The following essay is both sad – as it follows the passing of a long-term and cherished colleague – and uplifting, as it celebrates his years of contributing at FHL and to marine science education.  The authors continue both of these by teaching “John’s course” every other summer at FHL, for which we are grateful!

Best,
Dr. Megan Dethier, FHL Director
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A Tribute to John Fleng Steffensen

by Paolo Domenici and Jacob L. Johansen

This article is a tribute to John Steffensen, who was a fish physiology expert from the University of Copenhagen.  John taught the Fish Swimming course at FHL from 2002 to 2025, and passed away unexpectedly on September 27^th 2025 after a back surgery.  We would like to begin with this image of John teaching outdoors at Friday Harbor Laboratories.  It captures, better than almost anything else, his enthusiasm for both science and teaching.  The story behind the image is simple, and quintessentially John.  One day during our 2013 Fish Swimming course, we found ourselves without a lecture room.  This was likely the result of a scheduling mix-up (probably of our own doing), not uncommon during the busy summer season with its shifting weather and fishing schedules.  After a few minutes of searching, John decided that a classroom was unnecessary.  All he needed, he said, was a bit of open space.  He found a whiteboard, set it up in one of the most beautiful spots at FHL, picked up a felt pen, and began teaching.  With nothing more than that board and dry-erase marker, he delivered a lecture that was as engaging, insightful and entertaining as any he ever gave.

That moment reflects John’s relationship with technology.  He was remarkably knowledgeable about the latest tools, cameras, computers, gadgets of all kinds, and if you needed advice, John always knew exactly what would suit your needs.  Yet at the same time he loved the simplest, most traditional ways of working.  Even during some of his most polished lectures, complete with Powerpoint slides, he would suddenly reach for a piece of chalk and begin drawing ideas, mechanisms, and questions on the board.  Therefore, we remember that outdoor lecture not as an exception but as a wonderful one, entirely characteristic of John.

Above all, John was defined by his infectious enthusiasm.  He carried it into every space, among colleagues, friends, and students alike.  John never lost the childlike curiosity that lives within all of us: the impulse to keep asking questions, to take things apart just to see how they work.  He was endlessly fascinated by animals and was curious about how they manage to achieve feats that humans can only dream of.

The most recent of John’s fascinations was the Greenland shark, an animal he suspected lived not only slowly, but extraordinarily long.  One of John’s most exciting lectures during the Fish Swimming course focused on his quest to unveil the secrets of longevity in these sharks.  He devoted much of his final work to this mystery, ultimately revealing that Greenland sharks are the longest-living vertebrates known, capable of living for more than 400 years.  Because sharks lack bones that can be used for aging, John turned – characteristically – to an unconventional solution.  He recalled a murder case in which the time of death had been determined by aging the cornea of an eye.  Following this lead, he contacted the coroners involved across international borders, and persuaded them to examine Greenland shark eyes.  That insight led to one of the most remarkable discoveries of his career: the astonishing age of these old-and-cold sharks.  Of course, studying the Greenland shark also meant traveling to Greenland and learning about the place and its people, something John loved deeply.

The scientific questions John asked were rooted in animal functions, particularly the links between physiology and behavior, and were usually the result of direct knowledge of the phenomena that he or others had observed in nature.  One of John’s favorite ideas – and the subject of one of his fascinating lectures – was the idea that large schools of fish use up so much oxygen that the fish at the back of the school experience hypoxia.  This idea traced back to a 1968 Science paper by Bill McFarland, whom John and Paolo had met at Friday Harbor Labs many years earlier, and whom they both greatly admired.  As in the case of the Greenland shark and the hypoxic fish schools, the history behind the ideas, and how we arrive at asking certain questions, was very important to John.  Thus, his lectures often began with observations or studies from the 1970s or earlier, and it was a privilege to hear him recount – sometimes from first-hand experience – how scientific questions evolved over decades into modern understanding.  This of course was an excellent lesson for students to learn, and not the sort of fundamentals one reads about in textbooks.

John co-designed the Fish Swimming course and he was a driving force for it.  As co-instructors, we learned a lot from him and he will always be an inspiring reference for us.  This course has been taught biennially since the early 2000s and is largely research oriented.  The instructors for the first two courses were John Fleng Steffensen, Paolo Domenici and Guy Claireaux.  Later courses were taught by Paolo Domenici and John F. Steffensen, with the addition of Jacob L. Johansen since 2013.

John was always eager to interact with students from all over the world, as he liked to travel and was curious to learn about different cultures.  Thus, the course was designed to be a melting pot of cultural exchange.  Over the years, the course has attracted graduate students and advanced undergraduates from more than 25 countries across Asia, South America, Oceania and Europe, as well as the United States and Canada.  This international breadth has been one of this course’s defining strengths, fostering interaction among students from diverse cultural and scientific backgrounds.  Lectures and explanations of experimental set-ups are concentrated in the first two weeks of the course, while the second part is devoted to student-led research projects conducted in small groups of three to four.  John particularly enjoyed working with students and colleagues to develop innovative approaches to studying fish swimming, especially at the interface of physiology and behavior.  Projects ranged from swimming tunnel respirometry under varied flow regimes to high-speed video analyses of schooling dynamics and escape responses.  He not only inspired students to formulate creative and rigorous experimental questions, but he was also instrumental in troubleshooting the inevitably technical and conceptual challenges that occur during early experimental design and implementation.

In many cases, the experiments carried out during the course were publishable material, and John remained generously available to guide the students after the course, mentoring them through conference presentations and the publication process.  His guidance contributed to more than 20 peer-reviewed papers in internationally recognized journals from the Fish Swimming course at FHL, leaving a lasting scientific and educational legacy.

John was equally at ease in the lab and in the field.  Fishing days based at the Labs were among his favorites.  He piloted the boat, ran the operations, and guided students with efficiency and confidence.  In addition to radiating enthusiasm in the lab and the field, John shared ideas freely, offered advice without expectation, and showed little interest in power, competition, or recognition.  What drove him was curiosity; pure and undiluted.

As Adam Summers wrote to us recently, John was an amazing combination of kind and brilliant.

John, you are deeply missed!

Paolo Domenici and Jacob L. Johansen,
FHL Fish Swimming course instructors

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