Day 8 – The second dredge and pressure changes
Menu:
Breakfast
- Veggie and egg scramble
- Grits
- Bacon and sausage
- Fresh fruit (the melon was primo!)
Lunch
- Grilled cheese
- Potato and leek soup
- Tuna wraps
- French fries
- Salad bar
- Roast beef
- Steamed broccoli
Dinner
- Spaghetti with meat sauce
- Green beans
- Rice pilaf
- Vegetarian lasagna
- Salad bar
Position
Southwest of the Sisters, at a small seamount named the Dory.
Weather
Winds up around 40-50, although a high-pressure front is coming through: hopefully we’ll be able to get in the water by tomorrow morning!
So we spent a good deal of the morning trying to figure out the dredge issue. Let me present it as a word problem: The dredge is strung out behind the boat by a length of wire, and since we’re moving on the surface, the dredge is trailing us at an angle, known as the “layback”. What we have to figure out is how long that wire has to be in order for the dredge to drag along the bottom. In order to determine this, we have a tool called a “pinger”, which sends pulses of sound that bounce off the seafloor and then back up to the ship. The pinger is attached to the dredge line, about 150 meters up from the dredge. To muddy the waters, this pinger, if not directly below the ship, bounces its sound waves at an angle off the seafloor back to the ship.
Fortunately, the layback is small enough that this bouncing does not confound our pinger measurements very much. The layback ratio, or the ratio between the length of wire out to the distance from the dredge to the surface, is around 1.02 – 1.08. If we do some calculations, we find that a change in this trawl ratio changes the pinger signal by under 10%, which is a small enough error to be able to still accurately determine our position on the seafloor.
Given this information, and a flat seafloor, it should be relatively easy using trigonometry to determine the depth of the dredge. However, the seafloor is anything but flat, especially when you want to dredge across a seamount. So how, given the uneven topography of the seafloor, do you pinpoint exactly where the dredge is on the bottom? Now, it is really a matter of lack of information; given enough iterations we should be able to generate a good intuition about where it is. If we know the starting position of the boat, and the exact heading, as well as the bathymetry (the topography of the seafloor), we should be able to narrow down our dredge mark.
Unfortunately, with the prevailing 35-knot winds, we’ve had to cancel operations for the rest of the day. However, it seems that a high pressure front is moving in sooner than expected, so with any luck, we’ll have Jason in the water again bright and early tomorrow morning.
To address Justin’s comment to this process of dredging and sampling from the seafloor, I’ve definitely had the same doubts about the costs versus the benefits of doing such sampling. Compared to the larger fishing trawls, our tiny dredge doesn’t make a huge dent in the reef. Although I must admit that science does also operate under the “bigger is better” mantra: there is a dredge called the “Sherman” that the group used last year on some seamounts that would no doubt leave a very large mark on whatever seamount it was sampling. Regardless of sample size, our focus is also primarily on fossil corals; that is, material that is not alive. There are other science crew aboard that collect live samples – you can get paleoclimate data out of live samples, especially to reconstruct reef position over time on the seamounts, and taxonomists are still discovering and describing species from this area.
I like to think that our sampling is sparse and selective enough to not have much of an impact on the reef community as a whole. However, in an already impacted zone that’s trying to recover from serious overfishing, our small sampling could seriously retard reef regrowth in this area. Indeed, this whole area is a Marine Protected zone, which means that no fishing of any kind can happen here, and any live coral taken from the seafloor must be catalogued and reported back to the Australian government. We had to get special permits to travel here – most Australian vessels are banned from operating in these waters. Moreover, our “sparse” sampling is potentially targeting very long-lived individuals that have been part of the reef community for decades, and maybe even centuries; it is no secret that corals take a very long time to grow, and their presence is an island to many fauna that are closely associated with it and he environment it creates.
The bottom line, though, is that our single cruise takes such small transects of the available ecosystem. We sample up and down contour lines with a field of vision of about five meters; we really have no idea if what we’re seeing is even indicative of the entire ecosystem on these seamounts. Even a large dredge like the Sherman, taking up literally a ton of material during a single dredge, impacts an incredibly small area of the Southern Hills.
The real ecological threat here might be the vast amount of carbon dioxide we’re releasing into the atmosphere by steaming in a big boat – the tank of the boat holds 285,000 gallons of diesel fuel (!!!!!) and even when idling in one place, the boat burns 1600 gallons of fuel every day.
Maybe we would be better off taking video of the whole thing and looking it at a screen, but there’s something about discovery that lends itself to dissection and destrucion – Human curiosity isn’t satisfied until we have really “gotten to the bottom of something”. And now I wax philosophical, which means I’ve spent way too much time thinking and writing about this. People’s thoughts and comments about this are encouraged, though! I’d like to hear what people think about the ecological friendliness of our cruise, dredging, sampling, or anything!