|Aquaculture/Fisheries Center - Extension - Fish Health|
|Collection of Samples|
In order to make a diagnosis and recommend treatment, samples of sick fish and water must be collected for workup at the diagnostic lab. Collection and handling of the sample is the most critical step in obtaining an accurate diagnosis of a fish disease problem. In most cases, only a relatively small percentage of all the fish in a pond are sick at one time and it is imperative that those sick fish are collected and sent to the Diagnostic lab. The best fish sample is sick fish that show obvious signs of disease or are alive but resting in the pond edge. Pond owners should walk around the pond and search for sick fish. It is a tedious job but saves considerable time compared to sending in a poor sample that does not provide a useful diagnosis.
It is frequently difficult to find sick fish even in a pond where many fish are dead. When faced with this situation, pond owners often submit "freshly dead" fish to the diagnostic laboratories. Samples of freshly dead fish may sometimes be useful as long as the fish still has clear eyes, red gills, and shows no signs of decomposition. However, freshly-dead fish are very inferior to live sick fish and changes that occur immediately upon the death of the fish may make accurate disease diagnosis difficult or impossible. When a fish dies, small external parasites may be lost almost immediately, environmental bacteria invade the fish and make it difficult to find disease bacteria, and viruses may die within a few hours. In addition, changes that rapidly occur in most organs make other diagnostic techniques (like histology) nearly impossible.
Another approach to not being able to find a sick fish is to try other sample methods. Random fish can be collected by snagging, seining, or cast nets but these samples will only be useful in cases where a very high percentage of fish in a pond have an active form of the disease. Fish that are caught using baited hooks, or that are netted or snagged while fish feed, are not useful. Only healthy fish feed, so a sample of fish that are feeding is useless. The figures below (links) show advantages and disadvantages of different collection methods.
Along with sick fish, a water sample from the effected pond should always be submitted to the diagnostic lab. This sample should consist of at least 250 ml (about 8 ounces) of water. The water should be collected from the pond away from the pond edge. The sample should be collected in a very clean. Traces of soda, juice, detergents, or other chemicals contaminating maple bottles may lead to inaccurate water test results. It is important to rinse the jar several times in pond water before collecting the sample. Water samples should be shipped along with the sick fish, but the water in which the fish are shipped is unsuitable. The water for analysis must be in a bottle separate from the fish. The container with the water sample should be kept cool and out of direct sunlight. If fish are shipped on ice, put the water sample on ice also. If samples of plants and algae are to be included, but them in their own container (a bag or bottle as appropriate), donít put them in the water sample. The plants may cause major changes to the water sample during shipping.