Blog from the Sea of Cortez: Exploring Isla San Marcos

Over the last few weeks we have done a lot of science: re-occupation of old CTD sites, eDNA sampling, fish surveys, etc. But it's all been pretty structured.

Today was different. It occurred to me that this was our first real day of pure exploration --- just going wherever out scientific instincts took us, with no particular agenda. Finally, 45 years after I got my PhD, I get to be a gentleman oceanographer.

The morning was spent looking for octopus. Gilly, a Stanford scientist and board member, had seen octopus kill a mollusk, then move into its shell. Once it got too big, it went looking for a bigger victim and so on up the housing chain. A few octopus were found, but they were just living under rocks, not in a victim's shell. We'll keep looking.

For the afternoon, we decided to explore an area north of Isla San Marcos. The charts of the area are not very good and some of them suggested that the ridge was actually two ridges or, possibly, seamounts, with a valley between them. The ridge is of interest because it is relatively shallow, often less than 40m, surrounded by deeper water, so it is likely to attract a lot of wildlife. We had also seen a few local fishermen out there. We decided to take a look with our echosounder and, possibly, ROV.

And, indeed, the area was incredibly abundant with wildlife. We saw hundreds of birds (mostly Elegant Tern, Yellow-footed Gull, Brown Pelican) and several humpback whales. There was a pod of about 100 dolphins swimming energetically around us all day --- occasionally they'd wander over for a closer look if we brought out a new piece of equipment.

We also saw many dense schools of fish on the echosounder, usually pressed up against the bottom, presumably hiding from the dolphins above.

The two seamounts turned out to be one long, continuous ridge trending to the NNW. Much of it was relatively shallow --- 40-60 m.

We collected enough echosounder data that we should be able to make a decent map of the area. That will require one of us mastering Echoview, a sophisticated piece of software, which will take a little time.

The day felt very "Steinbeck and Ricketts". We were out in the sun poking around all day, then retired to an anchorage in the evening for beers and talking.

Maddie taking a turn at the wheel of the ROV.

A pod of dolphins hung around the area all day.

Transect across the ridge, west to east. The lines SM1 and SM2 mark the location of two CTD casts. Note the "fish balls" above the ridge. Also, the deep scattering layer, located at 200m on the west, 350m on the east.

CTD cast "SM2". This shape is typical of the area: temperature, salinity, oxygen, and chlorophyll all decrease with depth. The thermal gradient is strong enough to overcome the haline gradient. Deep water tends to be hypoxic or even anoxic.

After a CTD cast, the line must be brought in and "level wound." That means laying the wraps hard against each other to get nice even layers. Captain Paul nails it every time with his Jedi-like focus.  Oh, and he does it with his bare hands. When it's my turn, it looks like spaghetti.

A reef just off Isla San Jose.

Isla San Marcos. I found its starkness beautiful, but then I've always loved deserts.

An evening of chatting and enjoying a cold beer.