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What is the shocking truth about surveying fish populations?

Electrofishing to survey fish populations

Fisheries officers stunning fish on the river Cam as part of a fish survey programme

Fisheries officers stunning fish on the river Cam as part of a fish survey programme. Note the generator, transformer and probe delivering the current to the water.

Where netting would be ineffective, electrofishing can be used to survey fish populations and to carry out fish rescues (e.g. during pollution incidents). Electrofishing involves putting an electric current into the water to temporarily disable the swimming muscles of fish. It is usually carried out from a boat. Larger fish are stunned more effectively than smaller ones. After a fish is stunned it is removed from the water with a net and transferred to a tank of water to recover, then returned unharmed to the river. A fish population survey involves counting fish, identifying species, measuring lengths to obtain a biomass estimate, and taking scales to determine age, growth and survival.