Mesophotic coral reef ecosystems (MCEs) occur in tropical regions extending from depths of 30m to the limit of zooxanthellate corals (approx. 150m). MCEs are often connected to shallow coral reef ecosystems, where it is suggested they may provide an important reservoir of recruits for threatened coral and fish populations, known as the deep reef refugia hypothesis. Yet their importance to overall reef resilience in the face of localised human disturbances such as overfishing and climate change impacts including coral bleaching and ocean acidification is largely unknown. Our work mostly focuses on whether coral and fish populations on shallow reefs and adjacent MCEs are connected, requiring the use of advanced survey methodologies and specialist lab techniques. The ORC team has experience in coordinating remote technical diving expeditions, including closed-circuit rebreather diving to 100m depth for survey work. Much of our current mesophotic work focuses on two regions: Utila, Honduras and the Bird’s Head Seascape, Indonesia.

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In Honduras we are working with Coral View Research Centre and Operation Wallacea to investigate coral and fish species distributions across the shallow to MCE depth gradient through the use of stereo-video systems, benthic video transects, baited camera drops, fish light traps and recruitment tiles. This partnership between ORC, Operation Wallacea and the Coral View Research Centre allows long-term trends in mesophotic reef health to be identified. During September 2015 ORC members also ran the Thinking Deep expedition to Utila, Honduras.

Our Honduran reef fish work is focused on: (i) differing behavioural responses of fish to survey techniques across the depth gradient, (ii) impacts of invasive lionfish (Pterois volitans) at mesophotic depths and (iii) the role of herbivores on mesophotic reefs. In early 2015 we deployed reef herbivore exclusion cages to investigate the long-term role of herbivores in structuring the benthic community.

Lionfish (Pterois volitans) at 60m on the south shore of Utila, Hondruas. (Photo: Ally McDowell)

Our Honduran coral research aims to understand how depth generalist coral species can survive under different light regimes. To this end we employ techniques such as scanning electron microscopy, bulk stable isotope analysis, PAM fluorometry, photography and aquaria trials. These data will reveal whether deeper corals are more efficient at photosynthesising, whether they are frugal and reduce their metabolic rate, or whether they increase the degree of filter feeding which occurs. The manner in which corals satisfy their energy demands at increased depth will in part dictate how likely the deep reef refugia hypothesis applies.

In Indonesia we are working with partners at Conservation International Indonesia and WWF-US to identify patterns in reef fish abundance and diversity in marine protected areas in the Bird’s Head Seascape, the location of the greatest fish diversity ever recorded. By using camera drop surveys both inside and outside of newly established MPAs we are involved in long term monitoring of the responses of mesophotic fish communities to protection.

Birds head seascape SVS
Assessing mesophotic reef fish species richness and abundance using baited remote underwater camera systems in the Bird’s Head Seascape, Indonesia.

Alongside this research we are interested in exploring patterns in ecological community data to see if mesophotic reefs can be characterised by the species present. Redirecting attention from specific depth bands and towards a particular type of reef will aid the design of future studies and allow more nuanced interpretations of reef responses to pressures.