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Midwater Ecology Expedition Summer 2019 – Log 2

Astrid Leitner and Rob Sherlock prepare a net for a late-night tow to see which animals are present during the daily vertical migration to the surface. The tow was lowered to a depth of about 400 meters during daylight and pulled up to the surface well-past dark. Photo by Rondi Robison.

Midwater Ecology Expedition Summer 2019 – Log 2

Every evening all over the world ocean a mass migration takes place during which animals from the deep swim upwards into the surface ocean to feed under the cover of darkness. When they are full, and the sun begins to light up the surface waters, these animals swim back down to their dark daytime homes mostly around 300 to 400 meters (980 to 1300 feet). These animals are known as diel vertical migrators. I am interested in how this nightly migration is impacted by the dramatic canyon topography here in Monterey Bay.

Canyons and seamounts can result in relatively shallow waters lying right next to the deep ocean, some 500 meters (1,600 feet) deep or more. When this happens we call it “topographic blocking” because such features can prevent the dawn-time descents for those animals that drifted over the shallows at night. I am interested in testing this idea locally and discovering if topographic blocking is a regularly occurring phenomenon on the canyon edges. If the canyon does block these migrators from descending to depth at dawn, we would expect to find vertical migrators “piling up” on or near the bottom, and possibly creating high-density swarms.

Krill are one very common and ecologically important member of the local vertically migrating community, which also includes a variety of small fishes, jellies, and shrimps. One night on this cruise we mapped the vertical migration over a particularly steep section of canyon wall using bioacoustics (similar to a “fish finder” used by fishermen). Then we used MBARI’s remotely operated vehicle Doc Ricketts to film this wall during the dawn descent.

When we arrived at the bottom, close to the edge of the canyon wall at a depth of 200 meters (660 feet) around 7:00 a.m., we were swarmed by krill! Although krill would be attracted to the ROV lights, they could not have swarmed if they hadn’t been there in the first place. The swarm was so thick we could hardly see anything in our cameras, and we observed several rockfish darting through the swarm. If this were a regular occurrence, it would make this an ideal feeding ground for important predators like rockfishes and whales. Time will tell!