Posted on July 29, 2010 by Sara Edge, Assistant Research Professor, HBOI/FAU
Before every submersible dive, scientists and crew on the R/V Seward Johnson transect an area of the Gulf of Mexico seabed looking for interesting features, such as sink holes, cliffs, peaks, and ledges, which might have a high diversity of benthic (bottom dwelling) species, such as corals, sea fans, and sponges, as well as high densities of fish and other pelagic (free-swimming) animals. The transect is done with a fathometer that scans the ocean floor with multibeam echosound to produce high resolution bathymetric maps. These maps are projected as colored images back to the ship’s bridge indicating depth and structure of the sea floor. Points on the map are selected and marked to create a route for the submersible to travel.
While working with John Reed to map the route I would be taking on yesterday’s JSL dive, we began to worry that the dive might be a bust due to the relatively flat topography of the sea floor and a lack of noticeable features. Since this was only my second submersible dive in the front bubble, I was even more worried. But, the area is important because Dr. Chris Koenig from Florida State University has documented sites that are breeding grounds for gag groupers and scamp in the winter and red snapper in the summer. For the past 10 years, the area has been designated a Marine Protected Area largely due to Dr. Koenig’s efforts and research. Regardless of the potential dearth in benthic diversity, I was excited to be venturing, yet again, into the blue abyss … and the research must go on despite expectations at the surface.
As we prepared for the dive at the pre-dive briefing, jokes and jabs were made about how exciting the dive would be (or not be). “Try not to fall asleep during the dive, Sara!” However, I was more concerned with making sure I would be taking the proper notes, photo documentation, and video footage than how exciting the dive would be. After the meeting, we boarded the submarine, prepped for the dive, and launched. Regardless of what is at the bottom, the thrill of sinking to the ocean’s depths, watching the light disappear, and starting to see a whole new world appear is thrilling every time. Once the submersible touched bottom, I noted the habitat type and geographical features. It was not very exciting at first, just a muddy, flat bottom with very few, dispersed bits of rubble containing sponges and other invertebrates. Then a 7-foot hammerhead shark passed over the sphere of the JSL. It was amazing!
After taking sediment and water samples for microbial analysis, the pilot took our bearings, and we began motoring to the first way point on our map. One of the interesting features we observed were small caves, possible made out of a type of clay that appeared to be fish burrows. How, or if, the fish actually excavate these caves is poorly understood, especially regarding the populations on mesophotic reefs. We videotaped and photographed a large snowy grouper at one of the caves. When groupers feel threatened or are confronted, they produce a loud, low frequency ”boom” sound, created by vibrating the swim bladder. Apparently the grouper felt the need to let us, or the submersible, know that this cave was his, because he postured, opened, and closed his mouth, then faced the JSL and gave us a nice boom. It was very interesting, but we decided to move on and observe the rest of the site, leaving the grouper to defend his cave.
As we traveled up a gradual slope, the topography began to change. Instead of a sediment plain, there were large rocky outcroppings which provided substrate for big sea fans and long whip corals, some reaching up to 2 meters. While I was fascinated with observing the diversity of the corals and sponges, Dr. Koenig in the aft compartment was thrilled with the diversity of fish in the area. As an invertebrate biologist, I am embarrassingly lacking in my knowledge of fish identification. However, Dr. Koenig began describing the different species of fish we were observing, such as scamp, red snapper, amberjack, snowy grouper, Warsaw grouper, blue angels, and big eyes among others. Some of the fish were more than a meter long and there were hundreds of them! The majority of the fish were red snapper since it is spawning season for this species, and we were at one of its breeding ground sites. The females were easy to identify since most were gravid and carrying an extended belly full of eggs. Dr. Koenig said that red snapper are always hungry and thus always feeding. This was evident by the fact that they chased the green measurement lasers projected by the video camera onto the sea floor. It was very entertaining watching them try to catch the green spots, like playing with a cat with a laser toy. I’m happy to say we have it all on video as “scientific evidence”.
The rest of the dive was spent recording fish species and sampling invertebrates, water, and sediment. But it was definitely a fun and exciting dive. As the snowy grouper let us know at the beginning, it was a BOOM!