I’ve been trying to get a post up, but our internet connection has been spotty, and it’s been awfully busy. We’re now at our northern station, anchored up in the “Tongue of the Choptank” (38° 31.3’N 76° 24.3’W). That’s not an official place name, but it’s a deep spot on the eastern side of the Chesapeake south of the mouth of the Choptank River. On the Charts it looks a little like a tongue coming out of the Choptank, so our Captain joked about that when we chose the station. Officially, we call it “North” station to distinguish it from the one off the Rapahannock, which we call “South”. Creative? Not really, but descriptive.
Don’t think we’re not a bit creative and silly though. Our summer students who are back at the lab holding down the fort while we get samples, made a bunch of new sieves for us and named them (see the picture on the right). Might not be to funny reading this at home (and I admit I might feel a little silly writing about this), but at 4 am when we’ve been hauling nets all night and someone calls out for the “Vanquisher” to filter a particularly mucky sample it makes me laugh.
But that’s not what I wanted to talk about. I wanted to talk about our nets.
We use three different types of nets to catch different types of animals. It’s a nutty system we’ve devised, and the crew has been a huge help getting this working. The three nets vary in size and complexity, and each requires a different winch and wire. The deck is pretty cramped, but as the data started to come in during the first few tows, we knew our efforts were worth it. I want to describe the three nets, though I have only pictures for two of them in action, but I’m going to spend a bit more time on my personal favorite, which I’ve written about before.
So what are these three nets and what are we catching? The biggest is the mid-water trawl (MWT), and we use it to sample fish. I don’t know that much about fish nets, but it’s about 18 feet wide and has two large metal “doors” that hold it open when it’s towed behind the boat. Our fisheries experts who are collaborators run this net, and it works pretty well to catch the big jelly fish (not ctenophores like I wrote about, but the cnidarian or “true” jellies that are the bane of many a bather this time of year), and small fish. Yesterday and last night we were able to set it 12 times and caught a myriad of finfish, jellies, and large invertebrates. Bay anchovies were the dominant animal, but we got a lot of spot, juvenile weakfish, harvest fish, butter fish, a few big sea nettles and moon jellies, some spider crabs, and even a couple of tiny squid. The squid I admit are my favorite, and they were identified as brief squid.
The next largest net is a Tucker trawl. This has a square opening and carries three nets. The first is called a drogue and is used mainly to get the net fishing properly as we send it down. Once it is at the desired depth, we send a messenger, a large brass weight that clips on the wire and slides down by sheer force o gravity. The messenger slams into the release mechanism which closes one net and opens the next. On this Tucker trawl we have two nets that are fished, and we bring the net up from the deepest point and can sample two distinct layers with each tow. The net is fitted with 1 mm mesh, so most of the tiny stuff (zooplankton) goes through and what you are left with are jellies (mainly ctenophores) and larval fish. Sometimes, if we are near the bottom some shrimp called mysids show up too, but in general this is used to target the ctenophores and larval fish. It’s a simple and elegant design that works very well. It is also the basis for the MOCNESS, our third net system.
MOCNESS has been discussed here before (see the comments), but we’ve made a few upgrades and it’s pretty tricked out now. We worked with the company that makes it to get a bunch of specific sensor onto it, and they are working really well for us, allowing us to get at our specific objectives in a very elegant way. This net fishes like the Tucker trawl, with a square opening and nets that can be opened and closed, but it has a few significant differences, mainly that it is entirely computer controlled. Now, don’t get me wrong, a lot of really great sciences is done with messengers and mechanical nets, but when the MOCNESS is working, it’s a torrent of data. Every four seconds the net is in the water, we get measurements of
• Volume of water flowing through the net
• Chlorophyll fluorescence
• Light reaching the net
In addition, it tells you when the nets fired properly and you electronically control when nets open. It can carry up to 10 nets depending on how it is configured, and we use 6 nets on it routinely in the Bay. Our Intrepid Research Assistant is using a MOCNESS in the Gulf of Mexico, probably as I write this, that is being fished with 10 nets to 500 m! Our net tows take about 20 minutes from the time it enters the water to the time we retrieve it. In the Gulf the tows will take more than 4 hours. Processing all those nets will take another solid hour or two, depending on what needs to be done with them, and that will just about fill up a watch. Here in the bay, we can do 12 in a 12 hour watch when things are going well.
Each of these nets brings us a collection of animals from under the surface of the water, and most of them get put into jars, preserved, and taken back to the lab. To do a proper job of cataloging where everything came from and what it is will take us a year or more, though we’ll take a good look at the data as we process it. This is a long slow process, but it gets us a tremendous amount of information. Other instruments we use can gather data over large areas, like the Scanfish, but it doesn’t get the species information we can get from net tows and microscope work. It’s a compromise we have to carefully weigh depending on the questions we’re asking, and so far, for this project, I think we’ve made some very good choices.