What can we learn about the open ocean from tracking white sharks? In this short video I provide an overview of some of the results of our work that investigates the use of mesoscale eddies by large pelagic fish.
In the coming week, we will again venture into the North Atlantic to continue our investigation of the role of eddies in structuring pelagic ecosystems. Even though it feels like we just returned from our first cruise (November-December, 2015), we have had a bit of time to digest some of the observations made during NAAMES-1. During our first expedition, we discovered that eddies indeed have a significant influence on mesopelagic fish. Eddies affected not only the depth at which these organism live, but also their abundance.
Using our custom-built deep-water echosounder (the RUMP), we observed that deep scattering layers, the acoustic signature of these mesopelagic ecosystems, are not only deeper in anticyclones, but also have stronger back scattering, when compared to cyclones (Fig. 5). This suggests that the observed pattern of white sharks preferentially using anticyclone eddies may be the result of these eddies affecting their prey. In the weeks to come, we will expand on these observations and surely gain more insight into the roles of eddies in structuring pelagic ecosystems.
Preparations are under way for the first cruise of the NAAMES field program. We will be leaving from Cape Cod, MA, on November 5th to conduct a 30-day cruise into the North Atlantic. Our mission is to characterize the marine ecosystem, from viruses to phytoplankton and all the way to mesopelagic fish, with the goal of quantifying the links between ocean variability (seasonality, eddies, and fronts), marine ecosystems and aerosols. We will be on board the R/V Atlantis, a 142-foot, steel-hulled research vessel operated by the Woods Hole Oceanographic Institution. Our cruise will take us from Massachusetts, up to about 48 degrees north, and then down along 41 degrees west to approximately 38 degrees north (see planned cruise track below).
The planned expedition path overlaid on satellite estimate of the April mean climatological chlorophyll-a concentration. The NAAMES program time line is shown along the bottom of the figure with blue triangle indicating planned cruise dates.
My primary research goals on this cruise will be to help guide the ship into mesoscale eddies using real-time satellite data. I will also be deploying 20 surface drifters that we will use to study the movement of the eddies and help guide the NASA C-130 aircraft to fly over and sample the eddies. In addition, I will be collecting multi-band acoustic observations to characterize deep scattering layers, layers of fish and squid below the sun-lit surface layer of the ocean (what is often called the “Twilight Zone”) inside and outside of the eddies. This research is motivated by recent observations of the use of eddies by white sharks. We have observed that sharks dive deeper and longer in anticyclonic eddies when compared to cyclones (more info here). Our hypothesis is that this is because deep scattering layers, where the sharks are feeding, are deeper and have higher biomass in anticyclones than cyclones. I will try to post semi-regular updates during the cruise, so please come back and see what we are doing in the coming weeks.
You can see where the ship is here.
Schematic representation of isopycnal surfaces (thin black curves), white shark dives (wavy black arrows) and the DSL (pseudo-color) in (a) anticyclonic and (b) cyclonic eddies. The detection of the DSL by ship-board acoustics is indicated by the cone of solid downward propagating waves with the returned signal indicated by dashed upward propagating waves.
Two days ago I reported that Lydia swam towards a large anticyclonic (clockwise rotating) eddy or meanders (see post here). She approached this anticyclone and swam northeast along its periphery. In the past 24 hours, Lydia has turned southeast of the previous anticyclone and has entered into the center of an adjacent anticyclonic eddy! Analysis of her interaction with eddies during her trip offshore in the summer and fall of 2013 (find more info here) suggests that she prefers the cores of anticyclones over cyclones and makes repetitive deep dives (often over 800 meters) while in the cores of anticyclones. Her activity in the past few days suggest that she might be honing in on some cues, be they potential prey, mates, or other, and is occupying the core of yet another anticyclonic eddy.
Lydia’s track overlaid on a map of sea level anomaly. Anticyclonic eddies and meanders are shown as orange and red features, cyclonic eddies and meanders are blue and purple features.
This begs the questions: Do white sharks seek out anticyclonic eddies because they are areas that concentrate prey and/or potential mates, or do they prefer the warm water found in anticyclones while swimming elsewhere? We’ll keep a close eye on where Lydia goes from here and hope to be able to address these questions in the future.