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Mesophotic coral ecosystems (MCEs) support diverse communities of marine organisms with changes in community structure occurring along a depth gradient. In recent years, MCEs have gained attention due to their depths that provide protection from natural and anthropogenic stressors and their relative stability over evolutionary time periods, yet ecological structures of fish assemblages in MCEs remain largely un-documented. Here, we investigated composition and trophic structure of reef fish assemblages in the Northwestern Hawaiian Islands (NWHI) along a depth gradient from 1 to 67 m. The structure of reef fish assemblages as a whole showed a clear gradient from shallow to mesophotic depths. Fish assemblages at mesophotic depths had higher total densities than those in shallower waters, and were characterized by relatively high densities of planktivores and invertivores and relatively low densities of herbivores. Fishes that typified assemblages at mesophotic depths included six species that are endemic to the Hawaiian Islands. The present study showed that mesophotic reefs in the NWHI support unique assemblages of fish that are characterized by high endemism and relatively high densities of planktivores. Our findings underscore the ecological importance of these undersurveyed ecosystems and warrant further studies of MCEs.

2014 marked the sixth and most widespread mass bleaching event reported in the Northwestern Hawaiian Islands, home to the Papahānaumokuākea Marine National Monument (PMNM), the world's second largest marine reserve. This event was associated with an unusual basin-scale warming in the North Pacific Ocean, with an unprecedented peak intensity of around 20°C-weeks of cumulative heat stress at Lisianksi Island. In situ bleaching surveys and satellite data were used to evaluate the relative importance of potential drivers of bleaching patterns in 2014, assess the subsequent morality and its effects on coral communities and 3D complexity, test for signs of regional acclimation, and investigate long-term change in heat stress in PMNM. Surveys conducted at four island/atoll (French Frigate Shoals, Lisianski Island, Pearl and Hermes Atoll, and Midway Atoll) showed that in 2014, percent bleaching varied considerably between islands/atolls and habitats (back reef/fore reef and depth), and was up to 91% in shallow habitats at Lisianski. The percent bleaching during the 2014 event was best explained by a combination of duration of heat stress measured by Coral Reef Watch's satellite Degree Heating Week, relative community susceptibility (bleaching susceptibility score of each taxon * the taxon's abundance relative to the total number of colonies), depth and region. Mean coral cover at permanent Lisianski monitoring sites decreased by 68% due to severe losses of Montipora dilatata complex, resulting in rapid reductions in habitat complexity. Spatial distribution of the 2014 bleaching was significantly different from the 2002 and 2004 bleaching events likely due to a combination of differences in heat stress and local acclimatization. Historical satellite data demonstrated heat stress in 2014 was unlike any previous event and that the exposure of corals to the bleaching-level heat stress has increased significantly in the northern PMNM since 1982, highlighting the increasing threat of climate change to reefs.

Extreme disturbances such as hurricanes can cause reductions in coral cover and three-dimensional (3D) structural complexity of coral reefs. We examined changes in structural complexity utilizing 3D reconstruction of a coral-reef site before and after Hurricane Walaka passed through Lalo of the Northwestern Hawaiian Islands. This event resulted in complete destruction of the coral-reef habitat, with dramatic changes in benthic cover from pre-hurricane tabulate coral to post-hurricane rubble. Rugosity and mean slope decreased after the hurricane, while structural complexity, captured by vector ruggedness measure (VRM), showed resolution-specific responses. This metric captured the structural complexity of rubble at a high raster resolution of 1 cm and that of tabulate coral at lower resolutions, resulting in decreases in mean VRM values at 2- and 4-cm resolutions but an increase at 1-cm resolution. Variability in profile and planform curvature was reduced after the hurricane due to a disappearance of extreme curvature values created by the tabulate coral after the hurricane. This study highlights the varying responses of habitat complexity metrics to the complete destruction of a coral reef and provides us with insights into how choices of habitat complexity metrics can affect quantitative assessments of 3D habitat structure.

Survey cruises by the National Oceanic and Atmospheric Administration (NOAA) in 2016 and 2019 yielded specimens of an undetermined red alga that rapidly attained alarming levels of benthic coverage at Pearl and Hermes Atoll, Papahanaumokuakea Marine National Monument, Hawaiʻi. By 2019 the seaweed had covered large expanses on the northeast side of the atoll with mat-like, extensive growth of entangled thalli. Specimens were analyzed using light microscopy and molecular analysis, and were compared to morphological descriptions in the literature for closely related taxa. Light microscopy demonstrated that the specimens likely belonged to the rhodomelacean genus Chondria, yet comparisons to taxonomic literature revealed no morphological match. DNA sequence analyses of the mitochondrial COI barcode marker, the plastidial rbcL gene, and the nuclear SSU gene confirmed its genus-level placement and demonstrated that this alga was unique compared to all other available sequences. Based on these data, this cryptogenic seaweed is here proposed as a new species: Chondria tumulosa A.R.Sherwood & J.M.Huisman sp. nov. Chondria tumulosa is distinct from all other species of Chondria based on its large, robust thalli, a mat-forming tendency, large axial diameter in mature branches (which decreases in diameter with subsequent orders of branching), terete axes, and bluntly rounded apices. Although C. tumulosa does not meet the criteria for the definition of an invasive species given that it has not been confirmed as introduced to Pearl and Hermes Atoll, this seaweed is not closely related to any known Hawaiian native species and is of particular concern given its sudden appearance and rapid increase in abundance in the Papahanaumokuakea Marine National Monument; an uninhabited, remote, and pristine island chain to the northwest of the Main Hawaiian Islands.

Quantifying the three-dimensional (3D) habitat structure of coral reefs is an important aspect of coral reef monitoring, as habitat architecture affects the abundance and diversity of reef organisms. Here, we used photogrammetric techniques to generate 3D reconstructions of coral reefs and examined relationships between benthic cover and various habitat metrics obtained at six different resolutions of raster cells, ranging from 1 to 32 cm. For metrics of 3D structural complexity, fractal dimension, which utilizes information on 3D surface areas obtained at different resolutions, and vector ruggedness measure (VRM) obtained at 1-, 2- or 4-cm resolution correlated well with benthic cover, with a relatively large amount of variability in these metrics being explained by the proportions of corals and crustose coralline algae. Curvature measures were, on the other hand, correlated with branching and mounding coral cover when obtained at 1-cm resolution, but the amount of variability explained by benthic cover was generally very low when obtained at all other resolutions. These results show that either fractal dimension or VRM obtained at 1-, 2- or 4-cm resolution, along with curvature obtained at 1-cm resolution, can effectively capture the 3D habitat structure provided by specific benthic organisms.

Long-term ecological monitoring of reef fish populations often requires the simultaneous collection of data on benthic habitats in order to account for the effects of these variables on fish assemblage structure. Here, we described an approach to benthic surveys that uses photogrammetric techniques to facilitate the extraction of quantitative metrics for characterization of benthic habitats from the resulting three-dimensional (3D) reconstruction of coral reefs. Out of 92 sites surveyed in the Northwestern Hawaiian Islands, photographs from 85 sites achieved complete alignment and successfully produced 3D reconstructions and digital elevation models (DEMs). Habitat metrics extracted from the DEMs were generally correlated with one another, with the exception of curvature measures, indicating that complexity and curvature measures should be treated separately when quantifying the habitat structure. Fractal dimension D64, calculated by changing resolutions of the DEMs from 1 cm to 64 cm, had the best correlations with other habitat metrics. Fractal dimension was also less affected by changes in orientations of the models compared to surface complexity or slope. These results showed that fractal dimension can be used as a single measure of complexity for the characterization of coral reef habitats. Further investigations into metrics for 3D characterization of habitats should consider relevant spatial scales and focus on obtaining variables that can complement fractal dimension in the characterization of reef habitats.

The architectural complexity of coral-reef habitat plays an important role in determining the assemblage structure of reef fish. We investigated associations between the reef habitats and fish assemblages in the Northwestern Hawaiian Islands (NWHI) using in situ fish counts and data on habitat metrics and benthic community composition that were obtained from three-dimensional (3D) photogrammetric reconstructions of the surveyed sites. The structure of fish assemblage as a whole on the basis of Bray–Curtis dissimilarity, species richness and the abundances of herbivores and piscivores were associated with habitat metrics, with higher levels of architectural complexity generally supporting greater numbers of fish species and individuals. Benthic cover did not explain additional variation in these variables after the effects of habitat metrics were taken into account. Corallivorous fish was the only group that showed positive associations with both habitat metrics and benthic cover (Acropora and Pocillopora corals). The total fish abundance and the abundances of planktivores and invertivores did not show associations with either habitat metrics or benthic cover. This study suggests that an appropriate combination of habitat metrics can be used to account sufficiently for the effects of habitat architecture on fish assemblages in reef monitoring efforts in the NWHI.

Although the existence of coral-reef habitats at depths to 165 m in tropical regions has been known for decades, the richness, diversity, and ecological importance of mesophotic coral ecosystems (MCEs) has only recently become widely acknowledged. During an interdisciplinary effort spanning more than two decades, we characterized the most expansive MCEs ever recorded, with vast macroalgal communities and areas of 100% coral cover between depths of 50–90 m extending for tens of km 2 in the Hawaiian Archipelago. We used a variety of sensors and techniques to establish geophysical characteristics. Biodiversity patterns were established from visual and video observations and collected specimens obtained from submersible, remotely operated vehicles and mixed-gas SCUBA and rebreather dives. Population dynamics based on age, growth and fecundity estimates of selected fish species were obtained from laser-videogrammetry, specimens, and otolith preparations. Trophic dynamics were determined using carbon and nitrogen stable isotopic analyses on more than 750 reef fishes. MCEs are associated with clear water and suitable substrate. In comparison to shallow reefs in the Hawaiian Archipelago, inhabitants of MCEs have lower total diversity, harbor new and unique species, and have higher rates of endemism in fishes. Fish species present in shallow and mesophotic depths have similar population and trophic (except benthic invertivores) structures and high genetic connectivity with lower fecundity at mesophotic depths. MCEs in Hawai‘i are widespread but associated with specific geophysical characteristics. High genetic, ecological and trophic connectivity establish the potential for MCEs to serve as refugia for some species, but our results question the premise that MCEs are more resilient than shallow reefs. We found that endemism within MCEs increases with depth, and our results do not support suggestions of a global faunal break at 60 m. Our findings enhance the scientific foundations for conservation and management of MCEs, and provide a template for future interdisciplinary research on MCEs worldwide.

Visual survey by divers using open-circuit (OC) SCUBA is the most widely used approach to survey coral reef fishes. Therefore, it is important to quantify sources of bias in OC surveys, such as the possibility that avoidance of OC divers by fishes can lead to undercounting in areas where targeted species have come to associate divers with a risk of being speared. One potential way to reduce diver avoidance is to utilize closed circuit rebreathers (CCRs), which do not produce the noise and bubbles that are a major source of disturbance associated with OC diving. For this study, we conducted 66 paired OC and CCR fish surveys in the Main Hawaiian Islands at locations with relatively high, moderate, and light fishing pressure. We found no significant differences in biomass estimates between OC and CCR surveys when data were pooled across all sites, however there were differences at the most heavily fished location, Oahu. There, biomass estimates from OC divers were significantly lower for several targeted fish groups, including surgeonfishes, targeted wrasses, and snappers, as well as for all targeted fishes combined, with mean OC biomass between 32 and 68% of mean CCR biomass. There were no clear differences between OC and CCR biomass estimates for these groups at sites with moderate or low fishing pressure, or at any location for other targeted fish groups, including groupers, parrotfishes, and goatfishes. Bias associated with avoidance of OC divers at heavily fished locations could be substantially reduced, or at least calibrated for, by utilization of CCR. In addition to being affected by fishing pressure, the extent to which avoidance of OC divers is problematic for visual surveys varies greatly among taxa, and is likely to be highly influenced by the survey methodology and dimensions used.

This study examined the effects of SCUBA bubbles on fish counts in underwater visual surveys conducted in the Papahānaumokuākea Marine National Monument (PMNM). Specifically, paired fish surveys were conducted at each survey site, utilizing two different gear types: open-circuit SCUBA (OC) and closed-circuit rebreather (CCR). Bubble exhaust released from the OC equipment is a potential source of bias for in-situ fish observations, as the associated audio and visual disturbances could either attract or repel fishes depending on whether their behavior is more driven by curiosity or caution. The study area, is a large (~1.5 million km2) and extremely remote marine protected area in which the response of coral reef fishes to divers represent natural behavior of naive fishes with little or no previous contact with humans. Historically, surveys conducted on OC in this area have shown an abundance of large roving piscivores and this study set out to determine the extant, if any, the audible and visual disturbances of OC bubbles have. The species typically seen in these prior surveys were Caranx ignobilis, Caranx melampygus, Aprion virescens, and a couple of species of sharks. We found differences in counts for some roving piscivores, including significantly more jacks observed on OC than CCR (Caranx ignobilis 57% more, and Caranx melampygus 113% more). Instance of first encounter, i.e. the time when a fish was first observed during a survey, also varied for some species. Higher numbers of Aprion virescens (p = 0.04), and C. melampygus (p = <0.001) were observed in the first 5-minutes of counts by divers on OC (i.e. when they were using breathing apparatus that produced noises that could be heard over long distances). Although not the focus of the study, we also assessed differences between OC and CCR counts for other groups of fishes. Estimated abundance of benthic damselfish was higher on OC than CCR, and counts of butterflyfish were lower on OC; but there were no significant differences for the other groups considered. This is an important control study that documents the natural responses of coral reef fishes to SCUBA bubbles generated by in-situ surveys.