INFINITESIMAL AND IN COMMAND
September 4, 2017
As wildlife biologists, it is in our nature to ask questions about the organisms that we observe. In fact, the fields of ecology and conservation have their roots in the naturalist movement of the 1800s, when simply spending hours outdoors observing different species was an acceptable form of science. Ironically, even basking shark hunters in this same period took a page from the book of the naturalists, relying on yearly observations to find the “hotspot” locations of these migratory animals. You can therefore imagine that it would be especially difficult to frame formal questions and develop hypotheses in the absence of your study organism.
You would be wrong.
It’s nearing the end of my first field season here in Ireland. I’ve met wonderful people, formed official collaborations with the likes of Queen’s University in Belfast and the Irish Basking Shark Study Group, and developed my project ideas for the next three years here. I’ve been to Derry-Londonderry, Belfast, Dublin, and all around the wild Ireland that is Inishowen. I’ve spent over 100 hours on the Swilly Goose, and probably that same amount of time drinking tea. Much of Ireland is what I expected: the generous hospitality, the vibrant green, the periodic rain and sunshine, the amazing historic sites amidst the fields of sheep and cows. But there is one thing that I didn’t anticipate. The basking sharks, as it turns out, are content to remain below the surface.
I’m not the only one surprised by this behavior. There is some truth to the term “basking shark,” originally named because they often were sighted basking in the sunlight at the surface. This is no sampling bias – indeed, they do spend a significant portion of their time at the surface when the sun is shining. Naturally, it is also easier to spot them in calm weather, and we have been buffeted by rain and wind since my arrival. Yet even in the most ideal conditions, barely a fin has appeared above the surface. Instead, the sharks appear to taunt us with their awkward leaps out of water, only to disappear below the surface again. This is odd even by the standards of the local fishermen, scientists, and citizens. All well in touch with their resident ocean-dwelling neighbors, people here are accustomed to yearly variation in shark sightings. And yet I constantly face the questions: “You have yet to see a fin? But you’ve seen how many breaches? What’s going on?” To a goal-oriented individual and PhD student, one could imagine that these circumstances would be frustrating. And yet, I surprise myself with my acceptance of this unexpected twist. It’s thoroughly out of my control. After all, I’m working in the North Atlantic – it’s not a zoo. More importantly, however, I am intrigued. My instincts as a scientist have kicked in, and as I tour Belfast, sip my tea in Buncrana, and walk along the beach, the question that plagues me is so irritatingly simple.
Naturally, the answers to this question are numerous. Fortunately, my experience on Irish waters over the past month has been necessary to help me formulate plausible theories about these animals, even in their absence. While there may be a variety of factors altering shark behavior in the 2017 season, there is one that inevitably plays a significant role (and one that, as humans, we can certainly relate to): food.
The basking shark is a massive animal. It’s average size is 5-7 meters, with maximum lengths recorded at 11 meters (just over 36 feet). Yet guiding most of its behavior is the tiniest of creatures. Zooplankton are found worldwide in the surface waters of the ocean. Their population contains both juveniles of larger fish and organisms that are destined to remain miniscule throughout their life cycle. In spite of their size, their presence is critical. Zooplankton serve as the foundation for almost every marine food web. Ireland is no exception, and basking sharks specifically have evolved to feed only on this food source. These sharks are filter-feeders, and their gills have special “rakers” that serve as filters to gather plankton as they swim with their mouths wide open. Ironically, it is only because they feed on zooplankton that basking sharks are able to reach such great lengths. Many of the juvenile organisms among the zooplankton will not survive until adulthood, and if they do, it will be several years before they reach a mass sufficient enough to sustain their predators. Yet there is no shortage to the number of organisms born in the ocean that can join the zooplankton rank, and so this is a food source that can continually replenish itself. Consequently, zooplankton has a huge collective biomass, more so than the populations of much larger fish simply because these tiny organisms are so numerous. It is due to the size and sustainability of this resource that they are the ideal food for most of the ocean’s largest animals, including the great whales (such as blue, humpback and sperm whales) and the three largest species of shark (whale, basking, and megamouth sharks).
Because zooplankton is often associated with the upper layers of the ocean, it would stand to reason that basking sharks would “bask” indefinitely. But here’s the catch: zooplankton can move. They exhibit their own sort of migration, which is the largest migration of biomass in the world. Diel vertical migration occurs when zooplankton retreat during the day into the darker reaches of the water column, where they can hide from the eyes of hungry fish. At night, they rise to the surface to feed on phytoplankton, the tiny plant matter that remains at the surface to photosynthesize. The extent of this migration may change seasonally or with environmental factors (temperature, salinity, water mixing, etc.). Of course, changes in the diel vertical migration of zooplankton would naturally alter the behavior of their largest predator. Why bask when the food isn’t at the surface? Yet even this explanation begs more questions. Have zooplankton truly changed their diel vertical migration cycle? If so, what is the cause – weather, climate change, temperature? Perhaps it’s not the migration, but a decrease in zooplankton populations themselves. Regardless of the exact mechanism, there is inevitably something sound in these hypotheses, because when it comes to the basking shark, the infinitesimal are in command.
These are some of the many theories that could explain a field season of mystery, and it’s evident that I have yet to learn most of the story. But what I have learned this year is that there is valuable information to be learned even in the absence of your study organism. There are questions that can arise only through the unexpected, when the sharks that bask dip below the surface. Somehow, I am even more excited to explore the answers to these when I return next July to the land of the leprechaun sharks, this time officially a visiting researcher at Queen’s University, Belfast.
For more information on the evolution of ecology and conservation in the United States, here’s a great accessible read:
Nature’s Ghosts: Confronting Extinction from the Age of Jefferson to the Age of Ecology, by Mark V. Barrow, Jr.
A SPECIAL THANKS...
for the wonderful support in Ireland this field season:
To Emmett, for his guidance, intellectual engagement, and generosity;
To Pauline, for being such a wonderful host and role model;
To Paul and Breda, for becoming my family away from home;
To Dr. Jonathan Houghton, for his scientific perspective, supervision and encouragement;
To Heather, Rosemary, Paul, Boyd, Natasha, and the rest of the lab at QUB - I look forward to working with you.