Coral reefs are undergoing changes caused by coastal development, resource use, and climate change. The extent and rate of reef change demand robust and spatially explicit monitoring to support management and conservation decision-making. We developed and demonstrated an airborne-assisted approach to design and upscale field surveys of reef fish over an ecologically complex reef ecosystem along Hawai‘i Island. We also determined the minimal set of mapped variables, mapped reef strata, and field survey sites needed to meet three goals: (i) increase field survey efficiency, (ii) reduce field sampling costs, and (iii) ensure field sampling is geostatistically robust for upscaling to regional estimates of reef fish composition. Variability in reef habitat was best described by a combination of water depth, live coral and macroalgal cover, fine-scale reef rugosity, reef curvature, and latitude as a proxy for a regional climate-ecosystem gradient. In combination, these factors yielded 18 distinct reef habitats, or strata, throughout the study region, which subsequently required 117 field survey sites to quantify fish diversity and biomass with minimal uncertainty. The distribution of field sites was proportional to stratum size and the variation in benthic habitat properties within each stratum. Upscaled maps of reef survey data indicated that fish diversity is spatially more uniform than fish biomass, which was lowest in embayments and near land-based access points. Decreasing the number of field sites from 117 to 45 and 75 sites for diversity and biomass, respectively, resulted in a manageable increase of statistical uncertainty, but would still yield actionable trend data over time for the 60 km reef study region on Hawai‘i Island. Our findings suggest that high-resolution benthic mapping can be combined with stratified-random field sampling to generate spatially explicit estimates of fish diversity and biomass. Future expansions of the methodology can also incorporate temporal shifts in benthic composition to drive continuously evolving fish monitoring for sampling and upscaling. Doing so reduces field-based labor and costs while increasing the geostatistical power and ecological representativeness of field work.

Increases in sea surface temperature impact animal metabolism, which in turn could influence benthic structure and resulting algal-coral balance. We utilized a long-term coral reef dataset from the west coast of Hawai‘i Island to investigate impacts of annual positive and negative sea surface temperature anomalies (SSTA) on benthic cover [algal turf, macroalgae, crustose coralline algae (CCA), and coral], herbivore density (sea urchins, grazers, browsers, and scrapers) and the relationship between benthic cover and herbivore density. Results showed significantly lower coral cover, but higher CCA cover with positive SSTA. Additionally, the density of sea urchins, grazers and browsers increased with increasing SSTA. Warming disrupted the normal relationship between herbivores and benthic cover on reefs, particularly for grazers where higher densities were coupled with lower algal turf cover only during negative SSTA. The direction of the relationship between benthic cover and herbivore type changed with positive SSTA, where increased algal turf cover was associated with increased herbivore density. Here, herbivores are likely responding accordingly to increases in food availability due to increased metabolism under warming. Despite herbivore populations increasing in density over the past two decades, algal turf cover remains on an upward trajectory. These results indicate that warming can alter herbivore-algal dynamics, where greater herbivore densities may be required to cause a reduction in algal turf cover. Protection of herbivores in addition to reducing nutrient input onto reefs will be essential in driving a reduction in algal turf cover on Hawaiian reefs.

Coral reef ecosystems are being fundamentally restructured by local human impacts and climate-driven marine heatwaves that trigger mass coral bleaching and mortality1. Reducing local impacts can increase reef resistance to and recovery from bleaching2. However, resource managers lack clear advice on targeted actions that best support coral reefs under climate change3 and sector-based governance means most land- and sea-based management efforts remain siloed4. Here we combine surveys of reef change with a unique 20-year time series of land–sea human impacts that encompassed an unprecedented marine heatwave in Hawai‘i. Reefs with increased herbivorous fish populations and reduced land-based impacts, such as wastewater pollution and urban runoff, had positive coral cover trajectories predisturbance. These reefs also experienced a modest reduction in coral mortality following severe heat stress compared to reefs with reduced fish populations and enhanced land-based impacts. Scenario modelling indicated that simultaneously reducing land–sea human impacts results in a three- to sixfold greater probability of a reef having high reef-builder cover four years postdisturbance than if either occurred in isolation. International efforts to protect 30% of Earth’s land and ocean ecosystems by 2030 are underway5. Our results reveal that integrated land–sea management could help achieve coastal ocean conservation goals and provide coral reefs with the best opportunity to persist in our changing climate.

Hawai′i coral reefs are essential ecosystems providing resources in the form of food and recreation as well as stabilizing nearshore biodiversity. The Seattle Aquarium has exhibited Hawai′i reef fishes and corals since the mid-1980s to educate guests about these critical ecosystems. In 2009, and in collaboration with Hawai′i’s Division of Aquatic Resources (DAR) and Washington State University, the aquarium expanded its conservation work in Hawai′i through annual surveying of eight reefs along the west coast of Hawai′i via SCUBA-based diver operated video (DOV). Five of the sites are in areas partially closed to most fishing while three sites are in areas partially open to most fishing. 100-meter DOV surveys took place a meter above a horizontal or vertical reef, and survey locations were marked with GPS and fixed underwater markings to enable annual surveys to occur in the same locations. Counts of fish species were subsequently made from the archived video. Over the 11 year dataset we documented increased total abundance at all sites and periods of increasing and decreasing species richness. Multivariate analyses comparing fish community structure before (2009-2012) and after an anomalous warm-water event (2013-2019) documented a persistent shift in community structure. This coincides with a documented marine heat wave in Hawai′i and associated coral bleaching events between 2013-2016. These results suggest that our long-term monitoring program captured a phase shift in community structure associated with changing environmental conditions. These persistent shifts may thus indicate hysteresis at relatively short temporal scales, and ongoing monitoring is required to observe whether the systems shift back to the pre-2013 community structure. As coral reef ecosystems face a multitude of stressors from warming waters to marine pollution, long-term monitoring programs are essential to illuminate trends that may inform conservation and management strategies to preserve these imperiled ecosystems.

Coastal ecosystems are disproportionally inhabited by global human population. Consequently, human impacts originating from land and sea combine with climate-driven disturbances to fundamentally restructure nearshore marine ecosystems. These coincident human stressors are especially acute in the tropics where population centres concentrate along shorelines and marine heatwaves increasingly trigger mass coral bleaching and mortality. However, despite decades of research, we lack consensus as to whether local management supports coral reef resilience under climate change. Here we combined recurring reef surveys with a unique time series of land-sea human impacts over a 20-year period that encompassed an unprecedented marine heatwave in Hawaiʻi. Reefs with high fish biomass and reduced land-based impacts such as wastewater pollution had positive coral trajectories pre-disturbance and experienced minimal coral loss following severe heat stress. Concurrent land-sea management resulted in a 3- to 7-fold increase in the probability of a reef having high reef-builder cover four years post-disturbance. International efforts to conserve 30% of Earth’s ecosystems are gaining momentum, but do not intrinsically link land and ocean conservation targets. Our findings suggest coupled land-sea policy measures are required to realise global conservation goals and provide coral reefs with the best opportunity for persistence in our changing climate.