Empirical Modelling of Solid-blocking Effect in a Blazka Respirometer for Gag, a Large Demersal Reef Fish

The effect of solid-body blocking in a large (257 l) Blazka-style respirometer was examined in gag Mycteroperca microlepis. Gag ranging from 376-690 mm total length were tested at velocities from 10 to 91 cm s -1 . The solid blocking effect (SBE) due to fish size, swimming speed, and water temperature was investigated by direct measurement of flow velocity at the area of the fish's maximum girth as compared to measurements at the rear (center) of the swimming chamber. Three models were compared as corrections for the SBE: the standard Bell and Terhune (BT), a novel modification of the Bell and Terhune (MBT) developed in this study, and the Bernoulli equation. Although the fit was good, the commonly used BT model consistently underestimated solid-blocking effect over the speeds tested (R 2 adj = 0.980) whereas the MBT and Bernoulli models provided better fits (both with R 2 adj = 0.993). The MBT model, with refined parameters for the fish body shape coefficient and fractional surface area exponent, yielded the best fit for observed data across all speeds for which gag were tested. This study also demonstrated that in Blazka-style respirometers, corrections were needed to account for SBE where fish occupied >10% of the cross-sectional area of the swimming chamber.