March 27, 2003: Leg 3, Day #17

Farallon Basin 

Well, here we are again, back in the saddle and back where the salps and sinkers have their own exclusive depth zones. Again today the wind was too high to allow blue-water SCUBA diving. 

This morning our pilots tackled the job of replacing a flooded cable in the VME can, a real challenge because it required precise and careful movements in a situation where access was often very difficult. Imagine trying to repair the plumbing under your kitchen sink without removing all of the junk you have stashed in there. The titanium can is tucked into the center of the vehicle with very little open space around it. Its contents are nested inside with incredibly tight tolerances. A single drop of water from the cable or from a sweating forehead could lead to a fried circuit board and major problems for the Expedition. The pilots worked last night, they worked this morning, and by early afternoon, they had it all buttoned up, tested, and ready to go. And they fly it, too.  

ROV pilots (left to right) D.J. Osborne, Dave French, Buck Reynolds, Buzz Scott, and Paul Tucker replacing a cable on Tiburon's VME can.

Our dive worked chiefly in the upper levels of the water column, examining the patterns of vertical stratification we observed here earlier. On descent, the first layer we encountered was a dense aggregation of Bathochordaeus, the larvacean we know from Monterey Bay but much shallower here, presumably because of the broad oxygen minimum zone. Unfortunately, we can't work on these animals here because they are above ROV Tiburon's operational depth minimum of 100 meters. Below the larvaceans was a region that contained mostly sinkers, the discarded feeding filters of Bathochordaeus. These large, carbon-rich packages are an important source of food for animals on the deep seafloor.  

Then came a layer thickly populated by salps. These were Cyclosalpa, a type which occurs as chains of individuals that coil into spiral shapes or "whorls." Also present were "nurse" cells, an alternate generation of these salps that occurs individually and produces the generation in chains (sounds like a book title). These salps, probably C. bakeri, have a small filament that projects from the rear of their bodies. When a group is coiled, with their bodies clustered and the filaments hanging down, they look just like a big medusa. Since salps have no defense mechanisms that we know of, perhaps this appearance protects them from would-be predators, who would prefer not to get a face full of stinging tentacles.  

Cyclosalpa—If you close one eye and look at this sideways, it looks just like a big medusa. Remember that where these animals live there is very little light, so that shapes and silhouettes are  important for target recognition.

Below the salps was a thin layer of ctenophores, mostly Thalassocalyce, the one we watched ingesting a euphausiid (krill) a few days ago. Then, although it was not immediately apparent to us, came the top of the sonic scattering layer (SSL). To detect this group of animals we had to switch sensory modes from visual to acoustic. SSLs are acoustic reflections of groups of animals in midwater. Most often these animals are vertical migrators. They occur near the surface at night where they do most of their feeding. At dawn they migrate downward into deeper, darker water, presumably to avoid being eaten by visually cued predators. These movements have intrigued scientists for years and nets dragged through SSLs have established that fishes, krill and a variety of other kinds of animals participate in the migrations—which on a global scale are surely the greatest mass migrations on Earth.  

Scientists and submarine skippers have been tracking SSLs acoustically ever since echosounders were developed way back in the last century. What we wanted to do today was see them, not just their echoes printed on a chart. Kim Reisenbichler set up the R/V Western Flyer's acoustic profiler (an expensive echo-sounder) so that we could see where Tiburon was relative to the SSL. We could then correlate the video images from the ROV with our position in the layer. At first, we didn't see anything obvious in the video, and the acoustic trace showed that the layer was avoiding the ROV. Then Kim began a series of maneuvers with Tiburon to let us catch the scatterers by surprise. We dropped slowly and obliquely through the layer (which was about 100 meters thick) then popped up quickly back into it. We turned out the lights and waited in the dark, then flew forward with the pedal to the metal and all our lights blazing.  

A digital shot of the acoustic profiler's data screen. Tiburon's path is the red and green line. Horizontal depth references are 50 meters apart. There is a blue band of surface scatter at the top of the chart. The principal scattering layer starts at about 250 meters and extends down to about 375 meters with a narrow layer deeper at about 425 meters. It looks as though the layer bulged ahead of the ROV as it came back up through the top of the layer.

Through the course of the afternoon, we began to see more and more of the animals that comprise the SSLs and to log them on video. Kim positioned Tiburon above the layer as the sky darkened after sunset, and we watched as the layer migrated up around us. It's early yet in the development of these methods, but in just a day we learned a great deal about the layers and how to go about studying them in the future.  

Jeff Drazen had our net in the water before the water stopped draining off Tiburon. He will be trawling until midnight, and we'll begin Tiburon's pre-dive at 0600 with launch time set at 0630. Watch this space... 

Bruce Robison

Karen Osborn's artwork was subjected to the pressure of the deep sea on one of ROV Tiburon's dives—before the dive (left) and after (right).