Plasma proteins,hepatic enzymes,thyroid hormones and liver histopathology of Cyprinus carpio (Linnaeus, 1758) exposed to an oxadiazin pesticide,indoxacarb

Indoxacarb is a relatively new pesticide from oxadiazin class, which is used near carp ponds for agricultural purposes. Thus, the aim of this study was to determine indoxacarb effects on common carp (Cyprinus carpio). The fish were exposed to 0 (control), 0.75, 1.5 and 3 ppm of indoxacarb over 21 days and plasma biochemical characteristics and liver histopathology were examined. Exposure to indoxacarb induced fall in total protein after 21 days. Globulin increased after 7 days and then decreased after 14‐ and 21‐day exposure. Plasma alanine transaminase and aspartate transaminase activities increased in 1.5 and 3 ppm treatments after 7‐day exposure. Indoxacarb exposure significantly decreased plasma alkaline phosphatase after 7 days with no change at the second and third samplings. After 7 days, plasma T₃ levels had no significant change; however, it decreased after 14 days in the 1.5 and 3 ppm treatments and also reduced after 21‐day exposure to the 3 ppm treatment compared to the control group. Plasma T₄ level only decreased significantly in the 3 ppm treatment compared to the control group after 21 days. Different histopathological symptoms such as necrosis, hyperaemia, sinusoidal space extension, pyknotic nuclei, leucocyte infiltration and melanomacrophage aggregates were observed after 21‐day exposure to indoxacarb. The symptoms intensity was dependent on indoxacarb concentration. In conclusion, the present results show that indoxacarb exposure adversely affects common carp health and welfare, which consequently may induce serious problems in this species aquaculture.     

Influences of dissolved oxygen on juvenile largemouth bass foraging behaviour

Summer stratification often leads to large areas of hypolimnetic hypoxia in lakes and reservoirs. These areas of hypoxia alter fish behaviours and distributions as well as restrict access to valuable prey resources, yet few studies have examined foraging behaviour of fish in response to low dissolved oxygen (DO) concentrations. We observed foraging behaviour of juvenile largemouth bass, Micropterus salmoides, in response to varying DO concentrations in tanks that simulated a stratified lake water column during the summer: 28°C oxygenated epilimnion, 15°C deoxygenated hypolimnion. Compared with saturated concentrations (8.0–9.0 mg/L), hypolimnetic oxygen concentrations of 3.0 mg/L and 1.5 mg/L resulted in a drastic decrease in prey consumption, handling efficiency and time spent below the thermocline mainly due to avoidance behaviour of hypoxic conditions. However, we found at high hypolimnetic prey densities, individual fish were more willing to venture into reduced oxygen concentrations. Several unique behaviours including transporting prey above the oxycline for consumption, aquatic surface respiration and gill flaring were employed by largemouth bass foraging in low oxygen environments. Reduced hypolimnetic oxygen concentrations may influence and alter feeding strategies, especially for fish that rely on benthic prey resources.     

Physiological stress responses in spotted wolffish (Anarhichas minor) subjected to acute disturbance and progressive hypoxia

The spotted wolffish (Anarhichas minor) is considered a promising species for coldwater aquaculture. It is a sedentary, bottom-dwelling fish which exhibits a calm and “non-stressed” behaviour in captivity. There are, however, no reports on the physiological responses to stressors in this species. In the present study we investigated primary (cortisol secretion) and secondary (glucose mobilization) responses to common aquaculture stressors like disturbance and hypoxia. Pre-stress plasma cortisol levels were within those generally considered representative for unstressed fish (~ 10 ng ml^− 1), whereas basal glucose levels were unusually low (0.3–0.4 mM). After exposure to a short, but severe disturbance challenge (emptying the tank of water for 10 min), the increase in plasma cortisol level was slow and relatively weak, reaching a peak level of 25 ng ml^− 1 4 to 8 h after disturbance. When the fish were exposed to a gradually decreasing oxygen level in the tank, a significantly elevated plasma cortisol level (35 ng ml^− 1) was seen in the fish that remained in the tank until oxygen saturation had decreased to 20% oxygen saturation after 2.5 h. However, a two-fold higher plasma cortisol level (~ 70 ng ml^− 1) was seen in all fish exposed to reduced oxygen levels (60, 40 and 20% oxygen saturation) after 3.5 h recovery in normoxic water. Plasma glucose levels showed only moderate increases (~ 70%) following disturbance and hypoxia challenges. An in vivo injection of ACTH caused a strong elevation of plasma cortisol (peak level ~ 170 ng ml^− 1), demonstrating a high capacity for interrenal steroidogenesis in the spotted wolffish. The slow and relatively weak cortisol response to stressors, and low plasma glucose levels, may relate to the sedentary lifestyle of the spotted wolffish. The stress-response is characterised by a passive (reactive) coping style, which is considered adaptive for farming of this species.     

Toxic effects of indoxacarb on gill and kidney histopathology and biochemical indicators in common carp (Cyprinus carpio)

The aim of this study was to determine the biochemical and histopathological effects of indoxacarb on an economic fish species Cyprinus carpio. The fish were divided into four triplicated groups exposed to either of 0% (control), 5% (0.75 mg/L), 10% (1.5 mg/L) or 20% (3 mg/L) of the pesticide LC₅₀. Blood sampling was performed after 7, 14 and 21 days exposure. There were no changes in blood calcium levels at any times. In the indoxacarb‐treated fish, chloride levels decreased at the first and increased at the second sampling compared to the control. In 3 mg/L treatment, phosphorus increased significantly after 7 days compared to the control. Plasma glucose levels decreased significantly at the 7th and 14th days sampling; however, it increased at the 21st day. After 7 days, creatinine level in 3 mg/L treatment was significantly lower than the control; however, the creatinine levels of indoxacarb treatments were higher than the control at the 21st day. The indoxacarb‐treated fish had higher plasma urea levels compared to the control at the 7th day. At the 21st day, plasma urea level at 3 mg/L was significantly lower than the control treatment. Edema, Lamellar curling, hyperplasia, lamellar fusion and hyperaemia were observed in the indoxacarb‐treated fish gill. Tubular necrosis, hematopoietic necrosis, melanomacrophage aggregates, Bowman's capsule edema, glomerulus degeneration and hyperaemia were observed in the indoxacarb‐treated fish kidney. Generally, sublethal concentrations of indoxacarb cause stress, hydromineral imbalance, metabolism alteration and gill and kidney damages in common carp.     

Stress response of juvenile rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>)to chemical cues released from stressed conspecifics

The objective of this study was to determine whether exposure of rainbow trout (Oncorhynchus mykiss) to water containing a stressed trout or skin extract from stressed and non-stressed trout would elicit a stress response in conspecifics. Juvenile rainbow trout were exposed for 1 hour to water containing a stressed fish, homogenized skin extracts from a non-stressed fish, skin extract from a stressed fish and water with none of these factors. The stress response was measured over a 24-h period (1, 6, 12, 24 h after exposure). Plasma cortisol levels increased at 12 h in fish exposed to water from a stressed fish and skin extract from a stressed fish. Plasma glucose and hepatic hsp70 levels were not affected by treatments. The results suggest that rainbow trout elicit a stress response when exposed to stress-related alarm cues released from conspecifics.     

A comparative study on the effects of a pesticide (cypermethrin) and two metals (copper, lead) to serum biochemistry of Nile tilapia, Oreochromis niloticus

The present study was designed to compare the responses in freshwater fish Oreochromis niloticus exposed to a synthetic pyrethroid, cypermethrin (CYP); an essential metal, copper (Cu); and a nonessential metal, lead (Pb). Fish were exposed to 0.05 μg/l CYP, 0.05 mg/l Cu, and 0.05 mg/l Pb for 4 and 21 days, and the alterations in serum enzyme activities, metabolite, and ion levels were determined. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities increased in response to CYP, Cu, and Pb exposures at both exposure periods. While elevations in alkaline phosphatase (ALP) and lactate dehydrogenase (LDH) activities and in cholesterol level were observed in pesticide-exposed fish at 4 and 21 days, they increased in Cu- and Pb-exposed fish at 21 days. Although metal-exposed fish showed increases in cortisol and glucose levels at 4 days followed by a return to control levels at the end of the exposure period, their levels elevated in pesticide-exposed fish at both exposure periods. Total protein levels decreased in Pb- and pesticide-exposed fish at 21 days. Na⁺ and Cl⁻ levels decreased in pesticide-exposed fish at both exposure periods and in Cu- and Pb-exposed fish at 21 days. The exposures of pesticide and metals caused an elevation in K⁺ level at the end of the exposure period. The present study showed that observed alterations in all serum biochemical parameters of fish-treated pesticide were higher than those in fish exposed to metals.     

The effects of exposure to water-soluble fractions of crude oil on selected histochemical parameters of the liver of Atlantic croaker, Micropogon undulatus L.

Abstract. Micropogon undulatus L. were sampled from Galveston Bay, Texas, and transported to the research laboratory. Groups of five fish were exposed to 5 and 10% dilutions of the water soluble fraction (WSF) of southern Louisiana crude oil for 1, 3, 7, 14 and 21 days. Selected histochemical parameters of liver sections from control and WSF-exposed fish were evaluated. Glycogen was reduced in amount while glucose-6-phosphate dehydrogenase, lipid and cholesterol were increased in liver sections of WSF-exposed fish. These observations support a hypothesis that exposure to petroleum hydrocarbons increases mixed-function oxidase activity. The alterations were more obvious in sections from fish exposed to the higher level of the WSF. The histochemical parameters appeared most altered in hepatocytes near afferent hepatic blood vessels. These hepatocytes were the first cells of the liver to contact the pollutants.     

Effects of acidic water,aluminum, and manganese on testicular steroidogenesis in <Emphasis Type="Italic">Astyanax altiparanae</Emphasis>

Metals can influence the gonadal steroidogenesis and endocrine systems of fish, thereby affecting their reproduction. The effects of aluminum and manganese in acidic water were investigated on steroidogenesis in sexually mature male Astyanax altiparanae. Whether mature male fish recover from the effects of metals in metal-free water was also assessed. The fish were exposed to 0.5 mg L^?1 of isolated or combined aluminum and manganese in acidic pH (5.5) to keep the metals bioavailable. The fish underwent 96 h of acute exposure, and samples were taken 24 and 96 h after the beginning of the experiment. The fish were then maintained in metal-free water for 96 h. Plasma levels of testosterone, 11-ketotestosterone, 17β-estradiol, and cortisol were measured. Acidic water increased the plasma concentration of testosterone and 11-ketotestosterone. Aluminum increased the testosterone levels after 96 h of exposure. Manganese increased the 17β-estradiol levels after 24 h of exposure and maintained at high levels until the end of the experiment. With the exception of acidic pH, which increased cortisol levels after 24 h of exposure, no changes were observed in this corticosteroid during the acute experiment. Aluminum and manganese together also altered steroid levels but without a standard variation. The fish recovered from the effects of most exposure conditions after 96 h in metal-free water. A. altiparanae could use reproductive tactics to trigger changes in testicular steroidogenesis by accelerating spermatogenesis and spermiogenesis, which may interfere with their reproductive dynamics.     

Influence of continuous magnetic field exposure on water properties and subsequent effects on the growth performance,plasma biochemistry,nutritive value and liver histopathology of Jade Perch Scortum barcoo in a recirculating system

A 150‐days study was conducted on the continuous exposure of magnetized water at 0.00, 0.10, 0.15 and 0.20 Tesla (T) on quadruplicate treatments of Jade Perch Scortum barcoo juveniles in a recirculating system. Each replicate consisted of 18 fish with an average weight of 7.52 g over all treatments. The feeding efficiencies, growth, whole‐body proximate plasma parameters, survival were measured and liver histopathology was observed. Our results show no significant improvement in water quality parameters, specific growth rate and body weight gain in this study. However, the best feed conversion ratio was recorded in fish exposed to 0.10, 0.15 and 0.20 T, which are significantly better than the control 0.00 T. Though survival was unaffected but plasma biochemistry and liver histopathology were affected even within the treatment. No liver, plasma or blood abnormalities were detected in fish exposed to 0.10 T and the control 0.00 T. But, one out of four livers from fish in the 0.15 T treatment showed a localized inflammatory response. These changes increased in the liver of fish exposed to 0.20 T, high AST and necrosis in this group is evidence of the liver cells damaged or been at risk. A significantly higher crude protein and lipid were noticed in the exposed fish compared with the control, 0.15 T had the highest crude protein and lipid while the control has the lowest. Based on the overall findings, on growth performance and looking at other factors like absence of any physiological disorder 0.10 T can be used as an effective and affordable technique improve the feeding efficiencies of Jade Perch.     

Efficacy of Benzocaine as an Anesthetic in Juvenile Tambaqui Colossoma macropomum

Abstract.— The present study investigated the use of benzocaine as an anesthetic for juvenile Colossoma macropomum (tambaqui). In the first experiment, fish were exposed to various doses of benzocaine for 10 min at 24 C. The second experiment examined the effects of duration of exposure to 100 mg/L of benzocaine. In the third experiment, fish were exposed to 100 mg/L at temperatures of 24, 27, and 30 C. Benzocaine concentrations of 100–150 mg/L were considered ideal for quickly inducing total immobilization and fast recovery. Fish exposed to 350 mg/L benzocaine exhibited 30% mortality. No changes in hemat-ocrit were recorded in fish exposed to different concentrations of benzocaine. Plasma glucose increased significantly when fish were exposed to benzocaine concentrations greater than 200 mg/L. Recovery time after a 30-min exposure to 100 mg/L benzocaine was significantly greater than after an exposure for 10 and 20 min. No mortality was observed 96 h after exposure to 100 mg/L benzocaine for 10, 20, and 30 min. Dosages in the 100–150 mg/L range were effective for periods of up to 20 min of anesthesia. There was no effect of temperature on the time required for fish to lose equilibrium. However, recovery was significantly faster for fish anesthetized at 30 C. Benzocaine is an effective anesthetic agent for tambaqui juveniles, providing rapid immobilization and rapid recovery. Benzocaine is also less expensive than other available anesthetic compounds.     

Alterations in blood electrolytes of a freshwater catfish <Emphasis Type="Italic">Heteropneustes fossilis</Emphasis> in response to treatment with a botanical pesticide, <Emphasis Type="Italic">Nerium indicum</Emphasis> leaf extract

The present study aimed at investigating the effects of Nerium indicum leaf extract on the blood electrolytes of Heteropneustes fossilis for short- and long term. Fish were subjected to Nerium indicum leaf extract for short term (11.27 mg/L i.e. 0.8 of 96 h LC₅₀) and long term (2.81 mg/L i.e. 0.2 of 96 h LC₅₀). Fish were killed on each time intervals from control and experimental (Nerium indicum) groups after 24, 48, 72, and 96 h in short-term exposure and after 7, 14, 21, and 28 days in long-term experiment. Blood samples were analyzed for calcium and inorganic phosphate levels. Acute exposure of Nerium indicum leaf extract caused a progressive decrease in the serum calcium levels after 48 h in fish H. fossilis, which persists till the close of the experiment (96 h). The serum inorganic phosphate levels remain unaffected till 48 h in the Nerium indicum leaf extract–exposed fish. After 72 and 96 h, the levels exhibit a decrease. Chronic Nerium indicum leaf extract treatment provoked a decrease in serum calcium levels at day 14. This decrease continues till 28 days. The serum phosphate level of the Nerium indicum leaf extract–treated fish decreases on day 14 and 21. However, on day 28, the levels become close to the normal values. We conclude that Nerium indicum leaf extract exposure alters the blood electrolytes of the fish, thus causing physiological disturbances which might affect seriously the normal vital functions, growth rate, reproduction, and their survival in nature.     

Evaluating protection against Loma salmonae generated from primary exposure of rainbow trout, Oncorhynchus mykiss (Walbaum), outside of the xenoma-expression temperature boundaries

A series of challenge and re-challenge studies was conducted in which juvenile rainbow trout were exposed to the pathogen Loma salmonae , a microsporidian which typically causes xenoma formation during sporogony and inflammation in the gills as the xenomas undergo dissolution. The specific goal was to determine if a primary exposure, conducted at a water temperature outside of the range which permits the parasite to undergo sporogony and form branchial xenomas, would stimulate a protective response in the fish to a later challenge conducted under temperature conditions optimal for the parasite. Primary challenge of fish to L. salmonae at 7 °C or 21 °C blocked or limited xenoma formation, as discussed in a previous study. However, these fish had a relative percentage protection (RPP) against a second optimized exposure which matched, or was not significantly less than, the degree of protection (100%) that developed in other groups of fish that received a primary exposure throughout the range of water temperatures which permits xenoma formation. When the primary exposure was conducted at 5 °C, the RPP against the second exposure was adversely affected and declined to 61%. These findings have application to the control of L. salmonae within aquaculture, in that it may be possible to expose hatchery stocks of susceptible salmonid species to spores of L. salmonae when hatchery water temperature is at 7 °C. At this temperature, the risks of disease stemming from this primary exposure appear minimal, since xenomas fail to form. However, the degree of protection appears promising, and may be sufficient to protect fish from spore exposure occurring at netpen marine sites where the parasite may be endemic.     

Effects of chronic and periodic exposures to ammonia on the eye health in juvenile Atlantic halibut (Hippoglossus hippoglossus)

The effects of chronic and periodic peaks of un-ionised ammonia (UIA-N) exposure on eye health and cataract formation in juvenile Atlantic halibut, Hippoglossus hippoglossus, were examined. Fish with mean initial weight 51.7 g (SD 13.2) were exposed to five treatments consisting of control group and three groups (ChronicLow, ChronicMedium and ChronicHigh,) chronically exposed with UIA-N of 0.06, 0.12 to 0.17 mg/l, respectively, for 62 days at 11.9°C, pH 8.0 and salinity 34‰. Furthermore, a fifth group (HighPulse) was exposed to the same high levels as ChronicHigh for a short daily period (peak of 15 mg/l 30 min after exposure, 10 mg/l 1 h after exposure and 1.2 mg/l 3 h after exposure). In the subsequent period of the experimental study (from day 63 until day 100), no ammonia was added to the water. Mean weights were significantly lower in groups exposed to chronically high ambient ammonia concentrations compared to corresponding control group throughout the experimental period. The sampled fish exhibited signs of mild cataract formation, although the results showed no clear evidence that the ammonia treatments contributed to differences. Minor differences were found in measured muscle free amino acids, which could be used to explain potential changes in buffering capacity. The eye histidine status differed significantly at day 62, and osmotic differences in the eye lenses (measured as differences in N-acetyl histidine) were found in all group exposed to chronic levels of ammonia.     

Temperature and dissolved oxygen influence growth and digestive enzyme activities of yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

A 5 week experiment was carried out with juvenile yellowtail kingfish Seriola lalandi to investigate the interactive effects of water temperature (21, 24, or 27°C) and dissolved oxygen regime (normoxic vs. hypoxic) on the growth rate, feed intake and digestive enzyme activity of this species. Specific growth rate (SGR) was highest at 24°C, regardless of oxygen regime, but the SGRs of the fish exposed to hypoxia at 21, 24 and 27°C were 13%, 20% and 17% lower, respectively, than the SGRs recorded for the fish reared under normoxic conditions. The digestive enzyme activities (i.e. trypsin, lipase and α‐amylase) were influenced by temperature but did not appear to be affected by dissolved oxygen concentration. Information about the effects of water temperature and dissolved oxygen on feeding, growth and digestive capacity of juvenile yellowtail kingfish could contribute to improving feed management decisions for production of this fish species under different environmental conditions.     

Energy balance of juvenile <Emphasis Type="Italic">Cyprinus carpio</Emphasis> after a short-term exposure to sublethal water-borne cadmium

Stress in fish can be assessed by means of a bioenergetic approach, based on the evaluation of changes in their physiological parameters. The objective of this study was to determine the impact of sublethal water-borne cadmium (Cd) on the energetic balance of juvenile Cyprinus carpio under laboratory conditions after a short-term exposure. Fish were exposed to a concentration of Cd (0.15 mg Cd l⁻¹) for 2 weeks. This concentration is environmentally realistic since it is usually found, even at higher values, in heavily polluted periurban water bodies of Argentina. No mortality was recorded among the animals used in the experiments. Food intake, food assimilation and assimilation efficiency, fecal production, liver glycogen content, oxygen consumption, oxygen extraction efficiency, specific metabolic rate, ammonia excretion and ammonia quotient (AQ), condition factor, and liver somatic index were determined. The overall balance was expressed as the scope for growth (SFG). The morphological indices and the liver glycogen content of Cd-exposed fish showed no significant differences when compared to those of controls. There was a significant decrease in the food intake, fecal production, and food assimilation rates as well as in AQ; the SFG exhibited a highly significant decrease. The remaining parameters (assimilation efficiency, oxygen consumption, oxygen extraction efficiency, specific metabolic rate, and ammonia excretion) increased after the exposure to Cd. We concluded that the sub-chronic exposure of Cyprinus carpio to a sublethal concentration of Cd causes important alterations in the energy-related homeostasis of fish. Most of the responses are indicative of physiological adaptations to compensate an increased energy requirement due to the impairments caused by the metal.     

Are brown trout Salmo trutta fario and rainbow trout Oncorhynchus mykiss two of a kind? A comparative study of salmonids to temperature‐influenced Tetracapsuloides bryosalmonae infection

Proliferative kidney disease (PKD) of salmonids caused by Tetracapsuloides bryosalmonae causes high mortalities of wild brown trout (Salmo trutta fario) and farmed rainbow trout (Oncorhynchus mykiss) at elevated water temperatures. Here the aim was to compare the temperature‐dependent modulation of T. bryosalmonae in the two salmonid host species, which display different temperature optima. We used a novel experimental set‐up in which we exposed brown trout and rainbow trout to an identical quantified low concentration of T. bryosalmonae for a short time period (1 hr). We followed the development of the parasite in the fish hosts for 70 days. PKD prevalence and parasite kinetics were assessed using qPCR. Exposures were performed at temperatures (12°C and 15°C) that reflect an environmental scenario that may occur in the natural habitat of salmonids. T. bryosalmonae infection was confirmed earliest in brown trout kept at 15°C (day 7 post‐exposure) while, in all other groups, T. bryosalmonae was not confirmed until day 15 post‐exposure. Moreover, significantly greater infection prevalence and a faster increase of parasite intensity were observed in brown trout kept at 15°C than in all other groups. These results indicate that PKD is differentially modulated by water temperature in related host species.     

Petroleum – induced free radical toxicity in African catfish (Clarias gariepinus)

The reaction products derived from lipid peroxidation, as well as superoxide dismutase and catalase enzyme activities were measured in tissues of catfish (Clarias gariepinus) exposed to oil in water dispersions of 0%, 0.5%, 1.0% and 1.5% for 0, 7, 14, 21 and 28 days. Lipid peroxidation significantly (P<0.05) increased in various tissues relative to control values after 14, 21, and 28 days of exposure to 1.0% and 1.5% oil in water dispersions. Catalase and superoxide dismutase activities decreased after 7 days of exposure but significantly (P<0.05) increased after 14, 21 and 28 days of exposure to 1.0% and 1.5% oil in water dispersions. The elevated levels of lipid peroxidation, superoxide dismutase and catalase activities in all tissues examined in C. gariepinus could be a reflection of oxidative stress on the fish. These observations seem to implicate that petroleum hydrocarbons act as a mediator in free radical generation. The increase in catalase and superoxide dismutase activities in these tissues could be an adaptive response to protect the fish from petroleum-hydrocarbon-induced free radical toxicity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of acid water on oxygen consumption,circulating catecholamines and blood ionic and acid-base status in rainbow trout (Salmo gairdneri,Richardson)

Chronically cannulated rainbow trout were exposed in acid water (pH 4.0) for 72h. The gill potential was strongly dependent on water pH, being blood side negative in neutral water, but positive in acid water. Catecholamine levels increased irregularly during acid exposure, and the Bohr and Root effects were not completely erased by the effect of catecholamines during acid exposure. Long term exposure to low water pH, although causing an acidosis in the fish, did not suppress resting oxygen consumption. Prolonged exposure to acid conditions, however, resulted in an increase in ammonia excretion. Changes in plasma sodium and chloride were similar to that reported previously for trout exposed to low calcium, acid water. We conclude that exposure of trout to pH 4 soft water, although impairing oxygen transport, does not limit resting oxygen consumption but does reduce the scope for activity. More extreme acid conditions do impair resting oxygen uptake.     

A behavioural sensor for fish stress

Due to water turbidity, fish stress might be difficult to observe. Evaluation of fish stress by blood sampling requires removing a fish from the water, which is in itself a stressful event. Therefore, we designed and built a sensor to detect fish behaviour that reflects stress. The electronic sensor detected early signs of fish stress by scoring the fish's inactivity. LEDs and detectors are embedded on a steel wand that is held underwater by an operator. In this preliminary (feasibility) study, the new sensor was validated for Tilapia (Cichlidae) and Hybrid Striped Bass (Morone). We induced stressful situations in the fish tanks by manipulating oxygen and temperature levels.ResultsLowering the temperature and oxygen levels both significantly increased the average number of signals identified by the sensor, which indicate stress. The effect of reducing water temperature from 24 °C to 15 °C was three times stronger than was the effect of lowering the oxygen saturation level from 85% to 50%. The difference in the number of signals between the good and stressful conditions was statistically significant, amounting to approximately eight sensor signals, 10.57 compared to 2.49 respectively. Lowering the temperature increased the mean number of signals by 5.85 and 6.06 at 85% and 50% oxygen saturation respectively, whereas lowering oxygen levels increased the mean number of signals by 2.02 and 2.23 at 24 °C and 15 °C, respectively. The results indicate that the stress status of cultured fish can be evaluated using the proposed behavioural sensor. The new sensor may provide an earlier indication of a problem in a fish tank or pond than was heretofore possible. This early warning can enable the fish farmer to take action before many fish are harmed.