Intertidal mudflats are important nursery grounds for juveniles of many fish species. However, they are being used increasingly to farm bivalve molluscs, which produce large amounts of organic "fluff", overlying the mud. Fish such as sole, Solea solea, hide in this fluff from potential predators, but the energy consumed by respiring the fluff may be high due to its biorheological properties. We developed an ichthyoviscometer. It incorporates a freshly killed fish as a viscometer, and we developed it to measure the rheological properties of fluids and suspensions, including fluff, at scales encompassing those in gill ventilation. We have shown that the rheological behaviour of fluff is close to that of a gel with a yield stress strongly dependent on particulate organic matter concentration ([POM]). This has allowed us to model fluff flow through the gill channels in living sole as a function of fish size and [POM], showing that in a 26-g sole, fluff would halve flow at a [POM] value of 3.2 g l(-1), and stop it at 3.4.