Cobb Seamount: Guided by Dolphins
During our transit from Corn to Cobb Seamount, we got quite the treat. A pod of a dozen or so dolphins approached the vessel and spent the next half hour bow-riding. Bow-riding is a common behavior by dolphins and porpoises during which they position themselves to be lifted up and pushed forward by pressure waves created off the bow (or front) of a large vessel. (Source = Encyclopedia of Marine Mammals).
That was pretty cool, but even cooler was finding out that seeing these dolphins here was a big surprise. At first glance, they looked like Pacific white-sided dolphins. But something didn’t seem right to Caroline Fox, the seabird and marine mammal observer on this survey from Environment and Climate Change Canada. Fortunately, I captured a full-body image of one leaping with my video camera. Caroline was able to send the images and the related video to her colleagues that specialize in marine mammals, and they confirmed that these are actually common dolphins. While common dolphins are some of the most abundant and familiar dolphins in the world, that’s not the case this far north. The last sighting in Washington State was in 2017. They hadn’t been seen in British Columbia since 2015.
Common dolphins are, however, often associated with underwater geologic features where upwelling occurs, such as seamounts. So, it’s exciting to see this information confirmed here—on the relatively unexplored Cobb-Eickelberg Seamount chain.
Seamounts Explained, and the Cobb Hotspot
In the last blog, I promised to explain how these seamounts formed. I mean, why would there be these groupings of underwater mountains out this far in the ocean? The answer is because of the Cobb hotspot.
The Cobb hotspot is a marine volcanic hotspot, which means that magma rising from the Earth’s mantle cools as it hits ocean water and creates underwater volcanoes. Over millions of years and because of plate tectonics, the volcanoes rising from the Cobb hotspot have moved to the northwest, forming an underwater mountain range called the Cobb-Eickelberg Seamount chain. So, seamounts are basically underwater mountains.
Though the Cobb hotspot is relatively close to the west coast of the U.S., this seamount chain extends over 1,000 miles to the Aleutian Trench in Alaska. And this hotspot is not alone. You can also see the Bowie hotspot on the map above, which has created its own underwater mountain range. The Kodiak-Bowie Seamount chain could be a target for future surveys of deep-sea coral and sponge habitat.
These geologic connections from the Aleutian Islands to the Pacific Northwest are one of the reasons these seamount chains have been targeted for exploration. Seamounts provide structure on what might otherwise be a monotonous seafloor. And variability of the seafloor provides variability in habitat, which in turn increases biodiversity. These seamount chains may also form distribution pathways for deep-sea corals, sponges, and other organisms.
Cobb Seamount Revisited – Refining Our Understanding
This research expedition is funded in part by a national NOAA program, the Deep-Sea Coral Research and Technology Program. For management purposes, NOAA Fisheries is split into five regions: Greater Atlantic, Southeast, West Coast, Alaska, and the Pacific Islands. And each region is funded on a rotational basis to conduct deep-sea coral research, which fuels the national database. This one-stop-shop for deep-sea coral information in the U.S. helps us understand where deep-sea corals occur and how they might be important as habitat for other species. All this information also helps scientists develop predictive models to speculate about unexplored areas, and for regional fisheries management councils to make more informed decisions to manage our nation’s fisheries sustainably.
The Deep-Sea Coral Research and Technology Program was established back in 2007. Alaska’s first rotation was 2012 to 2014 and focused on the Gulf of Alaska, Aleutian Islands, and the Bering Sea. The second rotation started in 2020 and will extend into 2023, and added this international partnership to study the Cobb-Eickelberg Seamount chain. Back in 2012, Fisheries and Oceans Canada was the first to explore Cobb Seamount in partnership with NOAA Fisheries’ Northwest and Southwest Fisheries Science Centers. They did so with a remotely operated vehicle (ROV) and an autonomous underwater vehicle (AUV). They found the seamount to be rich with deep-sea coral.
However, that 2012 expedition targeted specific sites on Cobb Seamount where scientists expected there to be coral. Our current effort is exploring the seamount using a random sample design. On this 2022 expedition, we were super fortunate to be joined by Dr. Janelle Curtis. She served as the chief scientist on that 2012 expedition, and she was quite surprised by what we found.
After her experience on the 2012 survey, Janelle was astonished to see so much sand and relatively little coral on Cobb Seamount. This proved the point of why this sort of randomized survey is so valuable. While the past expedition showed that corals can be abundant on Cobb Seamount, they are not everywhere here, and the predictive models actually expected this too.
But the models aren’t perfect. There were several sites where expectations did not match the observations. The best way to gain confidence in models is to ground truth them, which, again, is why this survey is so important. What we learn today will be incorporated into future models and help fine tune their accuracy. Visual observations also help us understand the true abundance of deep-sea corals and sponges on these seamounts, and perhaps add more value to those spots where we do find them.
Another notable observation on Cobb Seamount was seeing lots and lots of blackspotted rockfish. Typically, this particular species lives at depths of 350 to 500 meters. However, they were consistently much shallower on Cobb Seamount (in the 150 to 300 meter range). Another species was also particularly abundant—rosethorn rockfish. At one point, Chief Scientist Chris Rooper, estimated that at least 75 percent of the rockfish we were seeing were rosethorns.
With Cobb Seamount having the most surface area of the seamounts we’re surveying and rising closest to the surface, it’s also the seamount that experiences the most fishing effort. So, it was no surprise to see impacts of fishing gear, including some derelict gear that snags or breaks and ends up on the bottom.
Across our country, Regional Fisheries Management Councils are constantly evaluating our marine resources. The information and models that come out of surveys like this help management decide on future approaches, such as what areas might need some protection and what areas don’t.
In our fifth and final blog, we will wrap up with what we find on Brown Bear Seamount. Like Eickelberg and Corn Seamount, it has never been explored with cameras before. For now, feast your eyes on some more captivating photos from Cobb Seamount.