We’ve shown some amazing pictures of predatory sharks this week, but the biggest shark in New England’s waters is a gentle giant, feeding on tiny crab-like creatures called copepods. The basking shark, so named because it is often observed feeding near the surface,“basking” in the sun, is not only big, but is still a huge scientific mystery.
I recently caught up with one of our New England ocean celebrities, Greg Skomal, noted shark expert and Senior Fisheries Biologist with the State of Massachusetts, who filled me in on the state of basking shark knowledge.
Robin: How often do you encounter basking sharks in your work?
Greg: If we’re actively working on them, we find them frequently.
Robin: How do you know where to find them?
Greg: We use a spotter plane. Also, fishermen and whale watchers will help us figure out where the sharks are.
Robin: Have you spent much time in the water with basking sharks?
Greg: I have – some of that work has been to tag them underwater using a pole spear, but I’ve also filmed and photographed them.
Robin: How do they behave around humans?
Greg: They are generally shy, but I see two kinds of behavior. It’s either one or two sharks moving in straight line, searching or going someplace – cruising. If you want to dive with them you get the boat to put you right in their path, but they tend to get out of your way pretty quickly. The other kind is group behavior – small to large aggregations. You just jump into the water with them. Individuals will start to dive, but the group as a whole continues to feed.
The best way to dive on basking sharks is to find a group of them.
Robin: Do you ever feel any threat from them?
Greg: No, but I will say it’s an overwhelming experience because of their sheer size, so there’s some anxiety about just being in the water with such a big animal.
Robin: How big do they get?
Greg: They’ll get in excess of 30 feet long. I’ve seen two that big – beached animals. Much of what we know about basking sharks comes from beached animals.
Robin: What can you say about the shark in this photograph?
Greg: Here’s the fascinating thing about this picture – the snout on this animal is more pointed than on the average basking shark, which tells me it’s a juvenile and probably not very big. Basking sharks are born with a pointed snout, and they lose that over time and it becomes more rounded.
Robin: Why does that happen?
Greg: We don’t know a lot about the biology of these critters – about the changing of morphology and shedding of the gill rakers. For the second largest fish on earth we know amazingly little about its natural history.
Robin: What do you mean by shedding of gill rakers?
Greg: Gill rakers are used to sift plankton – like baleen in a whale. Basking sharks shed their gill rakers over the winter. Because of this, and their disappearance in the winter, scientists in the 1940s and 50s hypothesized that the sharks went to the bottom of ocean to hibernate.
This theory was published in a scientific journal, and was accepted for many years, until we started tracking them recently. It wasn’t until the turn of century that we started tagging these sharks and found out where they do go.
Previous scientists were correct that the basking sharks in the eastern Atlantic go deep and move off the shallow shelves, but they don’t go far. They just move to deep water where they don’t interact with humans and so were not encountered. This was discovered by David Sims and his crew in tagging studies. He was the first to demonstrate that they don’t hibernate.
So, we did same studies here on the western side of the Atlantic, but found that our basking sharks do something very different. They go to deep water, but they also go really far away. We published a paper in 2009 showing that they move to tropical areas: the Bahamas, the Caribbean, even south of the equator to Brazil. They make very broad migratory movements – some of the greatest ever described by science for a fish. But they still stay at great depths (3,000 feet or more) when they move.
Robin: Why do they go so deep?
Greg: You just asked the best question there is. We don’t know why. We hypothesize that movements of fish are driven by food and reproduction. We know they could go somewhere besides the tropics to get all food they need, so they might be traveling there to reproduce. They might come to the Gulf of Maine to mate, and then pregnant females might move south to gestate.
Robin: So they might be like right whales?
Greg: They do show up where right whales go, but they also go much further south. They go from the Gulf of Maine to south of equator. It must be energetically beneficial to do this, otherwise, why do it? But we can’t prove it yet.
Robin: Have you noticed a change in their numbers or distribution over the years?
Greg: It’s tough to know. The general thinking is populations are down in the Pacific, and perhaps the eastern Atlantic. But we don’t know about the western Atlantic. We don’t have populations trends here because we don’t have the data.
Robin: What are the greatest threats to basking sharks?
Greg: Bycatch, right now. They are sometimes taken by fixed gear as bycatch, in lobster, conch, or any other kinds of traps. Sharks inadvertently get wrapped in lines. Sometimes they get caught in bottom trawls and gill nets.
We don’t know what the implications of climate change will be. Maybe there will be a shift to the north, or a shift vertically in the water column. If you can determine what’s going to happen with the copepods they eat, you’ll have better sense of what might happen to basking sharks.
It’s an amazing species – phenomenal to be in the water with. I’ve put a lot of time into studying them and hope to continue to do so.