Do Thi Thanh Huong

Hypoxia tolerance and partitioning of bimodal respiration in the striped catfish (Pangasianodon hypophthalmus)

Air-breathing fish are common in the tropics, and their importance in Asian aquaculture is increasing, but the respiratory physiology of some of the key species such as the striped catfish, Pangasianodon hypophthalmus Sauvage 1878 is unstudied. P. hypophthalmus is an interesting species as it appears to possess both well-developed gills and a modified swim bladder that functions as an air-breathing organ indicating a high capacity for both aquatic and aerial respiration. Using newly developed bimodal intermittent-closed respirometry, the partitioning of oxygen consumption in normoxia and hypoxia was investigated in P. hypophthalmus. In addition the capacity for aquatic breathing was studied through measurements of oxygen consumption when access to air was denied, both in normoxia and hypoxia, and the critical oxygen tension, Pcrit, was also determined during these experiments. Finally, gill ventilation and air-breathing frequency were measured in a separate experiment with pressure measurements from the buccal cavity. The data showed that P. hypophthalmus is able to maintain standard metabolic rate (SMR) through aquatic breathing alone in normoxia, but that air-breathing is important during hypoxia. Gill ventilation was reduced during air-breathing, which occurred at oxygen levels below 8 kPa, coinciding with the measured Pcrit of 7.7 kPa. The findings in this study indicate that the introduction of aeration into the aquaculture of P. hypophthalmus could potentially reduce the need to air-breathe. The possibility of reducing air-breathing frequency may be energetically beneficial for the fish, leaving more of the aerobic scope for growth and other activities, due to the proposed energetic costs of surfacing behavior.

Oxygen delivery does not limit thermal tolerance in a tropical eurythermal crustacean

In aquatic environments, rising water temperatures reduce water oxygen content while increasing oxygen demand, leading several authors to propose cardiorespiratory oxygen transport capacity as the main determinant of aquatic animal fitness. It has also been argued that tropical species, compared with temperate species, live very close to their upper thermal limit and hence are vulnerable to even small elevations in temperature. Little, however, is known about physiological responses to high temperatures in tropical species. Here we report that the tropical giant freshwater shrimp (Macrobrachium rosenbergii) maintains normal growth when challenged by a temperature rise of 6°C above the present day average (from 27°C to 33°C). Further, by measuring heart rate, gill ventilation rate, resting and maximum oxygen uptake, and hemolymph lactate, we show that oxygen transport capacity is maintained up to the critical maximum temperature around 41°C. In M. rosenbergii heart rate and gill ventilation rate increases exponentially until immediately below critical temperatures and at 38°C animals still retained more than 76% of aerobic scope measured at 30°C, and there was no indication of anaerobic metabolism at the high temperatures. Our study shows that the oxygen transport capacity is maintained at high temperatures, and that other mechanisms, such as protein dysfunction, are responsible for the loss of ecological performance at elevated temperatures.

Ecological risk assessment of the antibiotic enrofloxacin applied to Pangasius catfish farms in the Mekong delta, Vietnam

Antibiotics applied in aquaculture production may be released into the environment and contribute to the deterioration of surrounding aquatic ecosystems. In the present study, we assessed the ecological risks posed by the use of the antibiotic enrofloxacin (ENR), and its main metabolite ciprofloxacin (CIP), in a Pangasius catfish farm in the Mekong Delta region, Vietnam. Water and sediment samples were collected in a stream receiving effluents from a Pangasius catfish farm that had applied ENR. The toxicity of ENR and CIP was assessed on three tropical aquatic species: the green-algae Chlorella sp. (72 h - growth inhibition test), the micro-invertebrate Moina macrocopa (48 h - immobilization test), and the Nile tilapia (Oreochromis niloticus). The toxic effects on O. niloticus were evaluated by measuring the cholinesterase (ChE) and catalase (CAT) activities in the fish brain and muscles, respectively, and by considering feed exposure and water exposure separately. Ecological risks were assessed by comparing maximum exposure concentrations with predicted no effect concentrations for cyanobacteria, green algae, invertebrates and fish derived with available toxicity data. The results of this study showed that maximum antibiotic concentrations in Pangasius catfish farm effluents were 0.68 μg L(-1) for ENR and 0.25 μg L(-1) for CIP (dissolved water concentrations). Antibiotics accumulated in sediments down-stream the effluent discharge point at concentrations up to 2590 μg kg(-1) d.w. and 592 μg kg(-1) d.w. for ENR and CIP, respectively. The calculated EC50 values for ENR and CIP were 111000 and 23000 μg L(-1) for Chlorella sp., and 69000 and 71000 μg L(-1) for M. macrocopa, respectively. Significant effects on the ChE and CAT enzymatic activities of O. niloticus were observed at 5 g kg(-1) feed and 400-50000 μg L(-1), for both antibiotics. The results of the ecological risk assessment performed in this study indicated only minor risks for cyanobacteria communities, suggesting that residual concentrations of ENR and CIP after medication are not likely to result in severe toxic effects on exposed aquatic ecosystems. However, more studies should be performed by considering other antibiotic treatments used in Pangasius catfish production and the potential ecotoxicological effects of relevant antibiotic mixtures on sediment communities.

Current status of freshwater prawn culture in Vietnam and the development and transfer of seed production technology

In Vietnam, the giant freshwater prawn Macrobrachium rosenbergii is becoming an increasingly important targeted species, as its culture, especially in rice fields, is considered to have the potential to raise income among improverished farmers. The production of M. rosenbergii based on aquaculture reached over 10 000 tons per year in 2002, having increased from about 2500 tons since the 1990s. Until recently, lack of a stable supply of seed had been an important obstacle to the further expansion and development of M. rosenbergii culture, but cumulative research on larval rearing, especially in the 1990s, has led to the development of new seed production technology based on the ‘modified stagnant green water system’. Following its disse mination by the efforts of provincial authorities, hatchery operators, and farmers, the freshwater prawn seed production industry developed rapidly in the Mekong Delta with over 90 hatcheries producing 76.5 million postlarvae in 2003. This is considered to have affected the expansion of rice-prawn farming in the Mekong Delta, leading to increased aquacultural production in the region. This paper reviews the current status of freshwater prawn culture in Vietnam and background history, and presents a socioeconomic evaluation of seed production technology implementation.

Some like it hot: Thermal tolerance and oxygen supply capacity in two eurythermal crustaceans

Thermal sensitivity of the cardiorespiratory oxygen supply capacity has been proposed as the cardinal link underlying the upper boundary of the temperature niche in aquatic ectotherms. Here we examined the evidence for this link in two eurythermal decapods, the Giant tiger shrimp (Penaeus monodon) and the European crayfish (Astacus astacus). We found that both species have a temperature resistant cardiorespiratory system, capable of maintaining oxygen delivery up to their upper critical temperature (Tcrit). In neither species was Tcrit reduced in hypoxia (60% air saturation) and both species showed an exponential increase in heart and gill ventilation rates up to their Tcrit. Further, failure of action potential conduction in preparations of A. astacus motor neurons coincided with Tcrit, indicating that compromised nervous function may provide the underlying determinant for Tcrit rather than oxygen delivery. At high temperatures, absolute aerobic scope was maintained in P. monodon, but reduced in A. astacus. However, A. astacus also displayed reduced exercise intensity indicating that impaired muscle performance with resulting reduced tissue oxygen demand may explain the reduced scope rather than insufficient oxygen supply capacity. This interpretation agrees with early literature on aquatic ectotherms, correlating loss of nervous function with impaired locomotion as temperatures approach Tcrit.

Effect of salinity on oxygen consumption in fishes: a review.

The effect of salinity on resting oxygen uptake was measured in the perch Perca fluviatilis and available information on oxygen uptake in teleost species at a variety of salinities was reviewed. Trans-epithelial ion transport against a concentration gradient requires energy and exposure to salinities osmotically different from the body fluids therefore imposes an energetic demand that is expected to be lowest in brackish water compared to fresh and sea water. Across species, there is no clear trend between oxygen uptake and salinity, and estimates of cost of osmotic and ionic regulation vary from a few per cent to >30% of standard metabolism.

Effects of nitrite exposure on functional haemoglobin levels, bimodal respiration, and swimming performance in the facultative air-breathing fish Pangasianodon hypophthalmus.

In this study we investigated nitrite (NO₂⁻) effects in striped catfish, a facultative air-breather. Fish were exposed to 0, 0.4, and 0.9 mM nitrite for 0, 1, 2, 4, and 7 days, and levels of functional haemoglobin, methaemoglobin (metHb) and nitrosyl haemoglobin (HbNO) were assessed using spectral deconvolution. Plasma concentrations of nitrite, nitrate, chloride, potassium, and sodium were also measured. Partitioning of oxygen consumption was determined to reveal whether elevated metHb (causing functional hypoxia) induced air-breathing. The effects of nitrite on maximum oxygen uptake (MO(2max)) and critical swimming speed (U(crit)) were also assessed. Striped catfish was highly tolerant to nitrite exposure, as reflected by a 96 h LC₅₀ of 1.65 mM and a moderate nitrite uptake into the blood. Plasma levels of nitrite reached a maximum after 1 day of exposure, and then decreased, never exceeding ambient levels. MetHb, HbNO and nitrate (a nitrite detoxification product) also peaked after 1 day and then decreased. Only high levels of nitrite and metHb caused reductions in MO(2max) and U(crit). The response of striped catfish contrasts with that seen in most other fish species and discloses efficient mechanisms of combating nitrite threats. Furthermore, even though striped catfish is an efficient air-breather, this species has the ability to sustain aerobic scope and swimming performance without air-breathing, even when faced with nitrite-induced reductions in blood oxygen carrying capacity. Our study is the first to confirm that high levels of nitrite and metHb reduce MO(2max) and thereby aerobic scope, while more moderate elevations fail to do so. Further studies are needed to elucidate the mechanisms underlying the low nitrite accumulation in striped catfish.

High capacity for extracellular acid–base regulation in the air-breathing fish Pangasianodon hypophthalmus

ABSTRACT The evolution of accessory air-breathing structures is typically associated with reduction of the gills, although branchial ion transport remains pivotal for acid–base and ion regulation. Therefore, air-breathing fishes are believed to have a low capacity for extracellular pH regulation during a respiratory acidosis. In the present study, we investigated acid–base regulation during hypercapnia in the air-breathing fish Pangasianodon hypophthalmus in normoxic and hypoxic water at 28–30°C. Contrary to previous studies, we show that this air-breathing fish has a pronounced ability to regulate extracellular pH (pHe) during hypercapnia, with complete metabolic compensation of pHe within 72 h of exposure to hypoxic hypercapnia with CO2 levels above 34 mmHg. The high capacity for pHe regulation relies on a pronounced ability to increase levels of HCO3− in the plasma. Our study illustrates the diversity in the physiology of air-breathing fishes, such that generalizations across phylogenies may be difficult. Summary: Pangasius has a much higher capacity for extracellular acid–base regulation during exposure to hypercapnia compared with other air-breathing fishes.

Chapter 8 The Effects of Hypoxia On Growth and Digestion

Here we review how hypoxia affects growth and digestion in fish. Thus, the growth effects of hypoxia are explained in terms of reductions of energy intake (appetite) and assimilation efficiency as well as in terms of the costs of digestion or specific dynamic action. It is clear that the most commonly documented cause of hypoxia-related growth retardation is through loss of appetite and the regulatory physiology of this effect is discussed. Finally, the effects of hypoxia on the growth of air-breathing fish are reviewed and the most promising areas for future research on oxygens role as a limiting factor for fish growth are highlighted.

Autonomic control of the heart in the Asian swamp eel (Monopterus albus).

The Asian swamp eel (Monopterus albus) is an air-breathing teleost with very reduced gills that uses the buccal cavity for air-breathing. Here we characterise the cardiovascular changes associated with the intermittent breathing pattern in M. albus and we study the autonomic control of the heart during water- and air-breathing. The shift from water- to air-breathing was associated with a rise in heart rate from 27.7 ± 1.6 to 41.4 ± 2.6 min(-1) and an increase in cardiac output from 23.1 ± 3.0 to 58.7 ± 6.5 mLmin(-1)kg(-1), while mean systemic blood pressure did not change (39.0 ± 3.5 and 46.4 ± 1.3 cmH(2)O). The autonomic control of the heart during water- and air-breathing was revealed by infusion of the β-adrenergic antagonist propranolol and muscarinic antagonist atropine (3 mgkg(-1)) in eels instrumented with an arterial catheter. Inhibition of the sympathetic and parasympathetic innervations of the heart revealed a strong vagal tone on the heart of water-breathing eels and that the tachycardia during air-breathing is primarily mediated by withdrawal of cholinergic tone.