Prochilodus scrofa exposed to graded hypoxia after changes in temperature. The measurements were performed on fish acclimated to 25°C and in four further groups also acclimated to 25°C and then changed to 15, 20, 30 and 35°C. An increase in o₂ occurred with rising temperature, but at each temperature o₂ was kept constant over a wide range of O₂ tensions of inspired water ( Pi o₂). The critical oxygen tensions ( Pc o₂) were Pi o₂= 22 mmHg for 25°C acclimated specimens and after transfer from 25°C to 15, 20, 30 and 35°C the Pc o₂ changed to Pi o₂= 28, 22, 24 and 45 mmHg, respectively. Gill ventilation ( _G ) increased or decreased following the changes in o₂ as the temperature changed and was the result of an accentuated increase in breath frequency. During hypoxia the increases in _G were characterized by larger increases in breath volume. Oxygen extraction was kept almost constant at about 63% regardless of temperature and ambient oxygen tensions in normoxia and moderate hypoxia ( P o₂∼70 mmHg). P. scrofa showed high tolerance to hypoxia after abrupt changes in temperature although its survival upon transfer to 35°C could become limited by the capacity of ventilatory mechanisms to alleviate hypoxic stress.     

Metabolic adjustments of an air-breathing teleost, Channa maculata, to acute and prolonged exposure to hypoxic water

Plasma and tissue metabolite levels were measured in the air-breathing Channa maculata during acute and prolonged exposure to normoxic and hypoxic water. Exposure of the fish to hypoxic water (water oxygen partial pressure, PwO ₂= 50 mmHg) for 1 h caused increases in plasma glucose and lactate, liver and brain lactate, liver a-amino acid, heart and brain alanine and brain succinate levels. The metabolic changes in heart, brain and muscle could only be detected when Pw O₂ was 30 or 10 mmHg. Heart glycogen and liver lipid decreased during acute exposure. Prolonged exposure to hypoxic water ( Pw O₂= 30 mmHg) for 3 days caused an increase in plasma glycerol and liver lactate dehydrogenase activity, and a depletion of glycogen store in all tissues investigated. However, metabolite levels which had been elevated during acute hypoxic exposure were observed to return to their normoxic values after prolonged exposure. It was concluded that anaerobic metabolism was triggered by acute exposure to hypoxic water. Prolonged exposure to hypoxic water induced a metabolic readjustment involving mobilisation of lipid and glycogen stores, which is probably a reflection of the high metabolic load of aerial respiration imposed on the fish during exposure to hypoxic water.     

Effects of aluminium in hard, acid water on metabolic rate, blood gas tensions and ionic status in the rainbow trout

Rainbow trout, Salmo gairdneri , cannulated in the dorsal aorta, were exposed to 2 mgl⁻¹ aluminium (added as Al₂(SO₄)₃, 18H₂O) at pH = 5.0 in hard water of very high calcium concentrations (3.3 mmol 1⁻¹). No changes were observed in the concentration of the major plasma ions. The fish, however, became hypoxic, as seen from a fall in the dorsal aortic oxygen tension from around 100 mmHg to 30–40 mmHg, and a simultaneous increase in carbon dioxide tension. Moreover, exposure resulted in a 15% increase in standard oxygen uptake. The ventilation frequency doubled and swelling of the erythrocytes was observed. The results are discussed in the light of some earlier data on the effects of acid/aluminium exposure in water of low calcium concentration, and reveal an important role for calcium in the physiological responses to low pH and aluminium.     

Implications of river damming: the influence of aquatic hypoxia on the diving physiology and behaviour of the endangered Mary River turtle

River impoundments are characterized by low oxygen levels as a result of reduced water velocity and increased water depth. Bimodally respiring turtle species are likely to be highly sensitive to changes in aquatic PO₂ with decreases in oxygen levels impacting upon their diving ability. The acute and long-term effects of aquatic hypoxia on dive duration, oxygen consumption and blood respiratory properties were examined in hatchlings of the endangered Mary River turtle Elusor macrurus . It was hypothesized that acute exposure to aquatic hypoxia would cause a decrease in dive duration as a consequence of a decrease in reliance on aquatic respiration. With long-term exposure to hypoxia, we predicted that Elu. macrurus would have the capacity to compensate for the acute effect of hypoxia and that dive duration would increase due to an increase in aquatic respiration, haemoglobin concentration and oxygen affinity (P₅₀). When exposed to hypoxic conditions, aquatic respiration in Elu. macrurus was substantially reduced resulting in a 51% decrease in dive duration. Contrary to our predictions, Elu. macrurus hatchlings did not acclimate, and long-term exposure to hypoxic conditions caused Elu. macrurus to lose significantly more oxygen to the hypoxic water than the normoxic acclimated turtles. The exacerbation of long-term hypoxia on the respiratory physiology and diving ecology of Elu. macrurus raises concerns about the impacts of long-term environmental change as a result of habitat alteration on the survival of freshwater turtle populations.     

Respiratory and acid-base disturbances in rainbow trout blood during exposure to chloramine-T under hypoxia and hyperoxia

Rainbow trout Oncorhynchus mykiss were exposed acutely to chloramine-T at a therapeutic concentration (9mg l⁻¹) under moderately hypoxic (water P o₂: l00 mmHg) or hyperoxic (water P o₂: 430mmHg) conditions and arterial blood gas tensions ( P ao₂ and P aco₂) and pH were monitored using an extracorporeal circulation. Hypoxia, alone, resulted in an increased ventilation frequency, a decrease in both arterial P co₂ and P o₂ and an increase in arterial pH. There was no effect of chloramine-T exposure on the measured variables as compared with pre-exposure (hypoxia baseline) values. Hyperoxia, alone, resulted in a decrease in ventilation frequency, an increase in arterial P co₂ and P o₂, and a decrease in arterial pH. Chloramine-T exposure under these conditions caused a significant increase in ventilation frequency, but no significant effect on arterial blood gases or pH as compared with the hyperoxia baseline values. Despite the increase in ventilation caused by chloramine-T during hyperoxia, there was no reduction in P co₂ or increase in P o₂. Although these results were of little pathological significance, this study suggests that chloramine-T, although stimulating ventilation, was impairing the diffusion of co₂ across the gill probably by the secretion of branchial mucus and enhancing the gill boundary layer.     

The occurrence of aerial respiration in Rhinelepis strigosa during progressive hypoxia

Rhinelepis strigosa did not surface for air breathing in normoxic or moderate hypoxic water. This species initiated air breathing when the P _io₂ in the water reached 22 ± 1 mmHg. Once begun, the air-breathing frequency increased with decreasing P _io₂. Aquatic oxygen consumption was 21·0 ± 1·9ml O₂ kg⁻¹h⁻¹ in normoxic water, and was almost constant during progressive hypoxia until the P _io₂ reached 23·9 mmHg, considered the critical oxygen tension (P_co₂). Gill ventilation increased until close to the P _co₂ (7·9-fold) as a consequence of a greater increase in ventilatory volume than in breathing frequency. Gill oxygen extraction was 42 ± 5% and decreased with hypoxia, but under severe hypoxia returned to values characteristic of normoxic. The critical threshold for air breathing was coincident with the P_co₂ during aquatic respiration. This suggests that the air-breathing response is evoked by the aquatic oxygen tension at which the respiratory mechanisms fail to compensate for environmental hypoxia, and the gill O₂ uptake becomes insufficient to meet O₂ requirements.     

Relationship of environmental pCO2 to selected blood parameters of Oncorhynchus kisutch (Walbaum)

Blood samples from sub-adult coho salmon ( Oncorhynchus kisutch ) were correlated with environmental conditions. Carbon dioxide tensions, pH and bicarbonate levels were determined in the blood for fish acclimated to environmental pCO₂ levels of 2.49 and 2.74 mmHg. Fish were removed from the high pCO₂ environment and placed in one with a low pCO₂. Their blood pH and pCO₂ were monitored during the course of acclimation. After 27 days of acclimation to a low pCO₂ environment, a group of fish was sampled and compared to a group of fish acclimated to a high pCO₂ environment. It was found that pCO₂ in the environment affects the rate of diffusion of carbon dioxide from the blood to the water. Under the environmental conditions prescribed in these experiments no significant change in bicarbonate was noticed. It is postulated that a bicarbonate chloride exchange is not a major pathway for the removal of carbon dioxide from the blood of O. kisutch .     

The effects of acute hypoxia and hypercapnia on oxygen consumption of the freshwater European eel

When exposed to hypoxia, eels Anguilla anguilla were able to regulate and maintain Vo₂ down to a water oxygen tension ( Pwo₂ ) of about 25 mmHg, a value far below those reported in other studies. When exposed to hypercapnia, eels showed a depression in Vo₂ as water carbon dioxide tension ( Pwco₂ ) increased. Faced with combined hypoxia-hypercapnia, eels showed an increase in their sensitivity to hypoxia, and the critical oxygen tension increased to 40–45 mmHg. The possible mechanisms underlying these responses were discussed, and the implications of such findings for extensive culture of eels were highlighted.     

Respiratory responses of Oreochromis niloticus (Pisces, Cichlidae) to environmental hypoxia under different thermal conditions

The oxygen uptake ( V O₂), breathing frequency ( f _R), breath volume ( V _S.R), gill ventilation ( V G) and oxygen extraction (%) from the ventilatory current of four groups of Oreochromis niloticus during graded hypoxia were measured under the following acclimation temperatures: 20. 25. 30 and 35°C. The critical oxygen tensions ( P O₂), determined from V O₂ v. P O₂ of inspired water at each experimental temperature were, respectively. 19±1±3±1. 18±0±4±9, 29±7± 4±1 and 30±2± 0.6 mmHg. The f _R remained nearly constant during the reductions of O₂ at all the temperatures studied. V G increased discretely from normoxic levels until the P O₂ was reached, below which it assumed extremely high values (17-fold higher or more). The increases observed in V G resulted, at all the acclimation temperatures, in an elevation in V _S.R rather than in f _R. The extraction of O₂ decreased gradually from normoxia until the P O₂ was reached, below which an abrupt reduction of extraction was recorded, except at 35°C when fish showed a gradual reduction in extraction just below the tension of 80 mmHg.     

A comparative study of blood oxygen transport in turbot and sea bass: effect of chronic hypoxia

A comparative study of blood oxygen binding and carrying capacities of turbot Scophthalmus maximus and sea bass Dicentrarchus labrax , two fish species differing in their demand for oxygen, was carried out under three levels of chronic hypoxia ( P o ₂ = 93, 65 and 40 mmHg) for 40 days. Blood O₂ affinity in normoxia was moderately high in both species ( P ₅₀ was c . 12–13 mmHg at pH 7·7). The Bohr factor was significantly lower in turbot (−0·52) than in sea bass (−0·85). In both species, blood O₂ affinity was not significantly affected by oxygen depletion whatever its level and duration. In turbot, however, P ₅₀ appeared to slightly decrease at the two more severe levels of hypoxia. In both species, blood O₂ carrying capacity was not affected by hypoxia and remained twice as high in sea bass than in turbot.     

Carbon dioxide anaesthesia in rainbow trout: effects of hypercapnic level and stress on induction and recovery from anaesthetic treatment

The physiological and anaesthetic effects of three different levels of air-saturated and buffered CO₂ anaesthesia, P _w co ₂=31, 78, or 125 mmHg, were examined in cannulated rainbow trout Oncorhynchus mykiss . Complete anaesthesia (no opercular movements) was not achieved by these hypercapnic levels after 20 min of CO₂ exposure. Although increasing P _w co ₂ reduced the induction times to the early stages of anaesthesia, it also resulted in increasing hyperventilatory, hypoxaemic, and acid-base disturbances. After a 10-min recovery period, while the respiratory acidosis component of the acid-base disturbance was corrected, there was a significant metabolic acidosis. Recovery time was longest in the high P_w co₂ treatment where 33% of the fish died. Two additional groups ( P _w co ₂=37 and 78 mmHg) were exposed to an acute stress prior to the anaesthetic treatment. Stress reduced the hypoventilatory effects of the low P _w co₂ treatment, increased the recruitment of anaerobic metabolism, and prolonged recovery time. Although the increase in plasma catecholamines elicited by the stress was small relative to the response obtained with the anaesthetic, stress prior to CO₂ anaesthesia impaired the efficiency of the treatment. Overall, our results suggest that P _w co₂ levels above 37 mmHg and/or stress prior to the anaesthesia impair the efficiency of air-saturated and buffered CO₂ anaesthesia by exacerbating the hypoxaemic effects of the hypercapnic treatment.     

The influence of dietary fatty acid composition on the respiratory and cardiovascular physiology of Adriatic sturgeon (Acipenser naccarii): a review

This paper reviews evidence that the fatty acid composition of dietary lipids influences the respiratory and cardiovascular physiology of Adriatic sturgeon {Acipenser naccarii) and, thereby, their tolerance of the stress of hypoxia. Sturgeon fed a commercial diet enriched in fish oil (menhaden oil as 15% of dry feed weight), with an elevated content of highly unsaturated fatty acids of the co3 series (ω3 HUFA), had a significantly lower standard metabolic rate (SMR) and routine oxygen consumption (Mo₂) than those fed a diet enriched with the same quantity of hydrogenated coconut oil, with an elevated content of saturated fatty acids (SFA). Both groups grew equally well. As a result of this difference in aerobic metabolism, sturgeon fed the w3 HUFA and SFA responded differently when exposed to hypoxic challenges. Sturgeon fed w3 HUFA exhibited no significant reflex hyperventilation when exposed to mild, moderate or deep hypoxia (30 min at water 0₂ partial pressures of 10.8, 6.6 and 4.6 kPa, respectively), no hypoxic depression of spontaneous activity during 3h in mild hypoxia, and no depression of Mo₂ during 3h in moderate hypoxia, unlike sturgeon fed SFA. The diets also influenced the performance of isolated hearts in vitro. Hearts from fish fed o3 HUFA maintained maximum in vitro cardiac power output unchanged when oxygen supply was reduced (O₂ content from 2.3 to 0.7 vol.%), unlike hearts from sturgeon fed SFA. Overall, the results indicate that dietary fatty acid composition can influence tolerance of hypoxia in sturgeon, through effects on SMR. When compared to sturgeon fed SFA, those fed co3 HUFA had lower SMR and were more tolerant of hypoxia, with effects both on the whole animal and on the isolated heart.     

Gill ventilation and respiratory efficiency of Sacramento blackfish, Orthodon microlepidotus Ayres, in hypoxic environments

Respiratory and gill ventilatory responses of Sacramento blackfish to three environmental temperatures (12, 20 and 28°C) and four environmental dissolved oxygen concentrations (130, 90, 65 and 40 torr P_O2) were examined to determine physiological strategies of survival in eutrophic lakes and suitibility for culture conditions. Situated in van Dam-type respirometers, experimental blackfish showed increased gill ventilatory flows from increased ventilatory frequencies and stroke volumes to meet higher respiratory oxygen demands at increased temperatures. Ventilation volumes also increased at reduced environmental dissolved oxygen levels by increased ventilatory stroke volumes alone, except at 28°C where frequency increases were also measured. Oxygen consumption rates remained essentially constant with declining dissolved oxygen, except at 28°C where excitement elevated respiratory metabolism at 65 and 40 torr. Percentage utilization of oxygen increased with temperature from 12°C, but levels at 20 and 28°C were insignificantly different. Contrary to most studies on other species, there was no change in percentage utilization under hypoxic conditions even with 4.7-fold increases in ventilation volume in excited fish at 28°C. The ability of blackfish to survive in hypoxic waters is quantitatively compared with other species by calculation of a respiratory efficiency index (I), which includes the relationship between ventilation volume and percentage utilization of oxygen under normoxic and hypoxic conditions as well as the half-saturation value (P₅₀) of the species' blood with oxygen.     

Hypoxia and interdemic variation in Poecilia latipinna

Variation in respiratory traits was quantified between two populations of the sailfin molly Poecilia latipinna (one from a periodically hypoxic salt marsh, Cedar Key, and one from a chronically normoxic river site, Santa Fe River). Two suites of characters were selected: traits that may show both short‐term acclimation response and interdemic variation in acclimation response (metabolic rate, critical oxygen tension and respiratory behaviour), and those that are not likely to respond to short‐term acclimation but may vary among populations (gill morphometric characters). Sailfin mollies from the salt marsh, acclimated to hypoxia (1 mg l⁻¹, c . 20 mmHg) for 6 weeks, spent less time conducting aquatic surface respiration and had lower gill ventilation rates than hypoxia‐acclimated conspecifics from the well‐oxygenated river site. Poecilia latipinna acclimated to hypoxia exhibited a lower critical oxygen tension ( P _c) than fish acclimated to normoxia; however, there was also a significant population effect. Poecilia latipinna from Cedar Key exhibited a lower P _c than fish from the Santa Fe River, regardless of acclimation. Cedar Key fish had a 14% higher mean gill surface area relative to fish from the Santa Fe River, a character that could account, at least in part, for their greater tolerance to hypoxia.     

Phosphorus transformations in the epilimnion of humic lakes: abiotic interactions between dissolved humic materials and phosphate

SUMMARY. 1. Sephadex gel filtration of filtered water from small, Finnish forest lakes demonstrated abiotic movement of ³³P from added PO₄ to two higher molecular weight fractions. This movement was most pronounced in waters with high humic content which also had high iron content. The two fractions which took up ¹³P had nominal molecular weights of > 100,000 and 10,000-20,000.
2. An equilibrium existed between free PO₄ and the two fractions. However, one fraction, at least, appeared to exist in two phases, with one phase in rapid equilibrium with free PO₄ but the other in only slow equilibrium.
3. Additions of ferric iron up to 1 mg Fe l⁻¹ to the filtered lake water stimulated movement from free PO₄, provided high concentrations of humic materials were present. In the absence of humic materials even 0.1 mg Fe 1⁻¹ would precipitate all added ³³PO₄.
4. The high molecular weight P was only partially reactive with standard molybdate reagents. Exposure of the high molecular weight P to sunlight caused a small release of PO₄ under the experimental conditions employed.
5. Possible implications for biological phosphorus demand of such sequestration of free PO₄ by humic materials in combination with iron are discussed.     

Endocrine, osmoregulatory, respiratory and haematological parameters in flounder exposed to the water soluble fraction of crude oil

Flounders Pleuronectes flesus with an implanted vascular catheter were exposed to a 50% dilution of the water soluble fraction (WSF) of Omani crude oil (c. 6ppm total aromatic hydrocarbons) and serial blood samples taken to determine their endocrine status (cortisol, catecholamines and thyroid hormones) and the resultant and/or causal physiological (haematological, ionoregulatory and respiratory) disturbances. This resulted in a progressive increase in plasma cortisol concentrations from 3 h onwards (rising from 18 to 51 ng ml⁻¹ after 48-h exposure), and increased plasma glucose concentrations indicating a generalized stress response. Plasma T3 and T4 concentrations of both control and WSF-exposed groups declined progressively over the experimental period; exposure to the WSF of crude oil further depressed plasma T4 concentrations but not plasma T₃ concentrations relative to those of control fish. Plasma osmolality and sodium and chloride concentrations were stable in WSF-exposed fish, however, plasma potassium concentrations were increased significantly at the 24-and 48-h sampling points. The most profound physiological disturbance in WSF-exposed fish was a dramatic decline in blood oxygen content (CvO₂) (from 2–8 to 0–8 ml 100 ml⁻¹ after 48-h exposure), which is likely to be the cause of the increased plasma noradrenaline concentrations from 3 h onwards. Increased noradrenaline is likely in turn to have been responsible for the significant increase in blood haematocrit and blood haemoglobin at the 3-h sampling point, although the dominant effect in the longer-term was a significant decline in both of these haematological parameters.     

Arterial Po2, acid-base status, and red cell nucleoside triphosphates in rainbow trout transferred from fresh water to 20‰ sea water

Transfer of rainbow trout from fresh water to 20‰ sea water decreased dorsal aortic Po₂ from 90 to 60 mmHg within 1 h. Arterial Po₂ was depressed for 24 h before it returned to control values. The internal hypoxia rapidly reduced the red cell nucleoside triphosphate content, suggesting prompt regulation of blood O₂ affinity. The extracellular acid-base status changed towards a small mixed metabolic and respiratory acidosis.     

The effects of temperature and hyperoxia on arterial PO2 and acid-base status in Piaractus mesopotamicus

The effects of hyperoxia and change of temperature (range 20–30° C) on blood gases were studied in the teleost fish Piaractus mesopotamicus , native to several major river systems in Brazil. Large hyperoxia-induced increases of arterial P o₂ ( P _ao₂) indicated that true branchial blood shunts are negligible in relation to total gill perfusion. This implies that blood gases will be influenced by ventilation rather than by shunts. Acute variations of temperature ( t ) were accompanied by changes of arterial blood pH (on the average Δ p H_aΔt⁻¹ of — 0·015 units °C⁻¹), due mainly to alterations of P _aco₂: 2·4 mmHg at 20° C, 5·0 mmHg at 30° C. Concomitantly, P _ao₂ declined from 116 mmHg (20° C) to 89 mmHg (30° C). The data suggest that temperature-induced changes of acid-base status depend mainly on gill ventilation and that the decrease of P _ao₂ with higher temperature is a result of this regulation.     

The effect of progressive hypoxia on swimming activity and schooling in Atlantic herring

Schools of herring exposed to progressive hypoxia show a peak in velocity during severe hypoxia, at 15–34% oxygen saturation, followed by a decrease in swimming speed until school disruption occurred. The observed increase in swimming speed during severe hypoxia reveals a graded response, since the lower the fish's swimming speed prior to severe hypoxia ( U ₉₅₋₅₀, the speed at oxygen saturations between 95 and 50%), the greater the relative increase in swimming speed. The oxygen saturations at which both peak velocity and school disruption occurred were lower for fish with lowest U ₉₅₋₅₀, suggesting that the fish with the slowest speed U ₉₅₋₅₀ reach their critical P_O2 (at which there is respiratory distress) last, i.e. at lower oxygen saturation. At a functional level, it is suggested that herring encountering hypoxia increase their speed in order to find more favourable conditions, and the magnitude of this increase is modulated by their respiratory distress. It is also hypothesised that the observed increase in speed may be related to an increase in the rate of position shifting within the school. Since the oxygen saturation at which the response to hypoxia occurs and the magnitude of the response are related to the fish's preferred speed prior to severe hypoxia, it is suggested that such a preferred speed should be measured in experiments testing the effect of hypoxia on fish behaviour.