Temporal variation and distribution of trace metals in freshwater and fish from Calabar River, S.E. Nigeria

An investigation on the abundance and distribution of trace metals (Fe, Cu, Zn, Mn, Cr, Cd and Pb) in water, and nine species of fish samples from Calabar river was carried out in 1992. The concentrations of iron (6000–7240gl^–1), zinc (4910–7230gl^–1), and cadmium (3–7gl^–1) showed moderate pollution while those of copper (420–630gl^–1), manganese (23–48gl^–1), chromium (<10–20gl^–1) and lead (<1–10gl^–1) in water were well below WHO permissible levels. Significant seasonal changes (0.001p0.25) were obtained for iron, copper, zinc, manganese and cadmium in water. Furthermore, iron, zinc and cadmium showed statistically significant spatial changes (0.005p0.10). Of the nine fish species studied, no statistically significant relationship between body weight and the concentrations of the metals was observed. The concentrations of the metals per mean total body weight apparently decreases in the order Fe>Zn>Cu>Mn>Pb>Cd=Cr and were within the limits that were safe for consumption.     

Influence of salinity and temperature on the oxygen consumption in young juveniles of the Indian prawn Penaeus indicus

Routine oxygen consumption of very young juveniles (0.1 g) of Penaeus indicus H. Milne Edwards was significantly influenced by ambient temperature and weight of the animal, but not by ambient salinity, when tested at salinities (7, 21, and 35) to which they had been long-term (over 10 days) acclimated. Standard oxygen consumption of young juvenile prawns (1 to 3 g), subjected to step-wise changes in ambient salinity, from sea water to low salinity waters (2 to 6), and measured after short-term (24 h) salinity acclimation at each step, was lowest at salinities where prawns such as those tested occur naturally (10 to 15). The metabolic rates do not appear to have a direct relation to the osmotic gradient, even when the influence of interfering activity is eliminated. It appears that factors other than osmotic gradient will have to be sought in order to explain the metabolic patterns of P. indicus in relation to salinity.     

Effects of salinity on fasted rainbow trout,Salmo gairdneri

Rainbow trout (Salmo gairdneri Richardson) which had been maintained for 120 days in salinities of fresh water, 7.5, 15.0 and 32.5 at 10°C were fasted for up to 48 days under these same environmental conditions. Live weight loss between Days 7 and 48 of starvation could be described by a straight line, as could the decrease in condition factor . Trout maintained in 32.5% S showed a significantly greater weight loss than those in salinities of 15.0 and below. Muscle water content increased slightly during fasting in fresh water, 7.5 and 15.0 S. In 32.5 S, however, muscle water fell significantly between Days 19 and 37. Liver water content also increased slightly during fasting, except in 32.5 S, where water content again decreased between Days 19 and 37. The volume of the gall bladder contents increased during fasting.     

Aqua regia extractable trace elements in surface soils of venezuela

Surface soils (0–15 cm) were sampled at 10–20 km intervals along two transects in Venezuela. One (1162 km, 70 samples) ran west to east parallel with the Caribbean coastline, the other (920 km, 92 samples) ran south to north from the frontier with Brazil to the Caribbean shore. Sampling took place in both a wet and a dry season. Trace metals were extracted from dried, sieved (<2 mm) soil with boiling aqua regia followed by analysis by ICP or flame AAS. Metal values did not differ significantly between the two seasons and dates were averaged. Geometric mean values for the west–east transect were: Cr=41.5, Cu 17.9, Cs 3.6, Li=13.9, Mn=294, Ni=21.3, Pb=17.4, Sr=39.4, V=60.4 and Zn = 83.7g g^–1, respectively. Similarly, for the south–north transect Cr=21.3, Cu=4.3, Cs=1.1, Li=2.0, Mn=55.7, Ni=4.4, Pb=6.1, Sr=13.3, V=28.2 and Zn=16.7g g^–1, respectively. A classification of samples by lithology showed surface soil composition to be related to rock composition. Metal values were low in the soils in the south of the country, in the Guyana highlands (Gran Sabana). Low Zn contents were prevalent. Lead contents were affected by roadside fallout from vehicles using leaded petrol except that high Pb contents of soils in the Gran Sabana were of more complex origin.     

Sodium regulation in the shore crab Carcinus maenas as related to ambient salinity

The activity of Na–K-ATPase was measured in crude homogenates prepared from various organs (leg muscle, pincer muscle, heart, testes, digestive gland, hypodermis, gills 1–9) of shore crabs, Carcinus maenas L., acclimated to salinities ranging between 10 and 50 S (in steps of 10 S). In all salinities tested, Na–K-ATPase activity was highest in posterior gills 7–9 (10–12 mol P_i mg protein^-1 h^-1), followed by anterior gills 1–6 (ca. 2.5 mol P_i mg protein^-1 h^-1) and the other organs (in most cases far below 2mol P_i mg protein^-1 h^-1). In gills only, Na–K-ATPase activity was salinity-dependent, with the highest values in the lowest salinities and vice versa. In gills 7–9, Na–K-ATPase activity was increased more than threefold following a reduction in salinity from 50 to 10 S. Na–K-ATPase activity, expressed as percentage of total ATPase activity, amounted to 60–80% in gills, about 60% in hypodermis and 20–40% in the other organs. Ouabain, a specific inhibitor of Na–K-ATPase activity, reduced serum osmolalities in crabs kept at 9–10 S only when injected into the hemolymph (1 and 5 · 10^-5 M), but had no effect when dissolved in ambient water (10^-4 M). The results obtained underline that crustacean gills are the main organs for ionic regulation, and confirm the hypothesis of the central role of the Na–K-ATPase in active Na uptake as the basic mechanism of hyperregulation in dilute media. Reduction of serum osmolalities following injection of ouabain into the hemolymph confirms previous reports on localization of the sodium pump in the basolateral parts of epithelial cells.     

Exosmosis and “free space” in marine benthic algae

In distilled water, marine benthic algae immediately lose ions. This can be demonstrated by means of chloride titration or conductivity measurements. The rapidity and the complete reversibility of this process show that ion movements out of and into the free space are involved. When Laminaria saccharina thalli, exposed to a series of increasing NaCl-concentrations, are subsequently transferred into distilled water, the external concentration increases proportionally to rate of ion loss. Through its free space, the alga establishes an ionic equilibrium with its external medium. If the alga thallus is killed (20 sec boiling in isosmotic sea water), the extent of chloride loss is much higher. Because of thermal destruction of the osmotic space, the chloride can then escape from the entirc thallus. Since sublittoral algae die upon drying, chloride loss from dry thalli is much higher than that from wet thalli: this difference is small in littoral algae, which tolerate short periods of dryness. A close relationship exists between extent of chloride loss and degree of resistance to drying.     

Genetic population structure and gene flow among deep-sea amphipods,Abyssorchomene spp., from six California Continental Borderland basins

Studies of geographic population variation needed to estimate gene flow are lacking in deep-sea biology. Using allozyme electrophoresis, I have studied population-level geographic variation among scavenging lysianassoid amphipod populations (Abyssorchomene spp.) inhabiting deep-water basins of the Southern California Continental Borderland. Samples were collected from November 1987 to November 1990, using baited traps, from six basins whose bottom depths ranged from ca. 1000 to 2100 m. Five basins (San Diego Trough, Santa Catalina, San Nicolas, Santa Cruz, Tanner Basins) could be grouped together as shallow-sill basins, with physical conditions distinctly different from a single deepsill basin (San Clemente Basin). Amphipods tentatively identified asAbyssorchomene sp. 1 collected from the shallow-sill basins were morphologically discriminated from those collected in the San Clemente Basin, which were identified asAbyssorchomene sp. 2. Results from eight enzyme loci revealed significant genetic differentiation [Nei's genetic distance (D)>0.155] of deep-sill basin-dwellingAbyssorchomene sp. 2 vsAbyssorchomene sp. 1 from the shallow-sill basins and low levels of gene flow (migration rate, <1). Comparisons of benthic fauna suggest the presence of an abyssal-related assemblage in the deep-sill basin isolated from the northern shallow-sill basins. Genetic distances among the five shallow-sill basin populations ofAbyssorchomene sp. 1 were very low (D < 0.003). Estimates of gene flow among these populations were very high ( > 24 to 170) and suggested weak isolation by distance.     

Populational distribution I: The digital simulation

This paper presents a method of investigating the distributional pattern of a biological population, using a technique of simulation. The method consists of the comparison of the empirical frequency curve of a population, obtained using the Method of Quadrats, with a simulated one, since the pattern of the simulated curve depends on the simulated distribution. An example is given.     

Grazing and ingestion rates of nauplii,copepodids and adults of the marine planktonic copepod Calanus helgolandicus

The average grazing and ingestion rates of all stages of the marine planktonic copepod Calanus helgolandicus (Calanoida) from nauplius stage IV to adults were measured experimentally at 15°C in agitated cultures. The chain-forming diatom Lauderia borealis and the unarmoured dinoflagellate Gymnodinium splendens were offered as food. The food concentrations were close to natural conditions and ranged from 36 to 101 g of organic carbon per liter. The medium body weights expressed in g of organic carbon of almost all larval stages raised at 49 g C/1 were identical with the weight of the same stages caught in the Pacific Ocean off La Jolla, California, USA. In a log-log system, grazing and ingestion rates increased almost linearly with increasing body weight. Grazing rates ranged from 4 to 21 ml/day/nauplius stage IV to 286 ml to 773 ml/day/female. Ingestion rates increased from 0.2 g to 0.8 g C/day/nauplius stage IV to 18 g to 69 g C/day/female. Grazing and ingestion rates per unit body weight decreased gradually with increasing body weight. The daily ingested amount of food decreased from 292 to 481% of the body weight (g C) of nauplius stage V to 28–85% of the body weight of adult females. Grazing and ingestion performances of all stages increased with increasing particle size. Grazing rates decreased and ingestion rates increased with increasing food concentrations. The published data on food intake of the different age groups of C. helgolandicus show that the young stages of herbivorous planktonic copepods can play a major part in the consumption of phytoplankton in the sea due to their high grazing and ingestion rates.     

Salinity tolerance,salinity preference and temperature tolerance in the high-shore harpacticoid copepod Tigriopus brevicornis

Tigriopus brevicornis (O. F. Müller) were collected in 1992 from rock pools close to U.M.B.S. Millport, Isle of Cumbrae, U.K. and acclimated to various combinations of salinity and temperature for at least 1 wk prior to laboratory experiments. Higher salinities of acclimation enhanced tolerance to high salinity stress, while tolerance of low salinities was hardly affected by acclimation salinity. Acclimation to low temperature (10°C) extended the survivable salinity range for T. brevicornis. High-salinity acclimation enhanced the survivable temperature range. Copepods acclimated to 60 survived significantly lower and higher temperatures than did 34-acclimated individuals. At high temperature, 75-acclimated female copepods had the highest median lethal temperature, 38.9°C. Females were significantly more resistant to high temperatures than males. The copepods were seen to have a very low median lethal temperature when frozen into solid ice for 2 h; 50% mortality occurred at-16.9°C in 10°C, 34-acclimated T. brevicornis. Salinity preference experiments demonstrated an ability to discriminate between salinities differing by as little as 3. Copepods acclimated to 34 chose salinities near their acclimation salinity; individuals acclimated to 5 favoured slightly higher salinities, while copepods acclimated to 60 chose rather lower salinities.     

Arsenic tissue concentration of immature mice one hour after oral exposure to gold mine tailings

A potentially high bioavailability of arsenic in gold mine tailings from a site in northern California has been suggested by solubility studies. To help address this issue, an in vivo dosing study was conducted using 12dayold Swiss Webster mouse pups (n=8/group). A sample of sizefractionated mine tailings from the site (<20m particle size, 691g g^–1 arsenic) was prepared as an aqueous suspension and administered by gavage in a volume that provided 4mg As/kg body weight. The control group received the same volume of a commercial soil (1g g^–1 As) of similar particle size (<60m). No mortality or toxic signs were noted in either group. Tissue samples were collected 1h after gavage, freezedried, microwavedigested and analysed for arsenic by ICP/MS (detection limit 2ng As g^–1 dry weight). Arsenic concentrations (ng As g^–1 dry weight) in tissues from the pups who received mine tailings were significantly higher than in control tissues. The mean elevation in arsenic concentration was highest in the liver (3364% of control, p<0.0001), followed by blood (818 of control, p<0.0001), skin (207% of control, p=0.07), and brain (143% of control, p<0.0001). The carcass arsenic concentration (excluding the GI tract, liver, brain and skin) was 138 of control (p=0.02). The data indicate uptake of arsenic from weathered mine tailings by the immature mouse pups after oral exposure.     

Primary productivity and sizes of pools of organic carbon in the mixed layer of the ocean

We estimated primary productivity and distributions of carbon in the phytoplankton, micro-zooplankton, and suspended and dissolved matter in various areas of the World Ocean to increase our information about the organic carbon cycle in the surface layer of the sea. Primary productivity ranged from about 0.1 gC m^–2 day^–1 in the Gulf of Mexico to 9 gC m^–2 day^–1 in nutrient-rich water off Peru. Phytoplankton carbon ranged from less than 10 g/l in the former to 750 g/l in the latter and in nutrient-rich water off southwest Africa. Micro-zooplankton carbon usually was less than 50 g/l in all waters, and was dominated by ciliates, copepodids, and copepod nauplii in all areas. Concentrations of particulate carbon ranged from 12 g/l off the east coast of South America to 850 g/l off southwest Africa. Concentrations of dissolved organic carbon varied between 0.5 and 1.5 mg/l in all areas except off Peru, where maximum values of 4.5 mg/l were observed. Turnover rates of carbon by small standing crops of micro-flagellates (1 to 5 longest dimension) and dinoflagellates in nutrient-poor waters were lower than those by large standing crops of diatoms and micro-flagellates in nutrient-rich waters. Concentrations of phytoplankton usually accounted for 20 to 55% and micro-zooplankton for 2 to 30% of the particulate carbon in the surface layer of the sea. Concentrations of dissolved organic carbon were not related to concentrations of particulate carbon in most waters except off Peru, where they appear to be directly related.     

Effects of salinity on respiration and nitrogen excretion in two species of echinoderms

The 30-d survival limit of Eupentacta quinquesemita and Strongylocentrotus droebachiensis is 12–13 S. The activity coefficient (1 000/righting time in seconds) of stepwise acclimated sea urchins declined from 16.3 at 30 S to 3.5 at 15 S. Oxygen consumption rates (QO₂) of both species held at 30 S and 13°C were highest in June and lowest in December. During the summer, when environmental salinity is most variable in southeastern Alaska, the QO₂ of both species held at 30, 20 and 15 S varied directly with salinity. Perivisceral fluid PO₂ varied directly with acclimation salinity in sea urchins, but not in sea cucumbers. Perivisceral fluid oxygen content of acclimated sea urchins was significantly lower at 15 and 20 S than at 30 S due to reduced PO₂ and extracellular fluid volume at the lower salinities. The QO₂ of both species varied directly with ambient salinity during a 30-10-30. semidiurnal pattern of fluctuating salinity. No change occurred in the average QO₂ of either species over a 15-30-15. semidiurnal pattern of fluctuating salinity. Sea urchin perivisceral fluid PO₂ declined as ambient salinity fluctuated away from the acclimation salinity in both cycles and increased as ambient salinity returned to the acclimation salinity. Total nitrogen excretion of stepwise acclimated sea cucumbers declined significantly from 30 to 15 S, but there was no salinity effect on total nitrogen excretion in sea urchins. Ammonia excretion varied directly with salinity in stepwise acclimated sea cucumbers (67–96% of total nitrogen excreted), but there was no salinity effect on ammonia excretion (89–95% of total nitrogen excreted) of sea urchins. Urea excretion did not vary with salinity in sea cucumbers (2–4% of total nitrogen excreted) or sea urchins (2–9% of total nitrogen excreted). Primary amines varied inversely with salinity in sea cucumbers (2–30% of total nitrogen excreted), but did not vary with salinity in sea urchins (2–4% of total nitrogen excreted). The oxygen: nitrogen ratio of both species indicated that carbohydrate and/or lipid form the primary catabolic substrate. The O:N ratio did not vary as a function of salinity. Both species are more tolerant to reduced salinity than previously reported, however, rates of oxygen consumption and/or nitrogen excretion are modified by salinity as well as season.     

Concentrations of Mn,Fe, Cu,Zn and Cd in the mesopelagic decapod Systellaspis debilis from the East Atlantic Ocean

Specimens of the oceanic decapod Systellaspis debilis were collected from six sites in the East Atlantic Ocean between 1970 and 1984, and were analysed for Mn, Fe, Cu, Zn and Cd. The data confirm that there are small but significant differences in mean metal concentrations from some sites which showed no obvious pattern in relation to geographic location of the samples. As a result, ranges of site means are quoted as baseline levels for each metal (g g^-1 dry wt): 2.3 to 2.9 g Mn g^-1, 31.2 to 77.8 g Fe g^-1, 25.9 to 83.4 g Cu g^-1, 41.9 to 92.9 gZn g^-1, 11.1 to 31.8 g Cd g^-1. The concentration of cadmium in S. debilis from all sites was raised relative to cadmium concentrations reported for coastal decapods, perhaps as a result of dietary enrichment. Metal accumulation may provide useful information for understanding the complex feeding behaviour of many oceanic animals.     

Growth and survival of Mytilus edulis larvae exposed to low levels of dibutyltin and tributyltin

Two studies were conducted to observe effects of dibutyltin (DBT) and tributyltin (TBT) on larvae of Mytilus edulis for an exposure period of 25 d. Endpoints for evaluation were shell growth and mortality measured at 33 d. Larvae were cultured in a new laboratory assay chamber in a recirculating static test. The control, 2, 20, and 200 g/l DBT-treated populations had mean shell lengths of 527, 523, 417, and 180 m, respectively. Survival was 1% for the 200 g/l DBT-treated population, but ranged from 73 to 83% for controls, 2, and 20 g/l treatments. The no-observed-effect concentration (NOEC) was 2 g/l for DBT, while the lowest-observed-effect concentration (LOEC) was 20 g/l. The chronic toxicity value was 6.3 g/l. In the TBT bioassay, mean shell lengths for the control, 0.006, 0.050, and 0.130 g/l-treated populations were 565, 437, 385, and 292 m, respectively. Control survival was 74%, whereas TBT-treated populations survival ranged from 52 to 58%. The NOEC for TBT was 0.006 g/l TBT and the LOEC was 0.050. A chronic toxicity value of 0.017 g/l was calculated. The results of this study indicated that the toxicity of DBT was less than that of TBT. It was concluded that shell length was inversely related to exposure level in both DBT and TBT bioassays. In this study, we have observed TBT effects at lower exposure levels in the laboratory than previously reported, and also report the first data for DBT effects on mussel larvae.     

Dependence of consumers on macroalgal (Laminaria solidungula) carbon in an arctic kelp community: δ13C evidence

Stable carbon isotope measurements (¹³C) were used to assess the importance of kelp carbon (-13.6 to-16.5) versus phytoplankton carbon (-25.5 to-26.5) to resident fauna of an isolated kelp bed community on Alaska's north arctic coast from 1979 to 1983. The predominant kelp, Laminaria solidungula, showed some seasonal variation in ¹³C which was correlated with changes in the carbon content of the tissue. Animals that showed the greatest assimilation of kelp carbon (>=50%) included macroalgal herbivores (gastropods and chitons,-16.9 to-18.2), a nonselective suspension feeder (an ascidian,-19.0) and a predatory gastropod (-17.6). Animals that showed the least incorporation of kelp carbon into body tissues (<=7%) included selective suspension-feeders (hydroids, soft corals and bryozoans,-22.8 to-25.1). Sponges, and polychaete, gastropod and crustacean omnivores exhibited an intermediate dependence on kelp carbon (15 to 40%). Within some taxonomic groups, species exhibited a broad range in isotopic composition which was related to differences in feeding strategies. In the polychaete group alone, ¹³C values identified four major feeding habits: deposit-feeders (-18.0), omnivores (-20.4), predators (-22.2) and microalgal herbivores (-23.0). Distinct seasonal changes in the ¹³C values of several animals indicated an increased dependence on kelp carbon during the dark winter period when phytoplankton were absent. Up to 50% of the body carbon of mysid crustaceans, which are key prey species for birds, fishes and marine mammals, was composed of carbon derived from kelp detritus during the ice-covered period.     

Salinity dependence of sexual and asexual reproduction in the rotifer Brachionus plicatilis

A salinity dependent mictic response was observed in a clone of Brachionus plicatilis cultured in the 2 to 4 salinity range. This response was related to asexual exponential reproduction rates (G) and could be divided into three categories: (a) no mixis occurred at a salinity of 35 S and above, where G values were lower than 0.30 d^-1, (b) low mictic levels in rotifers cultured at 2 and 30 S, where G values ranged between 0.40 to 0.50 d^-1, and (c) high mictic levels in rotifers cultured at salinities ranging between 4 and 20 S, where G values ranged between 0.50 to 0.85 d^-1. Fluctuations in mictic levels varied with time during the course of the experiments. Results suggest that salinity conditions leading to optimal parthenogenic reproduction also support mixis.     

Residues of Organochlorinated Pesticides in Eggs of Water Birds from Tai Lake in China

The levels of organochlorine compounds in eggs of water birds from the colony on Tai Lake in China were studied. The eggs were collected in 2000 and belonged to the following species: 65 samples of black-crowned night heron (Nycticorax nycticorax), 36 samples of little egret (Egretta garzetta), 26 samples of cattle egret (Bubulcus ibis) from 13 clutches and 43 samples of Chinese pond heron (Ardeola bacchus) from 17 clutches. Dichlorodiphenyltrichloroethane (DDT) and its derivates (DDE and DDD), hexachlorocyclohexane (HCH) and its isomers (-HCH, -HCH, -HCH, -HCH), heptachlor, heptachlor epoxide, aldrin, dieldrin, endrin, endrin aldehyde, -endosulfan, -endosulfan, and endosulfan sulfate were determined in the laboratory by gas chromatography. The data showed that DDE had the highest levels in all the samples, followed by -HCH. The mean levels of DDE among the water bird species were in the order as follows: black-crowned night heron (5464.26ng/g, dry weight) > Chinese pond heron (2791.12ng/g, dry weight) > little egret (1979.97ng/g, dry weight) > cattle egret (660.11ng/g, dry weight). DDT and its metabolites accounted for 90% of the total organochlorines, except that it was only 73% for cattle egret. The differences of the residue among the bird species were statistically significant and could be attributed to their variations in prey and habitat. Although the DDE burdens in Tai Lake were much lower than 8 g/g (wet weight) which are thought to have significant adverse effects on black-crowned night herons, they would be expected to increase the risk of adverse effects on survival of chicks of herons and egrets, particularly black-crowned night heron, based on the critical value of 1 g/g (wet weight) DDE. The burdens of HCHs in this study were higher and the cyclodienes were lower than those found elsewhere.     

Capillary supply and mitochondrial volume density in the axial muscles of the mesopelagic teleost Argyropelecus hemigymnus

Quantitative measurements have been made on the ultra-structure and capillary supply to the axial muscles of the mesopelagic hatchet fish Argyropelecus hemigymnus (Cocco, 1829). Fish were collected at Eastern North Atlantic Ocean Station 10244, 32°48N; 31°15W during November 1980, from a depth of 480 to 550 m. Mitochondria with densely packed cristae occupy 44.3% of slow-fibre volume. Each myofibril is in direct contact with a mitochondrion. Compared with other fishes studied, the capillary supply to A. hemigymnus slow fibres is poorly developed. The average number of capillaries per fibre is 0.9, such that each m of capillary contact supplies 0.011 m² of fibre cross-sectional area. The capillary surface area (m²) supplying 1 m³ of slow-fibre mitochondria is 0.17 in anchovy (Engraulis encrasicolus), 0.14 in rat-fish (Chimaera monstrosa), 0.14 in tench (Tinca tinca), 0.16 in catfish (Clarias mossambica), and only 0.025 for A. hemigymnus. It is suggested that, relative to the former species, some modifications in factors determining tissue oxygenation (e.g. myoglobin concentrations, blood flow, perfusion distribution or haemoglobin) and/or mitochondrial respiration rate are required in order to match oxygen supply and demand to the slow muscle in A. hemigymnus.     

The respiratory metabolism of Tilapia mossambica (Teleostei)

Tilapia mossambica (Teleostei) weighing 5 to 80 g were acclimated at 30°C to salinities of 0.4 (tap water), 12.5 (50% sea water) and 30.5 (100% sea water). Their respiration was measured at routine activity and the partial pressure of ambient oxygen gradually reduced from 250 to 50 mm Hg. Respiration is salinity-dependent; the proportionate ability to use oxygen in any one salinity is — above the critical pO₂ —the same in all experimental groups. This ability is a function of temperature and increases from 15° to 30°C, becoming temperature independent from 30° to 40°C as long as the pO₂ remains above 150 mm Hg. At 50 mm Hg pO₂, the limiting effect of oxygen causes a decrease in metabolic rate. This limiting effect is minimal in 80 g fish kept in an isotonic medium (12.5 S), allowing greater scope for activity and a higher rate of oxygen uptake.