In the recent times environmental resources (soil, water and ambient air quality) are threatened due to pollution resulting from anthropogenic activities 123456. The potability and domestic utilization of water resources especially by humans have been widely studied 7891011. Surface water exists as freshwater, brackish and marine water. They are basically classified based on their physicochemical characteristics especially salinity, conductivity, hardness etc. Each of these surface water resources have their unique features they possess. This surface water is the major recipients of wastes streams resulting from human activities. Studies have indicated that surface water such as Epie creek, receives different kinds of wastes resulting from anthropogenic activities which has led to changes in the physical, chemical and biological characteristics of the surface water of the creek 1213. Sewage is also discharged into the aquatic ecosystem through direct means especially in several coastal communities in Bayelsa state 1415. After rainfall, runoff carries several materials and deposits them in the aquatic ecosystem.

Instances of substances recalcitrant to degradation (such as heavy metals) has been reported in surface water system in the Niger Delta 216. Some of these toxic substances have the tendency to bioaccumulate in fishes 1718192021. Typically, fishes are among the common protein source, and harvesting from the wild is a source of livelihood to indigenous people in many coastal regions of Bayelsa state, Nigeria.

Pesticides are substances that are used for controlling pest in homes and agricultural field. In the recent times, pesticides use is on the increasing trend. As such, pesticides are among the important pollutants of the aquatic ecosystem. Despite their roles in controlling pests in the environment, many of them pose a great problem to the environment probably due to their toxicity to animals and human. Their toxicity level depends on their chemical compositions and as well as the immune system and other metabolic pathways of the target organisms.

Paraquat, a non-selective, quaternary nitrogen based herbicide is use in controlling weeds 2223. Paraquat dichloride act as a defoliant in plant tissues 2223. Paraquat dichloride kills herbs through contact processes. Apart from the toxicity in plants, its effects in human have been reported in literature. For instance, Ariyu et al. 24 reported that paraquat dichloride enter the body through inhalation and ingestion processes, and damaged skin contact and its detrimental to humans because it has the capacity to cause multi-organ dysfunctioning and respiratory problems.

The remains of cans of herbicides if not properly discarded could end up in the water ecosystem especially in coastal region where most of their municipal waste are dumped in aquatic ecosystem/ and or close to surface water resources. In addition, after rainfall, runoff could wash herbicides used in agricultural field close to surface water resources were they could impact on the receiving water quality.

Fish are sensitive to aquatic pollutants, and as such they have been widely used to ascertain the effects of contaminants on aquatic ecosystem. Many herbicides have the tendency to cause dysfunctioning in the reproductive, food conversion efficiency, growth and mortality rates in fishes. Fishes have been widely used as bio-indicator in assessing pollution level in aquatic ecosystem. Hence this study focused on the behavioural response and acute toxicity of Clariasgariepinus fingerlings exposed to paraquat dichloride.

The water quality characteristics of the aquarium for the various concentrations are presented in Table 1. The pH, temperature, conductivity and salinity of the water ranged from 6.3 – 6.8, 25 - 27 ºC, 63.2 – 131.5 µS/cm and 0.02 – 0.04 ‰, respectively across the various concentrations. The water quality results including salinity and conductivity showed that the water is a freshwater and within the range that support fish life as variously reported in surface water in Bayelsa state 7891011121314. The pH and temperature is within the concentration to support fish life. Slight variation exists among the different concentrations but this could be due to the effect of increased concentration of the toxicant and/ or unstable nature of these parameters.

The Behavioral response of Clariasgariepinusexposed to varying concentration of paraquat dichloride. The fishes showed moderate swimming and opercular movement at lower concentration which decreased as the concentration increased. Body pigmentation, Intermittent swarming and Jerky movement were not observed in the control, but their rate increased as the concentration of the toxicant increased. The rate of surfacing and air gulping increased as the concentration of the toxicant increased. Behavioural response and mortality are some of the parameters used to examine the early toxicity of a toxicant to fishes. Several authors have reported the role of fish in monitoring pollution in aquatic ecosystem 36373839. As such when the fishes are stressed they responds through their behaviours 29. The behavioral changes observed in this study has some similarity with previous reports. Akinsorotan et al. 23 reported behavioral response such as restlessness, erratic swimming, loss of equilibrium, discolouration, and sudden fish death among Oreochromis niloticusfingerlings exposed to paraquat. Ladipo et al. 32 reported that herbicides containing paraquat causes abnormal behaviour in fish: erratic swimming, sudden quick movements and restlessness. The authors further reported that high concentration of the toxicant caused general weakness and made the fish to settle at the bottom of the aquarium. In addition, the findings of this study also align with non-herbicide toxicants such as detergent as reported by Oyoroko and Ogamba 29, Ogundiran et al. 36, Chandanshive 3738, Ndome et al. 39. The behavioural characteristics observed in this study suggests toxicological stress on the test organisms.

The percentage mortality of varying concentration of paraquat dichloride exposed to the fingerlings of Clariasgariepinus is presented in Table 2. The mortality rate at 0.00ppm, 16.56 ppm, 22.08 ppm, 27.60 ppm, 33.12ppm and 38.64 ppm was 0.00 %, 23.33%, 43.33%, 50.00%, 56.67% and 76.67%, respectively at 96 hours; 0.00 %, 20.00%, 26.67%, 33.33%, 36.67% and 46.67%, respectively at 72 hours; 0.00 %, 13.33%, 20.00%, 23.33%, 26.67% and 36.67%, respectively at 48 hours; and 0.00 %, 10.00%, 13.33%, 16.67%, 20.00% and 26.67%, respectively at 24 hours. Statistically, there was significant variation (p<0.05) in mortality rate. As the concentration of the paraquat dichloride increased mortality rate of the fishes increased. In addition, the mortality rate also increased as the exposure period increased apart from the control. This trend has been reported by Ladipo et al. 32, Oyoroko and Ogamba 28, Seiyaboh and Izah 40, Aghoghovwia and Izah 2235. In fingerlings of Heterobranchusbidorsalisexposed to paraquat dichloride Aghoghovwia and Izah 22 reported percentage mortality of 0.00 %, 23.33%, 43.33%, 66.67%, 80.00% and 93.33% at 0.00ppm, 15.00ppm, 18.00ppm, 20.00ppm, 22.00ppm, and 24.00ppm respectively.

Presents LC₅₀ values of fingerlings of Clarias gariepinus exposed to paraquat dichloride. The LC₅₀ values at 24, 48, 72 and 96 were 59.95, 47.59, 38.12 and 26.18ppm, respectively (Figure1). In this study there was no mortality observed at 0.00ppm of the toxicant (control) from 0.00 hours to 96 hours. The LC₅₀ values observed in this study is higher than the value of 1.75mg/l at 96 hours of juvenile Clariasgariepinus exposed to paraquat 32. Again the findings of this study is comparable to value of 30.2, 22.72, 20.23 and 18.44 ppm at 24, 48, 72 and 96 hours, respectively in fingerlings of Heterobranchus bidosalis exposed to paraquat reported by Aghoghovwia and Izah 22. Akinsorotan et al. 23 reported that at 96 hours, LC₅₀ value of 40.768 mg/l. The variation could be due to the fishes species, age, length and weight, and biochemical makeup of the fishes as well as the chemical properties of the herbicide (toxicant) 222335. The decline in the LC₅₀ as the exposure period increased suggests the effect of stress 222335. Furthermore, paraquat containing herbicides could cause alteration in cellular and biochemical activities in the fish due to the effect of stress 41.

This study demonstrated that herbicides such as paraquat dichloride are toxic to fingerlings of Clariasgariepinus. The study also showed that the toxicity depends on fish species. The results showed that the mortality rate increased as exposure period and concentration increased. The LC₅₀ values showed that paraquat dichloride is lethal to fingerlings of Clariasgariepinus. Based on the results, Paraquat dichloride users need to properly discard the remains of empty cans to avoid contamination. Also there is need to excise caution when using paraquat dichloride based herbicides close to aquatic ecosystem.