Figure 1 Butterflyfish pair swimming by their coral reef ecosystem.
In coral reef ecosystems everythingis interconnected, from the individual corals themselves to all the reef dependant and associated organisms. This includes your Koh Tao locals: Butterflyfish, Bannerfish, Angelfish, Rabbitfish, Damselfish… the list goes on. The balance in this ecosystem is quite delicate; therefore, one’s vulnerability to a changing climate will ultimately affect the other. As a result of coral bleaching alone, coral reef fishes are facing changes in population and recruitment dynamics, connectivity, and overall loss of abundance, diversity, and changes in community composition (Koester et al., 2023). Declines in coral health alter reef food webs, reducing availability of coral associated prey and shifts toward algae dominated reefs.
This leads to a change in diet composition, favouring some species over others, creating less diverse and productive habitats (Munday et al., 2008)This poses a major challenge at individual and population levels, as there are over 4,000 species of fish associated with coral reefs (Allen, 2008). Consequently, as coral numbers decrease, so do settlement habitats, breeding grounds, and overall shelter from predators. Fish populations are predicted to migrate to cooler waters, which can lead to local extinctions and changes in community composition (Hu et al., 2022). Additionally, as fish populations decline, they fail to fulfil relevant ecosystem services such as maintaining reef health through feeding on algae — which heavily competes with corals for space and resources. Loss of these functions reduces reef resilience, accelerating coral decline and creating a negative feedback loop.
Figure 2 Diagram explaining what coral bleaching is, how to detect it and what causes it. Figure extracted from NOOA at https://oceanservice.noaa.gov/facts/coral_bleach.html
Moreover, there is a detrimental effect of elevated sea surface temperatures on fish populations, as elevated temperatures have been linked to physiological stress in fish, increasing metabolism and energy demands, disrupting spawning timing and reproductive success (Mugwanya et al., 2022). This results in decreased growth rates, and an impaired immune system which ultimately increases mortality.
On top of this, ocean acidification — which lowers the ocean pH due to increased uptake of atmospheric CO2 — presents a major challenge in coral conservation. Ocean acidification has been found to decrease coral growth and reef formation, in addition to affecting fish sensory abilities (e.g. smell and hearing) by disrupting the functioning of their nervous system (Cripps, Munday and McCormick, 2011;Rodríguez et al., 2026). This will ultimately have an effect on their ability to navigate and avoid predators, as their ability to follow physical and chemical cues is reduced.
Figure 3 Ocean acidification because of increasing atmospheric CO2. Diagram extracted from Shape of Life at https://www.shapeoflife.org/climate/ocean-acidification
Why is this important?
Climate change not only affects marine organisms, but also human communities. Coral reef ecosystems are a major driver for tourism, coastal protection services and as a food source. Coral reef fish are a primary source of protein for hundreds of millions of people; declines in fish populations will reduce catches, ultimately affecting the world’s economy as food prices increase and tourism decreases, reducing income for local and national economies (Eddy et al., 2021). In addition to their costal protection services from storm and erosion damage, the loss of coral reefs and their associated organisms play an important role in several cultures. Coastal and island communities in Thailand are strongly linked to coral reefs culturally, economically, and nutritionally, making climate change impacts on reef fish highly important not only on a global scale but also locally.
What is there to be done?
Despite current research efforts on these complex ecosystems, the extent of these negative effects is yet to be fully understood. Whilst scientists continue to do research on fish responses to a changing climate, it is unlikely that fish populations are going toadapt to the rapid changes in their environment due to human activities. Therefore, to protect coral reef fish, conservation efforts and policies must be implemented to reduce human-induced pressures on marine habitats. There is still much to be done, including promising research into heat-resistant corals naturally found in regions such as the Red Sea, and how these may be incorporated into reefs around the globe. Although this approach presents many challenges, continued conservation efforts, policy making, and research could provide the resilience needed to prevent further ecological, social, and economic impacts resulting from human activities.
Figure 4 Roctopusdive research interns conducting a fish survey on Koh Tao local fish populations.
By: Isabel Vizcaya, EcoDivemaster
Reference list
Allen, G.R. (2008) ‘Conservation hotspots of biodiversity and endemism for Indo‐Pacific coral reef fishes’, Aquatic Conservation: Marine and Freshwater Ecosystems, 18(5), pp. 541–556. Available at: https://doi.org/10.1002/aqc.880.
Cripps, I.L., Munday, P.L. and McCormick, M.I. (2011) ‘Ocean acidification affects prey detection by a predatory reef fish’, PloS One, 6(7), p. e22736. Available at: https://doi.org/10.1371/journal.pone.0022736.
Eddy, T.D., Lam, V.W.Y., Reygondeau, G., Cisneros-Montemayor, A.M., Greer, K., Palomares, M.L.D., Bruno, J.F., Ota, Y. and Cheung, W.W.L. (2021) ‘Global decline in capacity of coral reefs to provide ecosystem services’, One Earth, 4(9), pp. 1278–1285. Available at: https://doi.org/10.1016/j.oneear.2021.08.016.
Hu, W., Du, J., Su, S., Tan, H., Yang, W., Ding, L., Dong, P., Yu, W., Zheng, X. and Chen, B. (2022) ‘Effects of climate change in the seas of China: Predicted changes in the distribution of fish species and diversity’, Ecological Indicators, 134, p. 108489. Available at: https://doi.org/10.1016/j.ecolind.2021.108489.
Koester, A., Gordó−Vilaseca, C., Bunbury, N., Ferse, S.C.A., Ford, A., Haupt, P., A’Bear, L., Bielsa, M., Burt, A.J., Letori, J., Mederic, E., Nancy, E., Sanchez, C., Waller, M. and Wild, C. (2023) ‘Impacts of coral bleaching on reef fish abundance, biomass and assemblage structure at remote Aldabra Atoll, Seychelles: insights from two survey methods’, Frontiers in Marine Science, 10, p. 1230717. Available at: https://doi.org/10.3389/fmars.2023.1230717.
Mugwanya, M., Dawood, M.A.O., Kimera, F. and Sewilam, H. (2022) ‘Anthropogenic temperature fluctuations and their effect on aquaculture: A comprehensive review’, Aquaculture and Fisheries, 7(3), pp. 223–243. Available at: https://doi.org/10.1016/j.aaf.2021.12.005.
Munday, P.L., Jones, G.P., Pratchett, M.S. and Williams, A.J. (2008) ‘Climate change and the future for coral reef fishes’, Fish and Fisheries, 9(3), pp. 261–285. Available at: https://doi.org/10.1111/j.1467-2979.2008.00281.x.
Rodríguez, I., García-Pérez, I., Sadeghi, N., Montblanch, M., Gutiérrez, J., Navarro, I., Capilla, E. and Garcia De La Serrana, D. (2026) ‘Impact of ocean acidification on skeletal structures in gilthead sea bream (Sparus aurata): In vitro and in vivo studies’, Aquaculture, 610, p. 742919. Available at: https://doi.org/10.1016/j.aquaculture.2025.742919.
