Meet the Whale Shark’s Ugly Cousin

The basking shark, Cetorhinus maximus, is the second largest fish in the ocean. They are currently considered “vulnerable” by the IUCN.

My favorite fact about the basking shark is that they are the source of many sea serpent legends. This is because they have a habit of swimming nose to tail, and their long, skinny bodies give the illusion of a serpent-like appearance.

They have even been the source of modern sea serpent legends: The Ziuo-maru carcass was misidentified as a plesiosaur in the 1970s. I remember reading magazines as a kid that showed this photo as proof that sea serpents exist (my life has come full circle, hasn’t it?).

Feeding

Basking sharks feed on zooplankton, like whale sharks. This is a result of convergent evolution, meaning both species evolved this feeding strategy separately. Basking sharks are known to be picky eaters, with a preference for calanoid copepods above all other plankton.

One of the identifying characteristics of basking sharks are gills that wrap almost entirely around the head. When swimming with their mouth open, water is forced to pass through the branchial arches, where gill rakers capture and filter out plankton. This feeding strategy probably explains two of the most common basking shark behaviors: swimming at the ocean surface and swimming nose to tail.

The surface of the water is where plankton are most dense. Basking sharks get their common name because they “bask” in the sun; similarly, one of their Irish names, Liamhán gréine, means ‘great fish of the sun’. Swimming with a large mouth wide open creates a high amount of drag, and swimming nose to tail may help prevent that. 

Location

Basking sharks are migratory, though we have only recently begun to understand their migration patterns. For decades, scientists theorized that basking sharks hibernated in the deep ocean, but satellite tag data have disproven this. However, they have shown that basking sharks do in fact dive to deeper water in the winter months, with one study showing that they have gone to depths up to 1,200 meters. It is not clear why they dive so deep, but the same study found that basking sharks seems to follow a similar diel vertical migration pattern as plankton.

Basking sharks are also found far and wide. I was shocked to discover that they have been seen as far as South Africa. I had always assumed they were only in the northern hemisphere, but it turns out that is just where most of our data comes from.

Life History

Not much is known about the life history of basking sharks. We do know that they are ovoviviparous, that gestation may last up to three years, and that they may take up over a decade to reach sexual maturity. This makes basking sharks particularly vulnerable to fishing pressure.

British Columbia and Basking Sharks

While my master’s research is focused solely on basking sharks in EU waters, in my general research on them, I came across an unusually troubling account of species eradication. This is the story of the basking shark in British Columbia.

Have you ever heard of basking sharks in British Columbia?

No?

Yeah, there’s a reason for that. I will tell you a short but terrible tale. I warn you, it is not for the faint of heart.

As I previously stated, Basking sharks are found throughout the world, and have been documented along every continent (except Antarctica). They were frequently sighted along the West Coast of the United States and Canada. However, in the late 1950s, as Salmon fisheries grew, basking sharks often became tangled in their nets.

Net entanglement, by the way, is still a threat to basking sharks.

Damage to salmon nets was quite expensive, costing individual fishermen up to $3,000 (adjusted for inflation) per net, and so fishermen asked the government to step in (Scott Wallace & Brian Gisborne, 2006).

And step in they did!

In 1949, basking sharks were put on the Canadian federal fisheries department list of “Destructive Pests”, which meant that eradication of basking sharks was government enforced (Scott Wallace & Brian Gisborne, 2006).

What did that result in?

Boats were encouraged to ram into any basing sharks they encountered. The general public were also encouraged to shoot at, harpoon, or generally harass any basking sharks they came across.

But this was the 1950s we are talking about, a period of technological advancement. The Canadian government developed an efficient method of dispatching basking sharks: A literal “shark cutting blade” attached to the bow of fisheries patrol vessels. It’s sole purpose was to cut basking sharks in half.

This amazing piece of technology was even highlighted in the November 1956 issue of Popular Mechanics.

According to Scott Wallace and Brian Gisborne’s book, the Basking Sharks: The Slaughter of BC’s Gentle Giants (I highly recommend it), the shark cutting blade alone was responsible for killing 413 sharks between 1955 and 1969. Basking sharks are virtually unheard of in BC water’s now. This is a shame for BC, as basking sharks are more and more becoming tourist attractions.

I feel like this history is, if not forgotten, not exactly well known. As I continue to research basking sharks, I try to keep in mind that their perception in the public eye has only recently become favorable. This poses a challenge when it comes to not only conservation, but the idea of tourism dedicated to basking sharks.

Also, this brief historical period basically ruins my day every time I think about it, so I like to spread that misery around.

References:

For this page, I’ve generally tried to link to articles available to the public. However, here is a full list of the scientific publications that informed this research.

Basking Shark | CITES. (2003, February 13). Retrieved October 12, 2017, from https://www.cites.org/eng/gallery/species/fish/basking_shark.html

Colin Speedie. (2017). A Sea Monster’s Tale: In Search of the Basking Shark. Plymouth, UK: Wild Nature Press.

Cotton Peter A., Sims David W., Fanshaw Sam, & Chadwick Mark. (2005). The effects of climate variability on zooplankton and basking shark (Cetorhinus maximus) relative abundance off southwest Britain. Fisheries Oceanography, 14(2), 151–155. https://doi.org/10.1111/j.1365-2419.2005.00331.x

Crowe L. M., O’brien O., Curtis T. H., Leiter S. M., Kenney R. D., Duley P., & Kraus S. D. (2018). Characterization of large basking shark Cetorhinus maximus aggregations in the western North Atlantic Ocean. Journal of Fish Biology, 0(0). https://doi.org/10.1111/jfb.13592

Doherty, P. D., Baxter, J. M., Gell, F. R., Godley, B. J., Graham, R. T., Hall, G., … Witt, M. J. (2017). Long-term satellite tracking reveals variable seasonal migration strategies of basking sharks in the north-east Atlantic. Scientific Reports (Nature Publisher Group); London, 7, 42837. http://dx.doi.org/10.1038/srep42837

Gore, M. A., Rowat, D., Hall, J., Gell, F. R., & Ormond, R. F. (2008). Transatlantic migration and deep mid-ocean diving by basking shark. Biology Letters, 4(4), 395–398. https://doi.org/10.1098/rsbl.2008.0147

Harvey-Clark, C. J., Stobo, W. T., Helle, E., & Mattson, M. (1999). Putative Mating Behavior in Basking Sharks off the Nova Scotia Coast. Copeia, 1999(3), 780. https://doi.org/10.2307/1447614

Helfman, G., Collette, B. B., Facey, D. E., & Bowen, B. W. (2009). The Diversity of Fishes: Biology, Evolution, and Ecology. John Wiley & Sons.

Inishown Basking Shark Study Group. (2014). Malin Head Ocean Centre, Marine Park and Biosphere Area.

IUCN. (2005). Cetorhinus maximus: Fowler, S.L.: The IUCN Red List of Threatened Species 2005: e.T4292A10763893[Data set]. International Union for Conservation of Nature. https://doi.org/10.2305/IUCN.UK.2005.RLTS.T4292A10763893.en

Lucifora, L. O., Barbini, S. A., Di Giácomo, E. E., Waessle, J. A., & Figueroa, D. E. (2015). Estimating the geographic range of a threatened shark in a data-poor region: Cetorhinus maximus in the South Atlantic Ocean. Current Zoology, 61(5), 811–826. https://doi.org/10.1093/czoolo/61.5.811

Natanson, L., Wintner, S., Johansson, F., Piercy, A., Campbell, P., De Maddalena, A., … Wedderburn-Maxwell, A. (2008). Ontogenetic vertebral growth patterns in the basking shark Cetorhinus maximus. Marine Ecology Progress Series, 361, 267–278. https://doi.org/10.3354/meps07399

New England Basking Shark & Ocean Sunfish Project | Basking Shark Information. (n.d.). Retrieved April 13, 2018, from http://www.nebshark.org/Information_BS.html

Rus Hoelzel, A., Shivji, M. S., Magnussen, J., & Francis, M. P. (2006). Low worldwide genetic diversity in the basking shark (Cetorhinus maximus). Biology Letters, 2(4), 639–642. https://doi.org/10.1098/rsbl.2006.0513

Scott Wallace, & Brian Gisborne. (2006). Basking Sharks: The Slaughter of BC’s Gentle Giants. Vancouver, BC: New Star Books.

Simon D. Berrow, & Clare Heardman. (1994). The Basking Shark Cetorhinus maximus (Gunnerus) in Irish Waters: Patterns of Distribution and Abundance. Biology and Environment: Proceedings of the Royal Irish Academy, 94B, 101–107.

Sims, D. W. (1999). Threshold foraging behaviour of basking sharks on zooplankton: life on an energetic knife-edge? Proceedings of the Royal Society B: Biological Sciences, 266(1427), 1437–1443. https://doi.org/10.1098/rspb.1999.0798

Sims, D. W., Southall, E. J., Quayle, V. A., & Fox, A. M. (2000). Annual social behaviour of basking sharks associated with coastal front areas. Proceedings of the Royal Society B: Biological Sciences, 267(1455), 1897–1904. https://doi.org/10.1098/rspb.2000.1227

Sims David W., & Reid Philip C. (2002). Congruent trends in long‐term zooplankton decline in the north‐east Atlantic and basking shark (Cetorhinus maximus) fishery catches off west Ireland. Fisheries Oceanography, 11(1), 59–63. https://doi.org/10.1046/j.1365-2419.2002.00189.x

Southall, E. J., Sims, D. W., Witt, M. J., & Metcalfe, J. D. (2006). Seasonal space-use estimates of basking sharks in relation to protection and political–economic zones in the North-east Atlantic. Biological Conservation, 132(1), 33–39. https://doi.org/10.1016/j.biocon.2006.03.011

Valeiras, J., López, A., & García, M. (2001). Geographical, seasonal occurrence and incidental fishing captures of basking shark Cetorhinus maximus (Chondricthyes: Cetorhinidae). Journal of the Marine Biological Association of the United Kingdom, 81(1), 183–184. https://doi.org/10.1017/S0025315401003605

Welton, B. J. (2013). Cetorhinus cf. C. maximus (Gunnerus) (Lamniformes: Cetorhinidae), A Basking Shark from the Late Miocene Empire Formation, Coos Bay, Oregon. Bulletin, Southern California Academy of Sciences, 112(2), 74–92. https://doi.org/10.3160/0038-3872-112.2.74