What are the causes of copper sulfate-induced fish poisoning?

Copper sulfate is a commonly used drug for treating fish diseases, but because some users are unfamiliar with its properties, it often causes harm.

1 Case reports

Case 1: In March 1987, Liantang Aquatic Farm used copper sulfate at 0.7g/m3 to kill algae in a broodstock pond, and three days later all the broodstock died. Water quality analysis showed an ammonia content of 0.35g/L.

Case 2: From August 30 to 31, 1999, a 0.67hm2 fattened fry pond with a water depth of 1m owned by a farmer in Bayi Township, Nanchang County, due to sultry weather and heavy rain, fish died for two consecutive days, about 10kg per day, mainly silver carp and grass carp fry. On the 31st, because a small number of wheel worms were found during examination, copper sulfate 4kg (equivalent to 0.6g/m3) and crystallized trichlorfon 3kg (equivalent to 0.45g/m3) were applied. On September 1 and 2, fish died over a large area, totaling about 1000kg. Examination of the fish bodies revealed no other symptoms.

Case 3: A fry pond of the Jiangxi Academy of Agricultural Sciences, 0.5hm2, with a water depth of 0.9m, which had not been desilted for many years, had a total fish death of 145kg from August 30 to September 3, 1999, mainly silver carp. Because the water quality was relatively fertile, copper sulfate 8kg (equivalent to 1.5g/m3) was applied at noon on the 3rd to kill algae, and from the 4th to the 5th the total fish death in the whole pond was 580kg. The order of fish deaths was silver carp, common carp, crucian carp, and grass carp, while freshwater pomfret had very few deaths. Upon examination, apart from gill congestion, no other abnormal disease symptoms were seen on the body surface.

Case 4: On September 1, 1999, two adult fish ponds of Wanhu Aquatic Farm in Nanchang County, with a combined area of 0.67hm2 and a water depth of 3m, were treated with copper sulfate 11.5kg (equivalent to 0.6g/m3) to treat dactylogyriasis, and the next day fish deaths reached 500kg. At the scene, we noted and observed that dying fish had noticeably accelerated respiration, and some fish bodies also showed convulsive symptoms. Case 5: On July 25, 1999, Guan Bing, a specialized household from Sanshan Village, Fushan Township, Nanchang County, used copper sulfate 100kg (equivalent to 0.75g/m3) in a contracted enclosed old river channel with a water surface of 6.67hm2 and a water depth of 2m. After the copper sulfate was sprinkled, large numbers of grass carp, common carp, blunt snout bream, and crucian carp were found dead, reaching 1500kg, while no silver carp or bighead carp died. The next day, no further fish deaths occurred.

The copper sulfate used in the above cases all had an effective content of 92-96%. Except for one case, none exceeded the conventional dosage.

2 Causes of poisoning

2.1 Compound poisoning It is mainly caused under conditions of high temperature and hypoxia. Cases 1 and 2 are typical examples. Some domestic scholars once conducted experiments on silver carp and found that within the water temperature range of 16-30°C, for every 10°C increase in temperature, the toxicity of copper sulfate increased by 1.7-1.8 times. Experiments on bighead carp weighing 12g showed that when the temperature rose from 22.9°C to 26.7°C, the safe concentration of copper sulfate dropped from 0.71g/m3 to 0.5g/m3.

[1] The toxic effect of copper sulfate on fish is mainly to destroy the respiratory function of the gills, and under hypoxic conditions its toxicity is even greater.

[2] There are many factors that cause hypoxia in water bodies. When copper sulfate is used to kill algae, it can cause severe hypoxia in the water body, and at the same time also produce toxic substances that poison fish. For example, ammonia has a very strong toxic effect on fish. According to Yuan Baoshan (1999), when the ammonia concentration in water reaches 0.289mg/L, all common carp die.

[3] We also noted and observed at the scene that fish poisoned by copper sulfate floated in large numbers at the water surface before death. Respiration accelerated, and at places where new water was introduced they gathered in groups to "struggle for water," similar to symptoms of hypoxia. It is precisely because of the side effects of high temperature and hypoxia that the toxicity of copper sulfate is greatly increased, so even conventional dosages often cause acute poisoning in fish.

2.2 Overdose poisoning Case 3 is clearly caused by fish poisoning due to excessive use of copper sulfate. According to reports, freshwater pomfret is not sensitive to copper sulfate, and the whole-pond sprinkling concentration can reach 1.5g/m3, while common carp, crucian carp, and silver carp are more sensitive to it, and fry are more sensitive than adult fish

[4]. This is basically consistent with the phenomenon in Case 3. Under laboratory conditions, copper sulfate (calculated as copper ions) against silver carp (weight not calculated) 96LC50=0.062mg/L

[5] (equivalent to copper sulfate 0.25g/m3). In natural water bodies, although organic matter, suspended matter, and calcium, magnesium and other ions can reduce the toxicity of copper sulfate and increase its safe concentration to some extent, the whole-pond sprinkling concentration for common carp, crucian carp, and silver carp should still not exceed 0.7g/m3 [4].

Of course, there are also cases in which high concentrations (above 1g/m3) of copper sulfate have been used to treat fish diseases with good results, such as 1g/m3 to control blue-green algae, 1g/m3 in combination to treat explosive infectious diseases, and under long-term low-temperature rainy conditions, 1-2g/m3 to treat wheel worm disease

[6] and other successful examples. However, all these high-concentration applications were carried out under strictly controlled environmental conditions, and the medication time must be short, while simultaneously increasing oxygen, changing water after medication, and minimizing the stress consumption of the fish.

2.3 Deep-water poisoning As for the cause of fish deaths in Case 4, the author believes that first, the water quality was lean and the bottom mud was scarce, so the water body had a weak capacity to consume the drug. Second, the water depth reached 3m, and there was a thermocline; combined with different densities of the upper and lower water layers, this formed a high drug concentration in the upper layer and a low drug concentration in the lower layer, so even conventional medication can cause fish poisoning in the high-drug layer.

2.4 Ingestion poisoning From Case 5, it can be found that in the water body only feeding fish such as grass carp, blunt snout bream, common carp, and crucian carp died, while filter-feeding fish such as silver carp and bighead carp did not die. Therefore, we confirmed that it was poisoning caused by feeding fish accidentally eating undissolved copper sulfate particles. Foreign reports have also stated that common carp, after ingesting copper at 400mg/kg, all died three days later.

The residue and enrichment of copper is also a factor that cannot be ignored in causing fish poisoning. For example, scallops cultured in water with copper ion concentrations of 0.005mg/L and 0.01mg/L for 45d contained 17.03mg/kg and 30.81mg/kg of copper in their bodies, with enrichment coefficients reaching 2900-3040 [5]. When fish eat this kind of food, it can also cause ingestion poisoning. Ingestion poisoning mainly harms feeding fish.

3 Issues to note when using copper sulfate

Copper sulfate causing fish poisoning is mostly caused by improper use. Therefore, correctly mastering the method of using copper sulfate is particularly important.

3.1 Use the drug flexibly according to water temperature, organic matter and suspended matter content, dissolved oxygen, ph, hardness, as well as climate, stocking, and fish body activity. When mixed with other drugs, the concentration should be reduced. Specifically, the use of copper sulfate can take 0.7g/m3 as the standard, fluctuating above and below. Measure the pond water volume accurately and calculate the dosage accurately. Medication should be applied on a clear early morning when fish are not gasping at the surface. After application, if conditions permit, aeration should be used to increase oxygen to prevent algae death from consuming oxygen and deteriorating water quality.

3.2 When dissolving the drug, metal containers should not be used, and the temperature of the dissolving water should not exceed 60°C, to prevent loss of effectiveness. Under no circumstances should undissolved particles be sprinkled into the water, to prevent poisoning by accidental ingestion. When using copper sulfate in deep water, it should not be sprinkled all at once according to the conventional dosage; it is better to use it in two applications separated by 4-5 hours.

3.3 Because copper residues last a long time and also have adverse effects on fish feeding and growth, it should not be used frequently.