The use of clove oil to induce anaesthesia,and its effects on blood chemistry,in Lates niloticus from Lake Victoria,Uganda

The efficacy of clove oil as an anaesthetic and its effects on blood parameters in Nile perch Lates niloticus were evaluated in 2010. Clove oil concentrations of 49.3, 73.9 and 98.5?mg l^?1 induced anaesthesia in <3?min, while the average recovery time from anaesthesia was 11?min 22 s. The optimal oil clove oil concentration was 49.3?mg l^?1, inducing anaesthesia in 4?min 33 s, with recovery in 3?min 31 s. No stress response was elicited. Clove oil at a concentration of 24.6?mg l^?1 was an effective sedative, whereas a concentration of 49.3?mg l^?1 was sufficient for measuring fish and stripping gametes. A concentration of 73.9?mg l^?1 induced anaesthesia within 4?min and fish recovered in 10?min. Therefore, clove oil was an effective anaesthetic and sedative for the handling of Nile perch within a mass range of 0.4–12?kg fish^?1.     

Influence of temperature on clove oil anaesthesia in flounder (Platichthys flesus Linnaeus, 1758)

The efficacy of clove oil as an anaesthetic in juvenile flounder was evaluated at two temperatures (10–12 and 16–18 °C) and the effective concentration determined. Flounder juveniles with a body length of 9.0–10.2 cm and a weight of 6–14 g were individually exposed to clove oil at concentrations ranging from 200 to 1000 mg L^?1 in a 5‐L bath with three replicates per treatment with 10 fish. The induction and recovery times were recorded in seconds for all doses and temperatures. Temperature had no effect on induction, but higher temperature shortened the recovery time. The effective concentration (EC₅₀) was estimated as 753 (692–820) mg L^?1 for ≤3 min with 95% confidence limits. The study demonstrated that clove oil can be used as an effective anaesthetic in juvenile flounder and that temperature has an effect on recovery time from anaesthesia with clove oil.     

Effect of short‐term (0–72 h) fasting on serum biochemical characteristics in rainbow trout Oncorhynchus mykiss

An experiment was carried out to investigate the effects of short‐term fasting periods on the serum biochemical characteristics of rainbow trout, Oncorhynchus mykiss. For this purpose, fish were fasted 0, 6, 12, 24, 48 and 72 h before blood sampling. Thereafter the serum levels of thyroxine (T₄), 3,5,3′‐triiodothyronine (T₃), cortisol, glucose, lactate, triglyceride, cholesterol, total protein, albumin, globulin and albumin : globulin ration (A : G) were determined. Results show that serum levels of T₄ (4.60–8.77 ng ml^?1), T₃ (7.50–13.3 ng ml^?1), cortisol (7.91–24.5 ng ml^?1), glucose (18.5–80.1 mg dl^?1), lactate (12.7–29.6 mg dl^?1), triglyceride (171–500 mg dl^?1), and cholesterol (321–535 mg dl^?1) were significantly affected by the fasting period. However, there were no significant changes in serum total protein (3.03–3.68 g dl^?1), albumin (1.78–2.01 g dl^?1), globulin (1.15–1.70 g dl^?1) or A : G (1.13–1.93) among the fish fasted 0–72 h. Results clearly show the importance of a fasting period on the serum biochemical properties of rainbow trout. According to the results, 24 h fasting is suggested as a pre‐sampling fasting period in rainbow trout to measure serum levels of T₄, T₃, cortisol, glucose, lactate, triglyceride, and cholesterol. Potential mechanisms related to the changes in biochemical properties are discussed.     

Kocuria SM1 controls vibriosis in rainbow trout (Oncorhynchus mykiss,Walbaum)

Aims: To develop probiotics for the control of vibriosis caused by Vibrio anguillarum and Vibrio ordalii in finfish. Methods and Results: Kocuria SM1, isolated from the digestive tract of rainbow trout, was administered orally to rainbow trout (Oncorhynchus mykiss) for 2 weeks at a dose equivalent to c. 10⁸ cells per g of feed and then challenged intraperitoneally with V. anguillarum and V. ordalii. Use of SM1 led to a reduction in mortalities to 15–20% compared to 74–80% mortalities in the controls. SM1 stimulated both cellular and humoral immune responses in rainbow trout, by elevation of leucocytes (5·5 ± 0·8 × 10⁶ ml^?1 from 3·7 ± 0·8 × 10⁶ ml^?1), erythrocytes (1·2 ± 0·1 × 10⁸ ml^?1 from 0·8 ± 0·1 × 10⁸ ml^?1), protein (23 ± 4·4 mg ml^?1 from 16 ± 1·3 mg ml^?1), globulin (15·7 ± 0·2 mg ml^?1 from 9·9 ± 0·1 mg ml^?1) and albumin (7·3 ± 0·2 mg ml^?1 from 6·1 ± 0·1 mg ml^?1) levels, upregulation of respiratory burst (0·05 ± 0·01 from 0·02 ± 0·01), complement (56 ± 7·2 units ml^?1 from 40 ± 8·0 units ml^?1), lysozyme (920 ± 128·8 units ml^?1 from 760 ± 115·3 units ml^?1) and bacterial killing activities. Conclusions: Kocuria SM1 successfully controlled vibriosis in rainbow trout, and the mode of action reflected stimulation of the host innate immune system. Significance and Impact of the Study: Probiotics can contribute a significant role in fish disease control strategies, and their use may replace some of the inhibitory chemicals currently used in fish farms.     

The efficacy of the aquatic anaesthetic AQUI‐S® for anaesthesia of a small freshwater fish,Melanotaenia australis

For stage four anaesthesia of the western rainbowfish Melanotaenia australis the most efficacious concentration of AQUI‐S^® was found to be 80 mg l^?1 when five different doses were tested at 27° C (range 26·5–27·5° C). This gave a mean ±s.e. (n = 10) induction time of 140·4 ± 8·9 s and recovery time of 180·2 ± 8·9 s and resulted in 10% mortality when fish were continuously exposed for 15 min. All fish given this dose during induction and recovery time trials ate within 1 h of recovery, and there was zero mortality during a subsequent 1 month monitoring period.     

Testing clove oil as an anaesthetic for long-distance transport of live fish: the case of the Lake Victoria cichlid Haplochromis obliquidens

Clove oil can be used as an anaesthetic in the handling of marine and freshwater fish. Few studies report on its use for periods up to 48 h, for example, under long‐distance transport conditions. This study tested the effect of different clove oil concentrations for 1–48 h on recovery and survival of the cichlid Haplochromis obliquidens, an ornamental fish species endemic to Lake Victoria. Haplochromis obliquidens were anaesthetized for 1 h using 5–25 μl L^?1 clove oil. There was no correlation between clove oil concentration and post‐anaesthesia recovery time (P = 0.15). On average, fish recovered within 9.5 ± 2 min, and no fish died within 24 h after recovery. Results from exposure of fish to 18–20 μl L^?1 clove oil for up to 48 h suggested a narrow margin of safety as this concentration range induced mortality. At 18 μl L^?1 recovery times ranged from 3 to 43 min between 24 and 36 h exposure, while fish exposed longer than 36 h recovered within 1–10 min, or within 1–2 min after 44–48 h. At the end of a 48‐h transport experiment total ammonia levels were higher in transport water containing anaesthetized fish than for non‐anaesthetized fish (1.65 ± 0.19 and 0.54 ± 0.08 mg L^?1 NH + NH₃, respectively). The combined use of clove oil and the selective ammonium ion exchanger zeolite was considered feasible as ammonia levels could be reduced by up to 82% compared to control bags without zeolite.     

Basic physico-chemical parameters of milt from sea trout (Salmo trutta m. trutta), brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss)

The aim of the study was to compare the physico‐chemical parameters of milt from sea trout (Salmo trutta m. trutta), brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss). Milt was collected by stripping and spermatozoa concentrations, were determined and compared with sperm motility and spermatocrit values along with seminal plasma indices (pH, osmolality, sodium, potassium, chlorine, calcium, magnesium, glucose and protein concentrations). The highest spermatozoa concentration of 22.3 ± 6.7 × 10⁹ ml^?1 was found in the sea trout milt, and was significantly different of those observed in brook trout (11.9 ± 3.3 × 10⁹ ml^?1) and rainbow trout (10.7 ± 4.4 × 10⁹ ml^?1). The values for pH and K⁺ did not differ significantly among species. The mean pH was 8.0 in the milt of each species and the K⁺ concentrations ranged from 24.8 ± 7.2 to 30.5 ± 7.6 mm L^?1. Considerable differences were determined for the Ca²⁺ ions concentrations. The highest value was found in sea trout (1.7 ± 0.3 mm L^?1), while in the rainbow trout it was 0.7 ± 0.5 and in the brook trout 0.4 ± 0.1 mm L^?1. The most pronounced differences were found in the glucose concentration cause of its unnaturally low concentration in rainbow trout of the mean value of 6.0 ± 15.2 mg L^?1. The mean value in sea trout and brook trout was 185.0 ± 172.4 and 231.2 ± 148.4 mg L^?1 respectively. For all species, protein mean values were below 1.3 g L^?1. The mean osmolality was between 230.6 ± 98.6 and 272.0 ± 26.4 mOsm kg^?1 in the species studied. No correlation was found between any components determined in milt and the spermatozoa motility (P > 0.05). The sperm concentration was positively correlated with the protein content in the milt of the three species studied, other less exhibited correlation was found.     

A note on copper bioaccumulation in Mozambique tilapia,Oreochromis mossambicus (Osteichthyes: Cichlidae)

Copper (Cu) is one of the most commonly reported metal pollutants in African water bodies, but there are few studies on African freshwater fish species of copper accumulation and copper toxicity. Adult O. mossambicus were exposed to 0mg l^?1 (control) and 0.75mg l^?1 Cu for 96h and 0 (control), 0.11, 0.29 and 0.47mg l^?1 copper for 64 days. Samples of liver and gills were collected after 96h, and after 1, 32 and 64 days, respectively. There were significant differences in the mean Cu accumulation values in the liver and gills between the control and the Cu-exposed fish after the 96-h exposure. In fish exposed to 0.11 and 0.29mg l^?1 Cu for 64 days there was an increase in copper level in the tissues. In fish exposed to 0.47mg l^?1 Cu the concentration in the gill and liver tissue did not increase between Day 1 and Day 32. At this time, Cu accumulation in the liver was higher than for fish exposed to 0.11 and 0.29mg l^?1 Cu for 64 days. Exposure to approximately 0.47mg l^?1 Cu for more than 32 days induced mortality.     

Susceptibility of the Antarctic fish Pagothenia borchgrevinki to MS-222 anaesthesia

Summary Induction of anaesthesia and distribution of the routinely used fish anaesthetic, MS-222 (ethyl m-amino-benzoate methanesulfonate), were monitored in the Antarctic fish Pagothenia borchgrevinki. As expected from the extreme low metabolic rate of Antarctic fish, induction times were significantly longer than those from rainbow trout. Onset of anaesthesia in the Antarctic fish also failed to correlate with either brain or blood MS-222 concentrations, in contrast to the trends observed in freshwater salmonids. High levels of free MS-222 in liver and in the aglomerular kidney, and the presence of acetylated derivatives in the blood, suggest these organs as primary sites of drug metabolism. MS-222 anaesthesia in P. borchgrevinki is also accompanied by dose-dependent changes in haematological parameters.     

Factors limiting fish populations in the Loch Fleet system, an acidic drainage system in south-west Scotland

Loch Fleet is an oligotrophic upland lake in Galloway, south-west Scotland. It once supported a brown trout, Salmo trutta L., sport fishery with low annual catches (< 150 fish year ^?1) but catches declined markedly after 1950 and no fish were caught after 1975. Diatom records for the lake sediments indicate acute acidification since 1975, pH changing from c. 5–8 to 4–6. In 1984 a project was set up at Loch Fleet to investigate techniques of acidity mitigation, with a view to restoring fisheries in this and similarly affected waters. An underlying assumption of the project was that fish had been lost as a direct result of acidity and associated factors. Studies were therefore undertaken during a 2-year baseline period (1984–1986) to validate this assumption. Fish surveys using a variety of techniques (gill-netting, trapping, electrofishing) confirmed that trout were absent from the Loch and its afferent streams, and also from its main outlet stream, the Little Water of Fleet, for a distance of 7 km downstream. Trout were present below this point but are prevented from passing upstream by a 5-m waterfall. Eels, Anguilla anguilla L., were present throughout the Little Water of Fleet, though not in the Loch itself. Population densities of both species were low, with less than 7 eels and 5 trout per 100 m². Survival studies using brown trout ova and yolk-sac fry indicated that conditions in the Loch and its afferent streams were acutely toxic to these stages as a result of the low pH (pH 4.5), low calcium (I mg l^?1) and high aluminium concentrations (200 μg 1 ^?1 total Al, 60 μg 1 ^?1 inorganic monomeric Al). Trout fingerlings could survive these conditions in short-term tests (9 days) but, in chronic exposure tests lasting up to 180 days carried out in situ in streams adjoining the loch, no fish survived this period. This toxicity was eliminated in experiments where pH was raised to 5.4 by KOH addition. It is concluded that the loss of the brown trout fishery at Loch Fleet occurred as a direct result of acidity and related factors, probably acting in the first instance on the sensitive intra-gravel ova and yolk-sac fry stages, leading to recruitment failure.     

Calcium‐dependent behavioural responses to acute copper exposure in Oncorhynchus mykiss

Using rainbow trout Oncorhynchus mykiss, the present study demonstrated that: (1) calcium (Ca) increased the range of copper (Cu) concentrations that O. mykiss avoided; (2) Ca conserved the maintenance of pre‐exposure swimming activity during inescapable acute (10 min) Cu exposure. Data showed that when presented with a choice of Cu‐contaminated water (ranging from 0 to 454 µg Cu l^?1) and uncontaminated water in a choice tank, O. mykiss acclimated and tested at low Ca concentration (3 mg Ca l^?1) avoided the 10 µg Cu l^?1 only. By contrast, O. mykiss acclimated and tested at high Ca concentration (158 mg Ca l^?1) avoided all the Cu concentrations ≥37 µg l^?1. The Cu avoidance was connected with increased spontaneous swimming speed in the Cu‐contaminated water. When subjected to inescapable Cu exposure (35 µg Cu l^?1), O. mykiss acclimated and tested at low Ca concentration reduced their spontaneous swimming speed, whereas no response was observed in O. mykiss acclimated and tested at high Ca concentration. Collectively, the data support the conclusion that in O. mykiss the behavioural responses to acute Cu exposure are Ca‐dependent.     

Copper exposure and the degeneration of olfactory receptors in rainbow trout (Oncorhynchus mykiss)

Abstract

Sublethal concentrations of copper in water cause the degeneration of olfactory receptors in rainbow trout (Oncorhynchus mykiss). Receptor cell loss has been correlated to the loss of olfaction in fish and may cause difficulties in olfactory mediated behaviors such as migration. This study investigated the effects of three levels of copper (100, 75 and 50 mg L^?1) on the olfactory epithelium of rainbow trout. Twenty fish randomly allocated between three exposure groups and one control were exposed for 24 hours under static renewal conditions. Light and scanning electron microscopic observations of olfactory tissue were taken to determine the extent of degeneration of receptors. In addition, levels of copper and zinc in the brain tissues were analyzed to determine if the olfactory route was a significant route of copper exposure and transfer to fish brain tissue. Results indicate that degeneration of receptors is related to the concentration of copper. Levels of copper in brain were found to be below detection of the instrument. Levels of zinc were extremely variable ranging from 52 to 132 ng zinc g^?1 brain tissue.     

A bioenergetic assessment of the influence of stocking practices on rainbow trout (Oncorhynchus mykiss) growth and consumption in a New Zealand lake

Summary

• 1 To investigate the carrying capacity and factors affecting growth of rainbow trout in Lake Rotoiti, we employed a bioenergetics model to assess the influence of stocking rates, timing of releases and prey abundance on growth and prey consumption. We hypothesised that stocking rates and prey abundance would affect growth and prey consumption by influencing per‐capita prey availability, and that the environmental conditions encountered by fish at the time of stocking would affect growth and consumption.
• 2 Prey consumption of stocked rainbow trout was calculated with the Wisconsin bioenergetics model. We calculated growth trajectories of released trout based on data from stocked trout that were released in spring and autumn from 1993 to 2009 and then re‐captured by anglers. Diet, prey energy density, body mass lost during spawning and lake temperature were measured locally.
• 3 Stocking timing had no effect on return rates to anglers or length or weight of caught fish. Although trout released in autumn were smaller than those released in spring, autumn‐released trout grew at a faster rate and had similar lengths and weights to spring cohorts after 2 years of growth in the lake. Modelled consumption parameters were negatively correlated with trout population size, suggesting that stocking rates (347–809 fish ha^?1 year^?1) caused density‐dependent effects on growth. Although common smelt (Retropinna retropinna) accounted for 85% of total prey consumption, no significant relationship was found between prey consumption by individual trout and adult smelt abundance, possibly because trout are targeting smaller smelt that our abundance estimate did not account for.
• 4 Releasing trout in autumn appears to be advantageous for growth, possibly because (i) temperature is more suitable for growth in autumn–winter than in spring–summer and (ii) prey for small trout is abundant in autumn. Mild winter conditions appear to enhance overwinter survival and growth of rainbow trout in warm‐temperate lakes compared to higher latitudes. This implies that moderately productive warm‐temperate lake ecosystems are highly suitable for trout growth in winter, but less so in summer, when lake stratification and high nutrient levels may create conditions suitable for algal blooms and hypolimnetic deoxygenation. High growth rates of trout in warm‐temperate lakes can therefore be supported by timing releases to coincide with favourable winter conditions.

     

Some aspects of the biology of the stargazer mountain catfish,Amphilius uranoscopus (pfeffer); (Siluriformes: Amphiliidae) indigenous to Kenya streams

A study on some biological parameters of the mountain catfish, Amphilius uranoscopus Pfeffer 1889 (Silurifomes: Amphiliidae), was carried out in the Thego stream on the slopes of Mount Kenya from February to December 2002. Physical and chemical profiles of the Thego show that the water quality parameters is typical of high altitude streams with temperatures rarely exceeding 18°C, DO ranging from 7.9 to 8.2 mg l^?1 and relatively high conductivity (97–137 μS cm^?1) typical of perturbed lotic environments. A total of 1010 fish were caught by an electro‐fisher, with sizes ranging between 8 and 24 cm fork length. The population structure had a unimodal distribution with maxima at 14–16 cm. The length–weight relationship showed relatively narrow range in the slope ranging from 2.61 in April to 2.98 in February 2002, thereby suggesting isometric growth pattern. The fitted growth pattern of A. uranoscopus showed an asymptotic length (L_∞) of 28.5 cm and a growth curvature (K) of 0.56 year^?1 resulting in an estimated natural mortality coefficient (M) of 0.90 year^?1. The Fulton’s condition factor (K) was also relatively stable with a peak in April (0.92 ± 0.21) and lowest value in June (0.86 ± 0.10). As A. uranoscopus is not under commercial exploitation, the seemingly depressed population is possibly attributed to the introduced exotic rainbow trout that heavily predates on the species and environmental perturbations arising from changes in land use. The implications of such changes on A. uranoscopus are discussed.     

Constructed wetlands with free water surface for treatment of aquaculture effluents

The aim of the study was to determine the reduction of the overall environmental load (in terms of organic and nutrient load) in effluents of a flow‐through trout farm. Effluents of a flow‐through system for rainbow trout (Oncorhynchus mykiss) production passed through constructed wetlands with free water surface. Removal of nutrients was determined in three wetlands of 350 m² each at hydraulic residence times (HRTs) of 3.5, 5.5 and 11 h. The areal load of total suspended solids (TSS), chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) varied in terms of HRTs from 12.3–36.8 g m^?2 day^?1, 21.7–65.2 g m^?2 day^?1, 0.23–0.70 g m^?2 day^?1, and 1.46–4.37 g m^?2 day^?1. Values for reduction of suspended solids, COD, TP, and TN were 67–72%, 30–31%, 41–53% ,and 19–30%, respectively. Significantly lower nutrient concentrations in the effluent among the wetlands were only found for nitrogen parameters: TN and ammonia concentrations were lower in the wetlands with a HRT of 5.5 h (0.89 mg L^?1, 0.11 mg L^?1) and 11 h (0.81 mg L^?1, 0.11 mg L^?1) compared with the one with 3.5 h (0.96 mg L^?1, 0.16 mg L^?1).     

The recovery and hematological rehabilitation of chlorine stressed adult rainbow trout (Salmo gairdneri)

Synopsis The ability of adult rainbow trout (Salmo gairdneri) to recover from acute chlorine exposure was tested. Six separate tests were carried out in duplicate. Blood was collected from each duplicate group of fish at the beginning of the test, at the appearance of the chlorine-distress symptoms, and at 24 and 48 h after exposures. The total residual chlorine (TRC) concentrations used in these six tests were 1.67, 3.50, 1.10, 1.25, 1.02 and 0.0 mg 1^–1, at water temperatures of 13.5, 14.6, 17.2, 22.6, 23.0 and 26.0° C, respectively. The cumulative fish mortality for these six tests at 48 h after exposure were 11.5, 100, 0.0, 67.1, 36.1 and 100 percent. Blood of chlorine stressed fish exhibited signs of hemoconcentration and hemolysis. All blood parameters with the exception of hemolysis returned to control levels within 24 hours after exporsure. High TRC concentrations in the range of 3.5 mg 1^–1 at 14.0° C, and TRC of 1.25 and 1.02 mg 1^–1 TRC at 23° C, significantly diminished the rainbow trout recovery.     

Altered burst swimming in rainbow trout Oncorhynchus mykiss exposed to natural and synthetic oestrogens

Juvenile rainbow trout Oncorhynchus mykiss were exposed to two concentrations each of 17β‐oestradiol (E2; natural oestrogen hormone) or 17α‐ethinyl oestradiol (EE2; a potent synthetic oestrogen hormone) to evaluate their potential effects on burst‐swimming performance. In each of six successive burst‐swimming assays, burst‐swimming speed (U_burst) was lower in fish exposed to 0·5 and 1 µg l^?1 E2 and EE2 for four days compared with control fish. A practice swim (2 days prior to exposure initiation) in control fish elevated initial U_burst values, but this training effect was not evident in the 1 µg l^?1 EE2‐exposed fish. Several potential oestrogen‐mediated mechanisms for U_burst reductions were investigated, including effects on metabolic products, osmoregulation and blood oxygen‐carrying capacity. Prior to burst‐swimming trials, fish exposed to E2 and EE2 for 4 days had significantly reduced erythrocyte numbers and lower plasma glucose concentrations. After six repeated burst‐swimming trials, plasma glucose, lactate and creatinine concentrations were not significantly different among treatment groups; however, plasma Cl^? concentrations were significantly reduced in E2‐ and EE2‐treated fish. In summary, E2 and EE2 exposure altered oxygen‐carrying capacity ([erythrocytes]) and an osmoregulatory‐related variable ([Cl^?]), effects that may underlie reductions in burst‐swimming speed, which will have implications for fish performance in the wild.     

Thermal tolerance and metabolic physiology among redband trout populations in south‐eastern Oregon

Streamside measurements of critical thermal maxima (T_crit), swimming performance (U_crit), and routine (R_r) and maximum (R_max) metabolic rates were performed on three populations of genetically distinct redband trout Oncorhynchus mykiss in the high‐desert region of south‐eastern Oregon. The T_crit values (29·4 ± 0·1° C) for small (40–140 g) redband trout from the three streams, and large (400–1400 g) redband trout at Bridge Creek were not different, and were comparable to published values for other salmonids. At high water temperatures (24–28° C), large fish incurred higher metabolic costs and were more thermally sensitive than small fish. U_crit(3·6 ± 0·1 L_F s^?1), R_r(200 ± 13 mg O₂ kg^?0·830 h^?1) and metabolic power (533 ± 22 mg O₂ kg^?0·882 h^?1) were not significantly different between populations of small redband trout at 24° C. R_max and metabolic power, however, were higher than previous measurements for rainbow trout at these temperatures. Fish from Bridge Creek had a 30% lower minimum total cost of transport (C_min), exhibited a lower refusal rate, and had smaller hearts than fish at 12‐mile or Rock Creeks. In contrast, no differences in U_crit or metabolism were observed between the two size classes of redband trout, although C_min was significantly lower for large fish at all swimming speeds. Biochemical analyses revealed that fish from 12‐mile Creek, which had the highest refusal rate (36%), were moderately hyperkalemic and had substantially lower circulating levels of free fatty acids, triglycerides and albumin. Aerobic and anaerobic enzyme activities in axial white muscle, however, were not different between populations, and morphological features were similar. Results of this study: 1) suggest that the physiological mechanisms that determine T_crit in salmonids are highly conserved; 2) show that adult (large) redband trout are more susceptible to the negative affects of elevated temperatures than small redband trout; 3) demonstrate that swimming efficiency can vary considerably between redband trout populations; 4) suggest that metabolic energy stores correlate positively with swimming behaviour of redband trout at high water temperatures; 5) question the use of T_crit for assessing physiological function and defining thermal habitat requirements of stream‐dwelling salmonids like the redband trout.     

Observations on limnological conditions associated with a fish kill of Oreochromis niloticus in Lake Chivero following collapse of an algal bloom

Possible causes of deaths of Oreochromis niloticus in Lake Chivero were examined in relation to changes in limnological conditions monitored over a 25‐month period. The fish deaths coincided with the collapse of an algal bloom that had developed and builtup in the lake for 8 months. Chlorophyll a and dissolved oxygen increased to average concentrations of 42.4 μg l^?1 and 10.9 mg l^?1 respectively prior to the collapse of the bloom. Dissolved oxygen decreased when the bloom started to die off and coincided with the fish deaths when the average surface dissolved oxygen concentration in the lake was 3.9 mg l^?1 and was at a depth of 5 m <2 mg l^?1. Mortality probably resulted from depressed oxygen levels caused by the high oxygen demand from the massive algal die‐off and released algal toxins. This is the first time that die‐off of algae has been linked to fish‐kills in Lake Chivero as occurs in other hypereutrophic systems.     

Response of juvenile tench Tinca tinca (L.) to the anaesthetic 2-phenoxyethanol

The response of tench Tinca tinca aged 40–171 days post‐hatch (22–49 mm TL) to the anaesthetic 2‐phenoxyethanol was studied at 25°C. The lowest effective concentration of 2‐phenoxyethanol increased with age, while the highest safe concentration decreased. The fish aged 40 days post‐hatch required a significantly (P ≤ 0.05) shorter time to become anaesthetized than did older fish. The recovery time after 15 min of exposure to 2‐phenoxyethanol at 0.45 g dm^?3 was significantly shorter in the 40‐day‐old fish than in older fish. In juveniles of the same age, induction time or recovery time did not depend on their size or condition (Fulton's coefficient). At 25°C, 2‐phenoxyethanol at 0.5 g dm^?3 may be used to efficiently and safely anaesthetize T. tinca juveniles.