**3. Concluding Remarks**

Current optical monitoring methods use a range of different approaches to answer the overly simplified question of 'Is this coral reef healthy?'. Due to coral reefs being physically and ecologically complex ecosystems, each technique offers a different piece of the puzzle, shedding light on parameters pertaining to overall reef health and status.

The Earth's coral reefs face a difficult and challenging future. If they are to survive the impending onslaught, effective monitoring and assessment will be fundamental to aiding in their recovery. A wealth of technological know-how is already being applied to study coral reefs from the colony to global scale. Consequently, our understanding of the problem and its severity continues to improve. New tools are being developed to help facilitate all survey requirements across all scales. On the larger scale, satellites and aerial multispectral/hyperspectral imaging provide the greatest spatial coverage with the trade-off of reduced spatial resolution. For colony or reef scale surveys, HyRi can produce

high spatial resolution but cannot replace remote systems. Rather, it is a tool to complement these assessments and provide ground truth data to look at environments in closer detail. The use of in situ imagers also enables areas to be studied where aerial imaging is not applicable such as deep reefs or reefs with complex structures (i.e., steep reef faces/walls and overhangs). Aerial based imaging is unsuitable for deep coral studies, but tends to work best in mapping shallow lagoons and reef flats as these are within the depth threshold for this type of imaging (20 m) [96].

In the near future, Bi-Frost DSLRs are predicted to be a powerful new addition in the diagnostic toolbox for coral health. The benefits of enabling rapid non-destructive and repeatable measurements [33] address many of the shortcomings of the current generation of UHI instruments. The 3D data obtained with a Bi-Frost DSLR can be used in many ways, making this single tool capable of recording data for several different diagnostic needs from health to population surveys.

**Author Contributions:** Conceptualization, J.C.C.D. and J.T.; Methodology, J.T.; Software, D.A.M.-S.; Validation, J.C.C.D., J.T. and D.A.M.-S.; Investigation, J.T.; Writing—original draft preparation, J.T.; Writing—review and editing, M.J.A., J.C.C.D., D.A.M.-S. and T.B.S.; Supervision, J.C.C.D. and T.B.S.; Funding acquisition, T.B.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** Pervioli Foundation, Roddenberry Foundation, and Bristol Alumni Foundation.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to acknowledge the editor Rupert Ormond for his guidance and wisdom to better focus the manuscript. We would also like to thank James Alexandroff for his continued support without which this project would not be possible.

**Conflicts of Interest:** The authors declare no conflict of interest.
