For the first time, Rasmus joins us on the blog!

Rasmus and Nancy

My name is Rasmus Swalethorp and I am a postdoctoral researcher at Scripps Institution of Oceanography, UC San Diego. I am a biological oceanographer with a particular interest in the ecology of planktonic organisms. Up until now most of my research has been done in the Arctic and Antarctic and this was my first research cruise to the Gulf of Mexico.

Fishing for copepods in the sea ice in the Artic Ocean

Compared to the polar seas it was astounding to see the number of planktonic species that lived in the Gulf. While, I am used to only seeing about tens of different species of fish larvae or copepods (small planktonic crustaceans), in the Gulf I was amazed to see hundreds. This high species richness leads to very complex ecosystems that my colleagues and I are working hard to disentangle!

A lovely bluefin tuna (Thunnus thynnus) larvae collected during our survey

This is the second cruise aimed at understanding what makes up a good habitat for larval Bluefin tuna (BFT) to survive and grow. We want to be able to tell where to find these habitats and how much will be available in the future ocean. Much of my research has focused on baby fishes, the larval period of a fish’s life. This is the time when fish, are the most vulnerable to starvation and predation. Therefore, it is also considered to be the period most critical to recruitment: which is the number of fish that make it to adulthood and reproduce themselves someday. Understanding what is needed for a larval fish to survive is of particularly importance to management of commercially exploited fish such as Bluefin tuna.

After all what would the world be without sushi?

That’s where my expertise comes at play. I study what they eat. The more the larvae eat, the faster they can grow to improve their hunting capabilities and to avoid being eaten themselves. So, how do we determine a good larval tuna habitat? First, we need to know how much food is available to the larvae, and then which sources of nutrients are sustaining the food chain sustaining the larvae.

Mike L. and Ramus recover and rinse the ring-net
that has sampled the zooplankton (prey of BFT)

You may imagine that these guys are just little eating machines consuming everything available around them. But despite being ferocious predators, we have found that they are actually quite picky with what they eat. So to determine the availability of larval tuna “food,” we first need to figure out what the larvae are actually eating. This is done through meticulously by dissecting the tiny stomach and intestine of the larvae under a microscope to analyze their diet. As many of the prey are crustaceans, they are difficult to digest, (imagine digesting a crab with its shell, ouch!), so we can identify the species and size based on the undigested remains. By comparing this diet information with what is collected in our zooplankton nets we can see which zooplankton the larvae prefer to eat and how much food is available to them.

We are also studying their feeding habits by looking at their Nitrogen (N) signature. What is a Nitrogen signature? We look at the two naturally occurring isotopes of N (that is different variants of N). This is done in specific amino acids, the building blocks of proteins that make up the body tissues of the fish larvae. The less common 15N isotope is heavier than the common 14N. This weight difference means that 15N assimilated from the food is moved around the larval body and ultimately excreted as waste products at a slower speed relative to 14N. As a result there is a buildup of 15N compared to 14N in the larvae relative to their food.

Nitrogen atoms

Because we know the amount of 15N in the prey, and that the buildup increases at a constant rate for each level of the food chain, from algae to herbivorous zooplankton to carnivorous fish, we can determine how high up the food chain the larvae are.

Some larger zooplankton collected by our ring net

This method gives information on what the larvae have been feeding on the past days to weeks, while looking at the stomach contents only tells us what they have eaten over the last few hours. Also this method gives us clues on other potential preys, such as smaller unicellular protozooplankton that have soft bodies and are rapidly digested and therefore cannot be seen in the stomachs analyzed under the microscope. By looking at 15N we can also tell from where the N is coming from that is taken up by the algae at the bottom of the food chain. We can see this by looking at certain amino acids in the larval tuna where there is no buildup of 15N. Therefore, these amino acids reflects the composition of the N taken up by the algae so if e.g., there is very little 15N compared to 14N it means that the plants are taking up N2 from the atmosphere, and if there is more it means they are taking up Nitrate brought up from the deep.
I am now back in the lab and I can’t wait to start analyzing all the tuna larvae we collected to unravel the secrets of their diet.

A better late than never post for #NF1802, Dr. Stukel #ontheblog

Stukel sets up the satellite-tracker inside the buoys

Although NF1802 has come to an end, we will continue to post for the rest of the summer! Stay tuned!

Today, Dr. Mike Stukel joins us once again on the blog! Mike is an Assistant Professor at Florida State University and one of the PIs on our joint NOAA RESTORE Science project. His PhD students, Tom and Taylor joined him in this survey.


“I study the intersection of plankton ecology and biogeochemistry.  I have a fascination for all of the microscopic critters (plankton) that drift constantly with the currents of the open ocean.  Much of my research focuses on either the role of plankton in converting carbon dioxide to organic matter and then storing that organic matter in the deep ocean (a process referred to as the biological pump) or determining how changes in the planktonic ecosystem affect the availability of prey for fish and other large organisms. During this project, my research goal is to understand how biogeochemical and ecological interactions at the base of the food web affect the survival of larval tuna.

The golden tufts in the bottom left are Trichodesmium, the long organism in the middle is a chaetognath, and many of the out-of-focus blobs are copepods

Specifically, I'm interested in two questions:

1. What processes supply nutrients to the algae at the base of the food web (upwelling that introduces deep nutrients to the surface ocean or nitrogen fixation that converts abundant nitrogen gas into plankton fuel)?  
2. How does the structure of the planktonic food web affect the efficiency with which primary production is converted to zooplankton biomass (i.e. fish food)?  

Illustration by Sabine and Baxter

On this cruise, one of my lab’s goals is to make simultaneous measurements of nitrate uptake and nitrogen fixation in tuna spawning habitat."

Mike looks for microscopic organisms in between stations

Stukel and the team during sediment trap operations on the back-deck

If you are a teacher or student and want to learn more about plankton, check out Mike's lab's website and this link and has lesson plan too!) developed by Ms. Colleen Miks.

PhD student Rachel Thomas on the blog!

Rachel filters water collected at 500 m in the GOM

Rachel Thomas from Florida State University's Earth Ocean and Atmospheric Sciences Department is a doctoral student in the Knapp Lab and she is our guest-blogger today sharing a bit of her research and cruise goals:

"My personal goal on this cruise is to be able to identify bluefin tuna through a microscope. I’ve never seen fish larvae before, so everything the “fish team” brings up in the nets is new and exciting to me! Most of my previous work is involved in the Southern Ocean, where there are unused nutrients in the surface ocean. This gives us a unique ability to look at how phytoplankton are reacting to different physiological stresses, such as light and nutrient availability."
One of these nutrients, is Nitrate (NO3) which is a form of dissolved inorganic nitrogen that is utilized by phytoplankton in the ocean (Malerba et al 2012). One of the aspects of our project (NF1704 & NF1802 is to examine the dynamics between these inorganic compounds at different depths of the surface water column (0-500 m).

Typical nitrate vertical profile comparison, (modified from A. Nahian Avro)

T. Kelly and Rachel Thomas recover the CTD rosette 

"The Gulf of Mexico has a very different light regime compared to the Southern Ocean, and little to no nitrate in the surface waters. Our research team hopes to explore how these contrasts are expressed in the isotopic composition of subsurface nitrate in the Gulf of Mexico. We will also use the isotopic composition of subsurface nitrate in the Gulf of Mexico to understand the nutrient sources for primary productivity fueling bluefin tuna growth in surface waters." 

Click here for last year's blog post from Rachel's experiences during NF1704.

A bit of trivia: the Southern Ocean is the fourth largest of the world's ocean basins and extends to Antarctica. Psst, how many oceans are there? (click here for an answer)

Rachel Thomas and Alanna Mnich ready to deploy an SVP drifter on the fan tail

Today on the blog: Kiana returns!

Kiana rinses the Bongo-90cm 

Today, one of our youngest scientists, Ms. Kiana Ford shares a brief post. This is Kiana’s second cruise aboard Nancy Foster and has been volunteering in our lab since January 2017. She is now a Senior at the University of Miami's RSMAS and got back from an amazing semester abroad with the UGalapagos & Isabella Oceanographic Institute in Ecuador just a few days before the ship started its journey.

Kiana makes friends with the locals in the Galapagos, Ecuador

Kiana shared with our blog “my goal for the cruise is to help find some bluefin tuna! I would also like to be comfortable jumping into any science role during the cruise, from deploying nets on the back deck to running the computer programs in the dry lab. My favorite experiences from participating in the oceanographic surveys is to get to know other scientists and crew members, and seeing how everyone contributes to some aspect of the expedition. My research goals when I return to land is to complete my senior thesis examining larval lobster populations collected in southern Cuba during NF-16-02-03. This summer I am eager to begin my internship with  Harbor Wildwatch, a non-profit organization in Gig Harbor, WA. They organize outreach events to educate the public on marine and environmental topics." 

Thanks Kiana! She disembarked during our port-stop in Pensacola, but we will see you soon in the NOAA FORCES Lab in the Fall semester.