Trout Need Water Flow

A brown trout waits for food to be delivered – then pounces! from the Cawthron Institute on Vimeo

Trout and native fish need more water than we think.

New research on New Zealand’s famed trout rivers has given scientific credence to a long-held angler suspicion _ that taking water from rivers is bad for trout.

Cawthron Institute project leader Dr John Hayes said “A river acts like a conveyor belt delivering the drifting food to the waiting fish,” . “We’ve shown that as flow declines, the diminished power and transport capacity of a river results in less drifting food. A new computer model that our team developed predicts that this translates to fewer, or more slowly growing, fish.”

Most models used to predict how much water a trout population needs use a 40-y-o Colorado model which uses physical habitat, including water depth and velocity, and substrate composition and cover, to figure how much was fish need, but drifting food availability is not considered.

According to the report the New Zealand model takes into account the difference between how much energy the fish consumes versus how much it expends, when foraging for food. The computer model simulates how water flow dislodges and transports aquatic invertebrates, how trout forage in the current on the drifting prey, and how this can be quantified in the currency of energy to predict fish numbers and growth rates. The model can be used to test hypothetical scenarios of stream-flow and habitat, and also takes into account depletion of the drifting invertebrates as fish eat them.

Read the full report here