Deep But Not Deserted – Exploring Deep-Sea Ecosystems Off the California Coast

A yellowtail rockfish swims past a pink landscape dominated by strawberry anemones and hydrocorals on Cordell Bank. Photo: MARE and NOAA

By Dr. Samuel Georgian, Marine Biogeographer at Marine Conservation Institute

In July 2018, Marine Conservation Institute staff scientist Samuel Georgian stepped on board the NOAA research vessel Bell M. Shimada, beginning a two-week expedition to explore deep-water coral and sponge habitats off the coast of northern California. In conjunction with NOAA’s Cordell Bank National Marine Sanctuary (CBNMS), Greater Farallones National Marine Sanctuary (GFNMS), and Marine Applied Research & Exploration (MARE), our aim was to explore and survey vulnerable deep-water habitats using a remotely operated vehicle (ROV), a mobile robot tethered to the ship and controlled by a pilot on board. Our research ended up being carried out across three national marine sanctuaries – Cordell Bank, Greater Farallones, and Monterey Bay. The national marine sanctuary program was created over 40 years ago in order to protect special underwater places across the United States, and manages sanctuaries and marine national monuments totaling an area covering over 600,000 square miles.

Off the coast of California, these national marine sanctuaries house an incredible array of deep-sea life, including cold-water corals and sponges that build crucial habitat structures for a large number of associated fish and invertebrate species. These habitats can be considered the ‘old-growth forests’ of the ocean – they are long-lived, slow growing, and extremely slow to recover following disturbance. Globally, these fragile deep-sea ecosystems are at significant risk from a growing number of threats including global warming, ocean acidification, deoxygenation, oil and natural gas extraction, seafloor mining, and fisheries. In many areas, including Californian waters, these habitats have also been significantly damaged by bottom-trawling fisheries that drag enormous nets along the seafloor, indiscriminately catching corals, sponges, and fish. Finding and surveying these diverse habitats is a critical step towards protecting and understanding their vulnerable ecosystems. 

A small mushroom coral in Cordell Bank National Marine Sanctuary. Photo: MARE and NOAA

Our expedition left the pier in San Francisco on a foggy morning in late July. Over the next 12 days at sea, we conducted 17 ROV dives, including many in completely unexplored regions. We witnessed an astonishing array of marine life including sponges, corals, octopuses, rockfish, sharks, seabirds, whales, and dolphins. Over thirty benthic samples were collected for later taxonomic identification and study, including one possible new coral species. Some of the surveyed areas were in Essential Fish Habitats and Rockfish Conservation Areas with recent or impending management decisions involving fishing closures or reopenings – making our survey work critical to effective ecosystem management. In some areas, these baseline and subsequent repeated surveys allow scientists to more accurately gauge the impacts of both fishing and management actions, improving our ability to conserve these and other areas.  

Ships never sleep. And although the scientists on board must, they make every effort to fit as much research as possible into every day at sea. With limited ROV pilots on board, ROV operations were generally limited to 12 hour days – but there were other ways to collect vital data. By the time the ROV was back on deck after each dive, the ship had already begun sailing towards the next survey site. Most nights, NOAA technicians spooled up the ship’s multibeam echosounder – a device that uses a specific type of sonar to map the seafloor at a very high resolution. Echosounders fire a swathe of soundwaves towards the seafloor, and measure the length of time it takes for the waves to hit the seabed and bounce back to the ship. These data allow us to map the depth of the seafloor with a very high precision and accuracy, and also gives us backscatter data that can provide insights into the type of sediments on the seafloor. Depth – and by proxy, physical features on the seafloor such as ridges or mounds – and sediment type are critical factors for determining whether corals and sponges are likely to be present, due to their need for hard substrate for larval recruitment and increased ability to filter feed from elevated seafloor features. These data are crucial for planning ROV dives, increasing our likelihood of choosing sites that will contain corals, sponges, and groundfish rather than the muddy seascapes that predominate much of the deep sea.

Marine Conservation Institute staff scientist Samuel Georgian displays his habitat suitability model for deep-sea corals on Cordell Bank. Photo: MARE and NOAA

The seafloor data collected during multibeam surveys, coupled with observational data on the locations of corals and sponges collected during ROV surveys, can be used to quantitatively model the expected distribution of species across large areas. Only a tiny fraction of the deep seafloor has been explored, largely due to the expense and technological difficulties associated with surveying deep waters. It is certainly not feasible to visually inspect all 600,000 square miles governed by the national marine sanctuary program! Further compounding the issue is the rare and patchy nature of deep-sea coral and sponge habitats – they typically cover relatively small areas of the seafloor, acting as oases of biodiversity in the otherwise sparsely populated deep sea. This scattered distribution creates practical problems, such as knowing where to plan ROV dives, and creates significant obstacles to effectively managing and conserving these important living resources. How can we adequately protect deep-sea species if we don’t even know where they are? That’s where habitat suitability modeling comes into play.

Habitat suitability models are tools that help scientists better map the areas where these deep-sea ecosystems will likely be found. The models link environmental data (such as seafloor features, water chemistry, and sediment type) with locations that are already known to be inhabited, allowing us to predict where we expect to find new communities in unexplored areas. Since these models can be developed using limited field surveys and remotely sensed data, and then used to predict distributions over large areas of the seafloor, they are particularly useful in improving our understanding of deep-sea distributions. Based in part on data collected during these expeditions with MARE, Marine Conservation Institute is developing habitat suitability models to predict the distribution of cold-water corals throughout Californian waters. These models will act as our best estimate of where we expect to find these taxa in completely unexplored areas, and will help us choose new survey sites and inform management decisions.

A large barrel sponge surrounded by a field of sessile sea cucumbers. Photo: MARE and NOAA

Marine Conservation Institute is a team of experienced marine scientists and policy advocates dedicated to saving ocean life. The organization’s goal is to create an urgently-needed worldwide system of strongly protected areas—the Global Ocean Refuge System—to protect marine biodiversity. Marine Conservation Institute also maintains the most accurate marine protected area database, the Atlas of Marine Protection.