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.