DIAGNOSING THE CAUSE OF FISH POISONING

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DIAGNOSING THE CAUSE OF FISH POISONING

The diagnosis of fish poisoning is a difficult and complicated task because there may be a delay in the discovery of the mortality, and the fish and water are then not sampled at the time when the pollution occurred. In such cases the patho-anatomic changes in the fish are obscured by the onset of post-mortem changes and the toxic conditions that caused the fish to be poisoned may have been carried away from the affected area with the water flow or, in the case of natural events, reverted to normal. Hence, it is necessary to use all the available information and all possible and relevant analytical methods to detect the cause of the harm to fish and, where appropriate, to aquatic invertebrates.

The analytical study should begin with an assessment of previous records of factors that might influence natural changes, and of recent discharges that may have been made, and then performing the necessary physico-chemical and hydrobiological analyses of the water. If necessary, the bottom sediments, the periphytes and then the fish themselves should be examined. Bioassays to measure whether the water has an acute toxicity is an important tool in the diagnosis of fish poisoning.

امنیت زیستی دامپزشکی

Examination in situ

If the fish are observed to be behaving strangely or are dying, the following important actions should be taken at the site.

(a) Define the area within which the fish are seen to die or change their behaviour.

(b) Catch some of the affected or newly dead fish and submit them to veterinary examination as soon as possible. Recognized procedures should be used for their storage and/or preservation.

(c) Record the status of the zooplankton, phytoplankton and benthic communities.

(d) Take water samples for hydrochemical analysis (some of the analyses and measurements must be carried out at the site e.g. O2, temperature, pH, transparency, smell, colour etc), and for the bioassay for acute toxicity.

(e) Draw a map of the affected area (and, if appropriate the surrounding areas) and record the site where water and sediment samples were taken. Fill in a form (an example is given below) for the in-situ examination.

If it is suspected that the fish might have died or changed their behaviour because of a discharge or use of a chemical in the vicinity of the affected water course or pond or lake, detailed information should be obtained of the time and method of the use and disposal, and of the identification and amount of the chemical applied.

If the fish are thought to have been killed by the use of a chemical in the vicinity of a water course or reservoir (e.g. a pesticide), the following data should also be included in the report of the site investigation:

  1. Day and hour when a crop was treated
  2. Chemical used (trade name, name and content of active ingredient, application rate, concentration)
  3. Method of application
  4. Identification at the land and type of crop treated, the area of the crop treated, distance from watercourse or reservoir, location of drainage ditches
  5. Name and address of the employer of the person who carried out the treatment
  6. Prevailing weather when the field was treated (wind direction, wind speed, rain, cloudiness) and rainfall records for the period between the application and the fish deaths.
  7. Map: A sketch map of the location, indicating the area where fish had died and the places where water samples were taken.

Hydrochemical examinatio  

The choice of the right sampling sites and the correct water sampling method is the main prerequisite for a successful diagnosis, so this must be given maximum attention. In flowing waters, water sampling sites should be distributed as follows:

(a) at the place of the incident where the fish died, or are dying, or exhibit a strange behaviour

(b) upstream of the above location:

-50 to 100 m downstream of a possible pollution source(s)

– at the place where the pollution source joins a watercourse stocked with fish

– 30 to 50 m upstream of the possible pollution source

(c) downstream of the place of the incident: at sites where the first signs of an unusual behaviour of the fish are observed. Sampling sites downstream of the place of the incident have to be determined by calculation if there has been a delay in arrival at the site: the front of the polluted area can be calculated from the time of discharge of the possible pollutant and the flow rate of the watercourse.

Fig. 6: Hrbáček’s displacement-type bottle for water sampling to determine the concentration of dissolved oxygen (Hrbáček et al. 1974)

Water samples

  1. In reservoirs and fish ponds, the places where water should be sampled should be located on the basis of each specific situation; in some cases the samples may have to be taken from different depths within the water column. Special sampling equipment (e.g. the Hrbáček displacement bottle, Fig. 6) are used to take water samples from different depths and from just above the bottom.

    The samples are poured into clean 1- to 2-litre bottles. It is not recommended that bottles are filled with water taken from near the bank or shore; it is usual to take the samples from 1–2 m away from the water’s edge. Water samples from near to the bottom must be taken with care in order not to include mud and other sediment. To obtain the maximum value in terms of analytical accuracy and usefulness of the data, the time between sampling and analysis must be as short as possible. Ideally, the samples should be transported in thermally insulated containers. In the laboratory the samples should be stored in refrigerators and kept at 3–4°C. However, these procedures may not be adequate to ensure the stability of some water parameters. In such cases, the samples must be analyzed as soon as they are collected, or they may be stabilized with a small amount of preservative. Detailed data on the preservation and treatment of the samples are given in Table 3.

The basic chemical analysis of the water includes the determination of the colour, odour, pH, acid capacity (alkalinity), concentration of dissolved oxygen, chemical oxygen demand (COD), biochemical oxygen demand (BOD5), ammonia, nitrites, nitrates, phosphates and total phosphorus. The need to analyze for any other chemicals depends

on the outcome of the local investigation into possible sources of pollution; the aim is to obtain chemical data which, together with ecotoxicological data, will identify the causes of the mortality or damage of the fish. When assessing the results of the physico-chemical analysis of water samples in order to identify causes of mortality, the parameters should not be evaluated in isolation; possible interactions have to be taken into account. The toxicity of the different chemicals and products to fish and aquatic invertebrates is influenced by the natural quality characteristics of the aquatic environment.

  Table 3: Maintenance and treatment of water samplesDIAGNOSING THE CAUSE OF FISH POISONING

Chemical examination of the water is carried out on site during the field investigation and in the laboratory. For field analyses, a portable chemical laboratory, such as Hach or the Combi kit, produced by the Central Laboratories of the Fish Culture and Hydrobiology Research institute at Vodňany, Czech Republic, can be used. The Combi kit can be used for the following determinations: Secchi disc water transparency, concentration of dissolved oxygen, pH, alkalinity, ammonia and phosphates.

Concentrations of metals in the water are measured by the atomic absorption spectrophotometry (AAS). The gas and liquid chromatography methods are used for the determination of organic compounds, e.g. the active ingredients of pesticides, surfactants, organic dyes, PCBs.

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آدرس :

خیابان آزادی، خیابان حبیب اللهی، خیابان قاسمی، کوچه گلستان، بن بست گل، پلاک ۲، طبقه اول، واحد ۱۱