Measuring the in situ tilt orientation of fish and zooplankton using stereo photogrammetric methods

Acoustic‐derived estimates of fish and zooplankton numerical and biomass density rely on knowledge of the sound scattered by an individual, known as target strength (TS). An important, but difficult to measure, factor in determining TS is the tilt orientation of animals in relation to the insonifying acoustic wave. Underwater stereo‐camera systems can provide in situ tilt measurements for fish and zooplankton. We describe a protocol to remove the effects of camera pitch and roll from tilt measurements using a low‐cost orientation sensor and a mathematical correction. A tank experiment indicated that tilt estimates made with corrected pitch and roll position data improved measurement accuracy as compared to those made with uncorrected position data. This experiment also provided empirical limits on the range of yaw values that allowed for accurate estimation of target tilt and length. We further tested this protocol using an in situ collection of krill stereo images collected in the Gulf of Alaska. Krill tilts calculated with pitch‐ and roll‐corrected position data were an average of 10° different than those collected with uncorrected position data. Our pitch‐ and roll‐correction improved the accuracy of stereo photogrammetric tilt estimation. Extending this method to appropriate spatial and temporal scales could allow for more accurate parametrization of TS models.