Fish-kill reporting
- What is a fish kill?
- What are common causes of fish kills?
- Reporting fish kills
- Taking samples
- How does DERM investigate a fish kill?
What is a fish kill?
A ‘fish kill’ is a significant and sudden death of fish or other aquatic animals, such as crabs or prawns. Such events are characterised by large numbers of aquatic animals dying over a short time, usually in a clearly defined area.
Some observations and reports of fish kill incidents, particularly where small numbers are involved (fewer than 50 or 10 kilograms in total), are found to be discarded fishing catches. These ‘false fish kills’ are often characterised by the presence of net or hook injuries.
What are common causes of fish kills?
Most fish and many other aquatic animals rely on being able to use their gills to ‘breathe’ oxygen dissolved in the water that surrounds them. A lack of dissolved oxygen in the water is one of the most common causes of fish kills, either caused by natural circumstances or associated with pollution by organic matter. Sometimes fish are poisoned by chemicals, such as pesticides. A description of all the likely causes of fish kills is provided in the Fish kill reporting and investigation manual (PDF, 350K)*.
Reporting fish kills
If you see a fish kill, call the Department of Environment and Resource Management.
Note: If a fish kill is not reported promptly, it is less likely that an investigation into the cause will be practical since possible causes, such as contaminated water, will have time to disperse.
What information should be reported?
When reporting fish-kills, record as much of the following information as possible:
- your name, address and contact number in case further information is required by the investigating scientist
- the exact location of the fish-kill and an estimate of the area affected
- the date and time of discovery and an estimate of when the fish-kill might have happened
- weather conditions at the time of discovery and for the 24 hours previously
- an estimate of the number and size of fish affected and the names or types of fish or other animals, such as crabs, involved
- whether sick or dying fish are also present and, if so, how they are behaving
- whether unaffected fish are also present. If only dead specimens are present, their state of decay and whether some are less decayed than others
- whether any unusual or abnormal materials were present, such as oil slicks, discoloured water, recently dumped rubbish
- whether any samples of dead fish, affected water or other materials have been taken, and where they are being kept
- any other factual information that could be relevant, such as industries or agricultural activities in the vicinity of the fish-kill
Taking samples
In addition to promptly reporting fish kills, what else can the public do to assist?
To provide further assistance to a subsequent investigation, take an on-the-spot sample as dead fish deteriorate and contaminated water flows away quickly.
What samples are useful for members of the public to obtain?
- both dead and dying fish (and any other animals affected)
- sediments (mud or sand) from the water where dead fish are found
- water
- any materials such as oil slicks or other foreign matter in the water.
How should samples be stored?
Clean containers should be used to store samples. Glass jars or bottles are best, but plastic may be used if glass is not available. Plastic bags are acceptable for dead fish. Large (1–2 litre) soft drink bottles are ideal for storing water samples. Anything smaller than these sizes is of limited value for chemical analysis. Bottles and jars should be pre-cleaned with hot water and detergent, then rinsed several times in the water being sampled before a sample is taken.
If possible, several samples of each kind should be taken, for example at least three fish, three sediment samples and three bottles of water. If a discharge or drain site is suspected as a source of contamination, samples should be taken both upstream and downstream of this, and clearly labelled with a waterproof pen or similar means.
All samples should be preserved by refrigeration or kept on ice. If the area is remote and/or collection of samples by an investigator is unlikely for more than 24 hours, samples should be kept in a deep freeze. DO NOT freeze water samples without leaving an airspace of about 20 per cent of the volume to allow for expansion.
How does DERM investigate a fish kill?
The steps taken in a science-based fish kill investigation are described in the Fish kill reporting and investigation manual (PDF, 350K)* published in 1998. It provides a standard science-based protocol for responding to reported fish kill incidents.
* Requires Adobe Reader
Last updated 11 February 2011