Percentage of PIT tagged adult lamprey selecting each stream branch, or moving downstream (black arrow) in the east branch of the Waikawa River. In 2017, 191 lamprey were releases and in 2018, 239 lamprey were released. Numbers in red indicate the mean femtomolar concentrations (10-15 Molar) of Petromyzonol sulphate (PS; ±1 standard error in brackets). The concentration of PS is measured by deploying POCIS sampler.