Reduction In Water Temperature Research Articles

Modeling the Effect of Water Diversion on the Temperature of Mountain Streams

Water diversion for hydroelectric power generation impacts the temperature of mountain streams. Such changes are estimated by using a coupled one-dimensional dead-zone heat balance model. In very steep river sections, the dissipation of kinetic energy is the dominant heat source. For such streams, water diversion has only a minor effect on water temperature, because dissipation-induced temperature changes are independent of discharge. In contrast, in river sections of gradual slope, the influence by solar radiation, long-wave radiation, and heat exchange with the streambed is stronger. In such cases, a discharge reduction can lead to significant temperature changes. For a small stream in the southern Swiss Alps, model results show that diversion increases temperature by about 3.7 (±0.9)°C in a 21 km long river section under high solar radiation during summer. During a cold winter episode, water temperature is estimated to be about 1.8 (±0.8)°C lower compared to natural conditions. This heat balance model can also be used to simulate the effect of different measures to reduce water temperature changes in affected streams.

Efficacy of UV irradiation in inactivating Cryptosporidium parvum oocysts.

To evaluate the effectiveness of UV irradiation in inactivating Cryptosporidium parvum oocysts, the animal infectivities and excystation abilities of oocysts that had been exposed to various UV doses were determined. Infectivity decreased exponentially as the UV dose increased, and the required dose for a 2-log(10) reduction in infectivity (99% inactivation) was approximately 1.0 mWs/cm(2) at 20 degrees C. However, C. parvum oocysts exhibited high resistance to UV irradiation, requiring an extremely high dose of 230 mWs/cm(2) for a 2-log(10) reduction in excystation, which was used to assess viability. Moreover, the excystation ability exhibited only slight decreases at UV doses below 100 mWs/cm(2). Thus, UV treatment resulted in oocysts that were able to excyst but not infect. The effects of temperature and UV intensity on the UV dose requirement were also studied. The results showed that for every 10 degrees C reduction in water temperature, the increase in the UV irradiation dose required for a 2-log(10) reduction in infectivity was only 7%, and for every 10-fold increase in intensity, the dose increase was only 8%. In addition, the potential of oocysts to recover infectivity and to repair UV-induced injury (pyrimidine dimers) in DNA by photoreactivation and dark repair was investigated. There was no recovery in infectivity following treatment by fluorescent-light irradiation or storage in darkness. In contrast, UV-induced pyrimidine dimers in the DNA were apparently repaired by both photoreactivation and dark repair, as determined by endonuclease-sensitive site assay. However, the recovery rate was different in each process. Given these results, the effects of UV irradiation on C. parvum oocysts as determined by animal infectivity can conclusively be considered irreversible.

Gastrointestinal uptake and distribution of copper in rainbow trout.

A single dose of radioactive copper ((64)Cu or new Cu) was infused into the stomach of rainbow trout (Oncorhynchus mykiss) to model dietary copper (Cu) uptake under conditions of a normal nutritional dose and optimum environmental temperature (16 degrees C, 0.117 microg Cu g(-)(1 )body mass). The distribution of new Cu to the gut and internal organs occurred in two phases: rapid uptake by the gut tissues (almost complete by 24 h post-infusion) followed by slower uptake by the internal organs. By 72 h, 60 % of the dose had been excreted, 19 % was still retained in the gut tissue, 10 % remained in the lumen and 12 % had been absorbed across the gut and partitioned amongst the internal organs. A reduction in water temperature of 10 degrees C (to 6 degrees C) significantly retarded components of new Cu distribution (movement of the bolus along the gut and excretion); nonetheless, by 72 h, the fraction absorbed by all the internal organs was similar to that at 16 degrees C. An increase in water temperature of 3 degrees C (to 19 degrees C) caused a pronounced increase in internal organ uptake by 24 h to approximately double the uptake occurring at 16 degrees C. The uptake of new Cu by the gut tissue had a low temperature coefficient (Q(10)<1) consistent with simple diffusion, while the temperature coefficient for transfer of new Cu from gut tissue to the internal organs was high (Q(10)>2), consistent with facilitated transport. Internally, the liver and gall bladder (including bile) were the target organs for dietary Cu partitioning since they were the only organs that concentrated new Cu from the plasma. Individual tissues differed in terms of the exchange of their background Cu pools with new Cu. The background Cu in the walls of the gastrointestinal tract (excluding stomach) exchanged 45-94 % with new Cu from the gut lumen, while tissues such as the stomach, gills, kidney, carcass and fat had 5-7 % exchangeable background Cu. The liver, heart, spleen, ovary, bile and plasma had only 0.2-0.8 % exchangeable background Cu. The gastrointestinal tissues appear to act as a homeostatic organ, regulating the absorption of nutritional (non-toxic) doses of Cu (0. 117 microg g(-)(1 )body mass day(-)(1)) by the internal organs. Within the dose range we used and at optimal temperature (16 degrees C), the new Cu content of the gut tissues fluctuated, but absorption of new Cu by the internal organs remained relatively constant. For example, predosing the fish with non-radioactive Cu caused new Cu absorption by the gut tissues to double and decreased new Cu excretion from 38 to 1.5 %, but had no effect on new Cu uptake by the internal organs. Feeding fish after application of the normal liquid dose of new Cu also had no effect on new Cu uptake by the internal organs, even though the presence of food in the digestive tract reduced the binding of new Cu to the gut tissues and assisted with the excretion of new Cu. The gut was therefore able to regulate new Cu internalization at this dosage. Higher new Cu doses (10, 100 and 1000 times the normal dose), however, evoked regurgitation and increased new Cu excretion within 4 h of application but did not elevate new Cu levels in gut tissue beyond a threshold of approximately 40 microg of new Cu. Only at the highest dose (1000 times the normal dose, 192 microg g(-)(1 )body mass), equivalent to toxic concentrations in the daily diet (7000 microg Cu g(-)(1 )dry mass food), was the buffering capacity of the gut overwhelmed, resulting in an increase in internal new Cu uptake.

Potential impact of groundwater augmentation of river flows on fisheries: a case study from the River Ouse, Yorkshire, UK

Many lowland rivers in the UK are under increasing pressure from abstraction of water for potable supply or irrigation. Many schemes, both existing and proposed, attempt to mitigate the loss of flow by augmentation from upstream reservoirs or pumping of groundwater into adjacent rivers. The impact of flow regulation by reservoirs is well documented, but little attention has focused on the effect of groundwater augmentation on the biota, particularly fisheries, in the receiving river. The present paper examines the potential effects of pumping groundwater into minor tributaries of the Yorkshire Ouse in England on the cyprinid fisheries downstream of the discharge points and in the main river channel. Concerns were raised with respect to poor water quality and reduced water temperature. In particular, it was predicted that small reductions in river water temperature brought about by discharge of cold (≤ 10 °C) groundwater could have serious detrimental effects on cyprinid fish recruitment and juvenile growth in the receiving river, leading to a decline in stocks.

Influence of environmental temperature and water oxygen concentration on gas diffusion distance in sea bass (Dicentrarchus labrax ,L.)

Gas diffusion distance (GDD) of sea bass was measured in fish bred under farm conditions, at different dissolved oxygen concentrations (DO): normoxia condition (80–100% of the saturation value) and `mild' hyperoxia condition (120–130% of the saturation value). Measures were carried out two times in a year (beginning of summer and autumn) in order to evaluate the effect of water temperature on GDD at the two different dissolved oxygen concentrations. There was a significant influence of both dissolved oxygen concentration (p ≤0.001) and environmental temperature (p ≤0.001) on GDD. In summertime it was 1.75 μm and 2.31 μm for fish reared under normoxia and hyperoxia, respectively, and in autumn 2.51 μm and 2.96 μm for fish reared under normoxia and hyperoxia, respectively. When DO was reduced at the higher temperatures, GDD decreased as well. Results lead to the conclusion that GDD increased with the increasing of DO, both due to reduced water temperature and to the mild oxygen hypersaturation following application of pure oxygen. The advantage for fish may be found in the compromise between maximising O2 diffusion at the gills and ions/water intake/loss, known as `osmoregulatory compromise'.

Relative sea-level changes in the last 5500 years in southern Brazil (Laguna–Imbituba region, Santa Catarina State) based on vermetid [formula omitted] ages

Twenty-six new radiocarbon dates from vermetid shells collected in the southernmost sector of the Brazilian rocky coast presented dates ranging from 5410±80 to 190±65 years B.P., with associated paleosea levels varying from +2.10 m to +0.20 m above present sea level. The overall suggested trend of the relative sea level (RSL), declining until at least 190 years B.P., is somewhat contradictory to a proposed RSL rise in the last 1000 years in southern Brazil. The data also seem to undermine a more widely accepted RSL trend that suggests that at least two negative RSL oscillations occurred between 4100 and 3800 years B.P. and between 3000 and 2700 years B.P. The maximum elevation of the RSL in the Holocene in southern Brazil was possibly lower than that observed in most of the Brazilian eastern coast. Discrepancies between ancient sea levels of similar ages are attributed to coincidental methodological problems, to imprecisions in determining past relative sea levels and to possible changes in the geomorphology and wave climate close to shore during the last 5000 years. A general trend of increasing δ 18 O with a reduction in age in the studied samples may suggest a gradual reduction of water temperature in the region during the same period.

Beneficial effects of blue-green algae and Azolla , excluding supplying nitrogen, on wetland rice fields: a review

The role of blue-green algae (BGA) and Azolla in supplying N to rice fields is well documented. In addition, they also bring about, directly or indirectly, a number of changes in the physical, chemical and biological properties of the soil and soil-water interface in rice fields. For example, BGA liberate extracellular organic compounds and photosynthetic O2 during their growth, while Azolla prevent a rise in the pH, reduce water temperature, curb NH3 volatilisation and suppress weeds; and both of them contribute biomass. On decomposing, they influence the redox activity and result in the formation of different organic acids in soil. All such changes brought about by BGA and Azolla in soil may ultimately influence plant-available nutrients and also soil characteristics. An attempt has been made in this review to highlight these effects of BGA and Azolla in rice fields and discuss their possible implications relating to management and productivity of rice-field systems.

DEVELOPMENT OF A SURFACE STERILIZATION SYSTEM COMBINED WITH A WASHING PROCESS TECHNOLOGY FOR LEAFY LETTUCE

Various washing systems combined with chlorinated water, ozonated water, ultrasonic waves, and spray washing were developed to increase the efficiency of surface sterilization of leafy lettuce. Leafy lettuce treated with 100 mg.L -1 , 150 mg.L -1 and 200 mg.L -1 chlorinated water for 20 min decreased microbial load by 90%-99%. Samples treated with ozonated water (1.0-1.5 mg.L -1 ) at 4°C for 30 min decreased microbial load by 99%. Ozone treatment for 60 min resulted in 99.9% reduction in coliforms. The solubility of ozone increased with the reduction in water temperature and is very compatible with a hydrocooling system. Ultrasonic wave, vortex and spray washing techniques decreased microbial load, however none achieved greater than a 90% reduction.

Evaporative cooling of a falling water film on horizontal tubes

Results of experiments on evaporative cooling of water falling freely from one horizontal tube to the next one below it are presented in the paper. Smooth, unheated tubes (diameter 19.5 mm) were arranged one upon the other in the same vertical plane. Water was uniformly distributed along the apex of the most upper tube, the water flow exposed to upwardly streaming air, and change of the water temperature in the flow direction was measured. At specified inlet conditions, the reduction of water temperature was observed to depend on the humidity and velocity of the air, and on the tube spacing. Based on the experiments, an empirical equation for calculating the evaporative water cooling is recommended.

Live transport of gravid Penaeus indicus using coconut mesocarp dust

At present, the transport of gravid Penaeid shrimp in polythene bags and styrofoam boxes often results in high mortality. A new method of transportation by packing the gravid Penaeid banana prawn ( Penaeus indicus) with coconut mesocarp dust in PVC tubes in a transportation tank resulted in high survival (99.4±0.23% over 42 h), reduced reabsorption of total and partial eggs (4.33±1.52% and 2.33±1.52%, respectively) and quicker spawning (90% within 7 days of stocking in maturation tank). The new transportation device has a double-jacketed box containing a space in between the two jackets for an ice and salt mixture to reduce water temperature, and provision for water exchange, aeration and securing the packed shrimp.

Physiological versus viscosity-induced effects of an acute reduction in water temperature on microsphere ingestion by trochophore larvae of the serpulid polychaete Galeolaria caespitosa

Physiological and viscosity-induced effects of an acute 10°C reduction in water tempera- ture on the feeding performance of trochophore larvae of Galeolaria caespitosa were separated. Experiments were conducted in which 3 and 10 um spheres were supplied to larvae separately and in combination. Both physiological and viscosity-induced components of the reduction in water temperature significantly reduced the number of microspheres ingested by larvae. When 3 and 10 um spheres were supplied to larvae both separately and in combination, reduced water temperature resulted in a 60% decline in the number of microspheres ingested. Increased water viscosity alone accounted for over half of these total declines. The remaining proportions of the total declines were attributable to the physiological effects of reduced water temperature. Increased water viscosity did not differentially influence the reductions in the numbers of 3 and 10 um spheres ingested by larvae.

Production and Extended Spawning of Cultured White Bass Broodstock

Abstract Our research focused on the development of domesticated broodstock of white bass Morone chrysops to reduce or eliminate the dependence of public and private hatcheries on wild-caught fish. Juveniles, obtained from spawning wild white bass, were reared to a mature size in an outdoor tank system under ambient environmental conditions. At 1 year of age, a few males were mature, and by 2 years of age all males and females were mature. Females were induced to ovulate with human chorionic gonadotropin (550 IU/kg) and then strip-spawned. Mean estimated egg production was 98,273 eggs/kg for 2-year-old fish and 127,805 eggs/kg for 3-year-old fish. Holding mature fish in reduced water temperatures extended the spawning period by 3 months. The use of cultured white bass broodstock should facilitate hatchery operations for stocking public waters and for hybrid striped bass aquaculture.

Metabolic stress during winter increases the toxicity of selenium to fish

This study examined the effect of reductions in water temperature and photoperiod, mimicking winter conditions, on the toxicity of combined dietary (5.1 μg/g dry weight) and waterborne (4.8 μg/l) selenium to juvenile bluegill ( Lepomis macrochirus). Elevated selenium caused hematological changes and gill damage that reduced respiratory capacity, while increasing respiratory demand and oxygen consumption. Elevated selenium in combination with low water temperature (4°C) caused reduced activity and feeding, depletion of 50–80% of body lipid, and significant mortality within 60 days. Fish in warm-water selenium exposures continued to actively feed and lipid depletion did not occur despite increased oxygen consumption. The combination of stress-related elevation in energy demand and reductions in feeding due to cold temperature and short photoperiod, leading to severe depletion of stored body lipid, is given the name Winter Stress Syndrome. This syndrome caused bluegill to undergo an energetic drain that resulted in death of about one-third of the fish. Results indicate that the current US national water quality criterion for selenium is not adequate to protect young bluegill and other species that substantially reduce activity and feeding during cold weather. Aquatic contaminants should be evaluated in the context of seasonal metabolic changes that normally occur in test organisms. Winter Stress Syndrome could be an important, but as yet unquantified mortality factor in many circumstances.

Application of heart rate transmitters in behaviour studies on Atlantic halibut ( Hippoglossus hippoglossus)

Implantation of acoustic transmitters in three specimens of Atlantic halibut ( Hippoglossus hippoglossus) was performed, and the surgical procedure is described. Acceptable transmittance of heart rate signals was dependent upon attachment of both tag electrodes in the pericardial wall close to the heart muscle. Strong muscular action during burst swimming and the capture and swallowing of food severely interfered with transmitted heart signals. The heart rate of one male halibut kept in a large tank with 50 other halibuts was recorded continuously for 4 months. A gradual decrease in pulse rate coinciding with a seasonal reduction in water temperature, day length and appetite was observed. A diurnal variation in heart rate was also detected. The fish showed little or no response in heart rate to certain farming procedures such as formalin treatment against parasites or cleaning of the tank. Bradycardia followed by increased heart rate after close interactions with larger fish was noticed in the first part of the recording period.

Alterations of Vertical Distribution and Migration of Zooplankton in Relation to Temperature

-Several crustacean zooplankters (Tropocyclops prasinus, Mesocyclops edax, Pseudosida bidentata ahd Bosmina longirostris) exhibited diurnal vertical migrations in early June at Belews Lake, North Carolina. These species spent daylight hours within the thermocline and nights in the epilimnion. Aeration of the lake hypolimnion lowered it from a depth of 11 m to 17 m by mid-July. After this restratification, B. longirostris, P. bidentata and M. edax migrated to greater depths, indicating that temperature may exert an important influence on vertical migratory behavior for these three species. Although vertical migration of Skistodiaptomus pallidus, Diacyclops thomasi and copepod nauplii was not demonstrated, these zooplankters consistently were found within or below the thermocline during day and night in June, and in July after restratification, suggesting distinct habitat preferences for colder waters. INTRODUCTION The vertical distribution and migration of zooplankton are often influenced by the temperature-depth profile in stratified systems and, indeed, thermal gradients may pose a barrier to migration (Wells, 1960; Hutchinson, 1967). Frequently, lakes are host to a number of different species exhibiting different thermal preferences (see Wells, 1960; Carter, 1974; Lane, 1978; Balcer et al., 1984, for examples). Of the species found in Belews Lake, North Carolina, the cyclopoid copepod, Diacyclops thomasi, is usually associated with cold waters (Yeatman, 1959; Carter, 1974; Moore, 1979), whereas Mesocyclops edax and Tropocyclops prasinus generally prefer warm waters (Yeatman, 1959; Carter, 1974; Balcer et al., 1984). The calanoid copepod, Skistodiaptomus pallidus, also is found and reproduces in warm conditions (Yeatman, 1959; Chapman et al., 1985). The cladoceran, Pseudosida bidentata, is restricted in geographic distribution to the southern United States (Brooks, 1959), which suggests that it also tends to be found in warm waters. In addition, some of the zooplankton species in Belews Lake have been reported elsewhere to undergo vertical migration. These groups include the cladoceran, Basmina longirostris (Wells, 1960; Wilson and Roff, 1973) and the copepods, Mesocyclops edax (Woodmansee and Grantham, 1961; Williamson and Magnien, 1982), Tropocyclops prasinus (Wilson and Roff, 1973) and D. thomasi (Wells, 1960; Wilson and Roff, 1973; Peacock and Smyly, 1983), as well as copepod nauplii (Wilson and Roff, 1973). Belews Lake serves as a cooling reservoir for the Belews Lake Steam Station operated by Duke Power Company. The thermocline is normally located between 7 and 11 m during the summer. Water intake valves at the steam station are located at approximately 9 m. To increase cooling efficiency, the power company installed a pair of air injectors at 17 m near the intake valves (Mitch Griggs, Duke Power Co., pers. comm.). Operation of the air injectors, which commenced on 9 June 1986, caused mixing of the cooler hypolimnetic waters with the warm epilimnion near the intake valves, thus reducing water temperature and increasing plant efficiency. As a result, the temperature profile in the rest of the main I Present address: Dept. of Fisheries and Oceans, Maurice Lamontagne Institute, C.P. 1000, MontJoli, Quebec, Canada G5H 3Z4.

Quantity and Quality of Summer Surface Net Zooplankton in the Kuroshio Current-induced Upwelling Northeast of Taiwan

This study compares the taxa distribution and biochemical composition of zooplankton from stations in coastal upwelling caused by the intrusion of the Kuroshio Current into the East China Sea shelf. Surface zooplankton samples were taken during summer cruise of 1989 from 21 stations using a conical net with a mesh size of 330μm. In addition to zooplankton taxonomical analysis, biochemical compositions of mixed zooplankton hauls including protein, lipid, ash and lipid classes such as phospholipids, free fatty acids, wax or sterol esters, sterols and triacylglycerides were studied. The results indicate that the copepod-dominant zooplankton biomass responded to the upwelling environment. Stations around the upwelling site showed markedly reduced water temperature, increased NO3-N, chlorophyll a and zooplankton concentrations. Zooplankton samples from the upwelled water contained higher proportions of protein and lipid and lower ash content than those from non-upwelling aregs. Higher free fatty acid level and lower phospholipids level were also observed in samples from upwellde water. These results suggest an enhancement in upwelling region of zooplankton production in both quantity and quality. Principal component analysis of zooplankton taxa and biochemical composition both indicated the existence of three distinct water masses, that can be related to the Kuroshio Current, the East China Sea continental shelf, and the upwelling. The existence of detritus in the zooplankton samples, which affected the measurement of zooplankton quality and its ecological implications, are discussed.

Reproductive periodicity of three species of chitons at a site in New South Wales, Australia (Mollusca: Polyplacophora)

Summary The reproductive cycles of Liolophura (L.) gaimardi, Cryptoplax mystica, and Ischnochiton (I.) australis are investigated using gonadal indices and histological features. L. (L.) gaimardi exhibits a clear annual reproductive cycle of spawning during autumn, resorbing over winter, and gametogenesis through spring and early summer. C. mystica and I. (I.) australis spawn primarily over extended periods through autumn/winter, and to a lesser extent during spring, resorption is limited, and gametogenesis proceeds at a constant rate throughout the year. The onset of gametogenesis in L. (L.) gaimardi coincides with increasing water temperatures, whereas the rate of gamete production in both C. mystica and I. (I.) australis appears to be independent of temperature. Spawning in all species is at its maximum during the period of maximum water temperature, though lesser spawnings may occur in C. mystica and I. (I.) australis at reduced water temperatures. Temperature is discounted as the direct cue initiatin...

GROUND‐WATER TEMPERATURE FLUCTUATIONS AT LYONS FERRY FISH HATCHERY, WASHINGTON1

ABSTRACT: The well field serving the Lyons Ferry Fish Hatchery has experienced reduced water temperatures following continued, periodic withdrawal of large volumes of water. In January 1985, the well field temperature was 49°F, which is less than the optimal 52°F for raising salmon and steelhead trout. The aquifer supplying the hatchery is in hydraulic and thermal connection with the Snake River and a flooded embayment of the Palouse River. Ground‐water temperatures in the well field cycle on an annual basis in response to changes in surface water temperature and pumping rate. Numerical simulation of the well field, using a simplified mixing cell model, demonstrates the coupling of well field hydraulics and aquifer thermal response. Alternative pumping schedules indicate that it is feasible to adjust ground‐water pumping to effectively store heat in the aquifer during the summer months when surface water temperatures are elevated. Sensitivity analysis of this model indicated that the primary controls of the system's thermal response are the volume of the aquifer assumed to contribute to the well field and temperature of the overlying surface water body.

Effects of Available Dietary Carbohydrate and Water Temperature on the Chronic Toxicity of Waterborne Copper to Rainbow Trout (Salmo gairdneri)

We used a factorial design to determine the effects of dietary carbohydrate content (0.1 or 25.6%) and temperature (10 or 15 °C) on the chronic toxicity of waterborne copper (0 or 144 μg Cu∙L−1) to rainbow trout (Salmo gairdneri) over a 12-wk period. Relative to those on the low-carbohydrate (LC) diet, trout reared on the high-carbohydrate (HC) diet developed enlarged, glycogen-filled livers and attained reduced final mean wet body weights. This response was more pronounced at 10 than at 15 °C. Whereas Cu exposure had no significant effect on the growth or mortality of either LC or HC fish at 15 °C, or LC fish at 10 °C, HC fish at 10 °C showed significantly reduced final weight and increased mortality. Trends toward reduced liver glycogen and increased serum glucose, characteristic of chronic stress, were apparent in all Cu-exposed fish. In Cu-free water, mean Cu concentrations in liver tissue of HC fish were significantly lower than those in LC fish at both temperatures. Copper exposure resulted in significant increases in liver Cu concentration for all diet treatments except HC at 10 °C. Elevated serum sorbitol dehydrogenase levels, indicative of hepatic damage, were apparent in Cu-HC fish at 15 °C and in HC, Cu-HC, and Cu-LC fish at 10 °C. No treatment effects on either serum protein or hematocrit were evident. The treatments during the growth period affected subsequent lethal tolerance of Cu. While the 96-h Cu LC50 was not significantly altered by diet alone at 15 °C, it was elevated in HC fish, relative to LC fish at 10 °C. The Cu LC50 of Cu preexposed fish was significantly elevated, relative to parallel groups held in Cu-free water, for LC fish at both 10 and 15 °C. This acclimation to Cu was not apparent in HC fish at either temperature: no significant elevation in LC50 occurred as a result of preexposure. We conclude that increased dietary carbohydrate can significantly increase the chronic toxicity of Cu to trout, and that the impact is enhanced by reduced water temperature.

Effects of Dietary 17α-Methyltestosterone and Bovine Growth Hormone on Growth and Food Conversion of Slow- and Normally-Growing American Elvers (Anguilla rostrata)

Dietary addition of 17α-methyltestosterone (MT) and bovine growth hormone (bGH) had a significant effect on growth and food conversion in American elvers (Anguilla rostrata). Both MT (1 μg/g) and bGH (2 and 10 μg/g) significantly increased the mean weight of elvers compared with controls. Higher concentrations of MT (10 μg/g) increased mean body weight, but not significantly. The effects of MT were more pronounced in normally-growing elvers compared with slow-growing elvers. Reduction of water temperature had a much stronger influence on weight gain in slow-growing elvers receiving MT than with normally-growing eels receiving MT. Food conversion was improved by feeding either MT or bGH. Food conversion was generally lower in slow-growing than in normally-growing elvers. Total body protein was related to body weight and was significantly lower in elvers fed MT than in control elvers. However, there was no significant difference in total body protein elvers fed bGH compared with the control group.