Pesticides have been found useful in the control of different kind of pest in both homes and agricultural fields 1. Pesticides are often classified based on the target organisms. For instance, the group of pesticides used to control, mitigate or eradicate pests include insecticides, acaricides, herbicides, fungicides, rodenticides, fumigants. According to Inyang et al. 2, pesticides can also be grouped based on the formulation (soluble in solvent or dust-like), origin (synthetic or organic) and mode of action (contact or systemic).

In recent times, the use of herbicides in controlling weeds in agricultural and home fields have increased. This has raised the concern of many environmentalists, possibly due to the toxicity nature of many herbicides to non-targeted organisms. Herbicides just like other pesticides (such as insecticides, acaricides, etc) tend to cause pollution in the environment depending on the concentrations and exposure duration. Over a prolonged period, herbicides could cause an alteration in the biochemical and physiological responses of exposed organisms especially fishes in the aquatic ecosystem and small mammals such as rabbits in the terrestrial environment.

Several herbicides have been studied in different parts of the world. But in Nigeria, some of the commonly used herbicides for controlling weeds in agricultural setting include paraquat dichloride 34, glyphosate 5 and 2,4-D Dimethylamine salt 6. The toxicity of some of these herbicides such as glyphosate and paraquat dichloride have been widely reported in the literature with respect to enzymatic, haematological, histopathological, biochemical, morphological/behavioural responses and mortality rate on fishes.

One of the herbicides that have not been commonly studied and are used for the control of weeds is 2,4-D Dimethylamine salt. Most herbicides that have 2,4-D Dimethylamine salt as the active ingredient is used as a selective, pre-emergent and post-emergent systemic herbicide which diminishes the number of broad leaves weeds and vegetation in agricultural field 6 of both annual and perennial plants.

Most chemical toxicants have the tendency to persist for some time in the environment depending on its nature, concentration and prevailing climatic/ environmental condition. 2,4-D Dimethylamine salt tends to persist in the soil for a long period. According to Inyang et al. 6, 2,4-D Dimethylamine could bioaccumulate in animals (including rabbits, fishes etc) and vegetation which are food source of humans.

Generally, herbicides may find its way into the aquatic ecosystem when they are applied close to surface water resources, and/ or when empty cans of herbicides are discharged into the water bodies directly. Runoff resulting from precipitation are the major methods through which pesticides enter the aquatic ecosystem 3, where they impact on the water quality and some of the associated fauna and flora.

Fish have been widely used to assess the effect of herbicides such as 2,4-D Dimethylamine salt on surface water resources. Toxicants such as herbicides could cause dysfunction in the reproductive, food conversion efficiency, growth, and mortality rates in fishes 3. Hence, this study assessed the mortality rate of Clariasgariepinus fingerlings exposed to 2,4-Dimethylamine salt.

About 0.00mls, 0.15mls, 0.25mls, mls, 0.35 mls, 0.45 mls and 0.55 mls of 2,4-D Dimethylamine salt (720g/L) (equivalent to 720000 mg/L) was pipetted into rectangular aquarium containing 10 litres of water (equivalent to 10000 mg/L). This brings the concentration to 0.00ppm, 10.80ppm, 18.00ppm, 25.20 ppm, 32.40 ppm and 39.60 ppm. Each of the experimental groups contains 10 fish. Each group was carried out in triplicate. The toxicant was renewed after 24 hours until the experiment was terminated at 96 hours. These concentrations were made by using the formula previously described by Inyang et al. 7891011, Aghoghovwia and Izah 45, Akinsorotan et al. 12:

mls x stock solution (mg/L) = aquarium water (ml) x desired concentration (ppm or mg/l).

Figure 1 shows the percentage mortality of Clariasgariepinus fingerlings exposed to 2,4-D Dimethylamine salt. The percentage mortality at 0.00ppm, 10.80 ppm, 18.00 ppm, 25.20 ppm, 32.40 ppm and 39.60 ppm was 0.00±0.00 %, 13.33±3.33%, 23.33±3.33%, 26.67±3.33%, 30.00±0.00% and 33.33±3.33%, respectively at 24 hours, being significantly different at p<0.05. Duncan test statistics showed that there is no significant deviation (p>0.05) between 18.00 ppm, 25.20 ppm and 32.40 ppm concentration of the toxicant. At 48 hours, the percentage mortality at 0.00ppm, 10.80 ppm, 18.00 ppm, 25.20 ppm, 32.40 ppm and 39.60 ppm was 0.00±0.00 %, 23.33±3.33%, 33.33±3.33%, 40.00±5.77%, 46.67±3.33% and 53.33±3.33%, respectively. There were significant discrepancies at p<0.05 across the various concentration of the toxicant. At 72 hours, the percentage mortality at 0.00ppm, 10.80 ppm, 18.00 ppm, 25.20 ppm, 32.40 ppm and 39.60 ppm was 0.00±0.00 %, 33.33±8.82%, 53.33±3.33%, 60.00±5.77%, 63.33±6.67% and 66.67±3.33%, respectively. Apart from 0.00 ppm and 10.80 ppm concentration, there was no significant deviation at p>0.05 among the different concentrations. The percentage mortality at 0.00ppm, 10.80 ppm, 18.00 ppm, 25.20 ppm, 32.40 ppm and 39.60 ppm was and 0.00±0.00 %, 36.67±6.67%, 56.67±3.33%, 63.33±8.82%, 73.33±8.82% and 83.33±8.82%, respectively at 96 hours. Statistically, there was dissimilarity (p<0.05) in the percentage mortality. The result showed that as the concentration of the toxicant increased the mortality rate of the fish also increased. This trend have been variously reported when fish are exposed to toxicants including the work of Aghoghovwia and Izah 45 that exposed Heterobranchusbidorsalis fingerlings to paraquat dichloride and glyphosate; Seiyaboh and Izah 17 that exposed Heterobranchusbidorsalis fingerlings to cassava wastewater; Aghoghovwia et al. 3 that exposed Clariasgariepinus to paraquat dichloride; Aghoghovwia et al. 15 that exposed Oreochromis niloticus and Clariasgariepinus to palm oil mill effluents; Akinsorotan et al. 12, Ladipo et al. 18, Ariyo et al. 19 that exposed Oreochromis niloticus, Clariasgariepinus and Labeorohitato paraquat dichloride; Ayoola 20, Nwani et al. 21 that exposed Oreochromis niloticus juvenile and Tilapia zillito glyphosate; Nwani et al. 22 that exposed Channa punctatus to Carbosulfan, Glyphosate and Atrazine; Ojesanmi et al. 23 that exposed Clariasgariepinus fingerlings to 2, 3- dichlorovinyl dimethyl Phosphate; and Oyoroko and Ogamba 13 that exposed Heterobranchusbidorsalis and Clariasgariepinus fingerlings to detergent containing linear alkyl benzene sulphonate. The mortality is often proceeded by behavioural changes which are characterized by moderate swimming and opercular movement, body pigmentation, Intermittent swarming and Jerky movement, increased surfacing and air gulping 3131415.

Figure 2, Figure 3, Figure 4, Figure 5 shows the LC₅₀ values of Clariasgariepinus fingerlings exposed to 2,4-D Dimethylamine salt. The LC₅₀ values at 24, 48, 72 and 96 were 86.15 ppm, 36.28 ppm, 18.72 ppm and 15.68 ppm, respectively. The LC₅₀ values decreased as the concentration of the toxicant increased. This indicates that as the concentration of 2,4-D Dimethylamine salt increased, its toxicity to fish increased. This trend is in line with the findings of authors when different fishes are exposed to different toxicants 345. Though, slight variation exists in the LC₅₀ values. Authors have attributed the variation to fish species, age, size and biochemical makeup of the fishes as well as the type of the toxicants 345. As the acute toxicity duration increases, it could alter the cellular and biochemical processes in fish due to possible changes in enzymatic, haematological, histopathological, physiological and metabolic processes due to hassle and stress.

In recent times, the rate of pesticides use especially herbicides have increased. Different brands of herbicides are marketed in Nigeria. This study evaluated the acute toxicity of 2,4-D Dimethylamine to Clariasgariepinusfingerlings. The study found that the mortality rate decreased as the concentration of the toxicant and exposure increased. LC₅₀ values of 15.68 ppm after 96 hours is an indication that the 2,4-D Dimethylamine salt is toxic to fingerlings of Clariasgariepinus. As such, there is the need to exercise caution in the use of 2,4-D Dimethylamine salt based herbicides for the control of grasses especially in the coastal region where surface water resources abound.