Description: Thermographic imaging has been shown to reliably detect marine mammals, both day and night, for operational mitigation in polar and subpolar waters (Zitterbart et al., 2013), with encouraging findings having recently been reported for temperate waters (NOAA Southwest Fisheries Science Center, 2015). As thermographic imaging is based on thermal contrast between whale body or blow and the sea surface, thermographic discriminability is expected to wane with increasing sea surface temperatures. ETAW explored the upper sea surface temperature limit of this approach by deploying high-end thermal cameras on North Stradbroke Island, Queensland, Australia (subtropical conditions) and on both the North and South shore of Kauai, HI, USA (tropical conditions). The study design included both acquisition of thermographic video as well as concurrent visual sightings, including double-blind setups. Our findings show, that cues of humpback whales are thermally discriminable even under the highest sea surface temperatures encountered (26°C / 79°F). Thermal discriminability and performance of computer based automatic detection of cues, appear, as based on the (subjective) experience gained in the field, to be more dependent on covariates such as camera height, sea-state and glare than on sea surface temperature. While increasing sea surface temperatures appear to reduce the operational radius of thermographic images (yet not below typical mitigation radii of 1-3 km), they did not render cues entirely indistinguishable from the background image, while glare and increased sea states caused increased numbers of false alerts. Comparisons of different IR technologies (scanning LWIR 8 – 12µm, focal plane array LWIR 8.0 – 9.4µm, focal plane array MWIR 3.7 – 5.5µm) suggest that the broadband LWIR sensor produced the clearest image least affected by glare. Tests of polarization filters in different orientations revealed that glare is somewhat, but not significantly reduced, for horizontal polarization orientation, but that benefits are outweighed by image degeneration due to the additional optics, at least for the high temperature resolution required in this application.