In my tenth shark piece for the Guardian I report on a recently discovered deep sea shark that glows in the dark…
The Basking Shark (Cetorhinus maximus) can reach lengths of up to 12m and is the largest shark in British waters and the second largest in the world after the Whale Shark. Both are plankton feeders, and it is the plankton rich water (primarily along the West Coast) during the spring and summer months which results in these giants visiting our shores.
Despite the basking shark belonging to the same family, as the great white shark (Lamniformes) it is in a genus of its own: Cetorhinidae. Of course being a plankton eater can make it rather more elusive than a great white and baiting it in is out of the question! However understanding more about how basking sharks feed and their prey certainly helps when trying to locate them in the Ocean. Dr Dave Sims and his team at the Plymouth Marine Laboratory have undertaken a substantial amount of work in this area;
Originally it was thought that basking sharks were indiscriminate filter feeders, engulfing whatever was suspended in front of them. However, Sims et al have shown that sharks elect to feed in waters which contain higher concentrations of their preferred prey species which happen to be planktonic shrimp. It is not known for certain how sharks actually locate high concentrations of these shrimp but there are currently a couple of theories. One theory is that sharks are capable of detecting the odour of dimethyl sulphide (DMS) emitted by phytoplankton when it is being grazed on by zooplankton. The second theory is that the sharks can detect activity of their prey using their electroreceptors known as Ampullae of Lorenzini.
Basking sharks feed at varying depths in the water column exploiting optimal food sources (deep sea shrimp have been found in their stomach contents). Sims et al reported that sharks do not feed when the plankton concentration is less than 1 gram of plankton per cubic meter of water, presumably because it is energetically not worthwhile. The higher the plankton concentration, the longer the sharks feed. When the plankton reach concentrations of 3 grams of per cubic meter of water the sharks will feed for up to two and a half times longer than when it’s at 1 gram. When they find a good place to feed they adopt a zigzag swimming pattern, this behaviour is termed “area restricted searching” or ARS. A preference for feeding occurs at current fronts where two water masses of different temperature meet. When the sea is calm less mixing occurs and the water stratifies into different layers, typically warmer on top, cooler below. This may result in the plankton experiencing low nutrient levels. Therefore plankton levels are higher where waters of different temperatures mix, such as at a thermal front. These fronts can be seen as almost slick lengths of still water and can be very useful for spotting sharks near the surface, these fronts can also collect quantities of debris such as jellyfish and seaweed which can make their identification even more obvious.
Basking sharks feed by a method known as obligate ram filter-feeding (Whale Sharks feed by a different technique known as suction feeding). They cruise along when feeding (typically around 1.9 miles per hour), with their mouth wide open, allowing the plankton rich water to pass through the gill slits where it is filtered out by gill rakers, near the rakers are cells which secrete large quantities of mucous when the shark closes its mouth (usually after 30-60 seconds), the rakers collapse squeezing the plankton mucous mixture into the mouth so it can be swallowed.
Many thanks to Richard Aspinall for the use of his images in this post.
References and Further Reading
Sims D.W. (1999) Threshold foraging behaviour of basking sharks on zooplankton: life on an energetic knife-edge? Proc. R.Soc.Lond. B.266:1437-1443.
Sims D.W.(2000) Filter-feeding and cruising speeds of basking sharks compared to optimal models: they filter-feed slower than predicted for their size. Jour. Exp. Mar. Biol. Ecol. 249: 65-76.
Sims D.W., Fox A.M. and Merret D.A. (1997) Basking shark occurrence off south-west England in relation to zooplankton abundance. J.Fish.Biol 51: 436-440.
Sims D.W. and Merret D.A. (1997) Determination of zooplankton characteristics in the presence of surface feeding basking sharks Cetorhinus maximus. Mar. Ecol. Prog. Ser. 158: 297-302.
Sims D.W. and Reid P.C. (2002) Congruent trends in long-term zooplankton decline in the north-east Atlantic and basking shark (Cetorhinus maximus) fishery catches off west Ireland. Fish. Oceanogr. 11:1: 59-63.
Sims D.W., Southall E.J., Richardson A.J., Reid P.C. and Metcalf J.D. (2003) Seasonal movements and behaviour of basking sharks from archival tagging: no evidence of winter hibernation. Mar.Ecol.Prog.Ser 248: 187-196.
Sims D.W., Southall E.J., Quayle V.A. and Fox A.M. (2000) Annual social behaviour of basking sharks associated with coastal front areas. Proc.R. Soc. Lond B. 267: 1897-1904.
Sims D.W and Quayle V.A. (1998) Selective foraging behaviour of basking sharks on zooplankton on a small scale front. Nature: 393: 460-464.
What does Hannibal Lecter, Ernest Hemmingway and I have in common? A taste for fava beans and a nice Chianti? (Chianti – yes), renowned literary skill? (pah! If only!). The answer is in fact the islands of Bimini, Bahamas situated 50 miles off the coast of Florida, North & South Bimini are the smallest habitable islands in the Bahamas with a total area of 9-square-miles and just over 2000 inhabitants.
Hemmingway began visiting the islands in 1935 and spent time fishing and writing, whilst angling he garnered knowledge which contributed to the creation of Old Man and the Sea and Islands in the Stream. An Atlantic Blue Marlin caught off Bimini weighing in at 500lbs was allegedly the inspiration behind these novels.
The final scene of Silence of the Lambs see’s Dr Lecter calling Agent Starling from a payphone on a tropical island, that island being North Bimini.
So what is it about Bimini that resulted in my visit, or to be more precise – my return visit? I had first gone to Bimini when I was conducting research for my PhD in 2006, I was staying at the Bimini Biological field Station (Shark-Lab) founded by “Doc” Gruber and was tracking juvenile Lemon Sharks and investigating their depth utilization.
Bimini is a well known Lemon Shark nursery area first recorded by Doc in the 1980’s, however in addition to the Lemon Sharks the Bimini Islands are a marine biologists and even more so a shark biologists dream, in shallow, clear waters you can see Nurse, Blacknose, Blacktip, Caribbean Reef, Tiger & Bull Sharks as well as Rays, Sawfish and the reason for my trip this time, the Great Hammerhead (Sphyrna mokarran).
There has been talk of Great Hammerheads visiting the waters around Bimini from January through to March for decades, and long is the legend of South Bimini Islands “Harbour Master” a huge Hammerhead patrolling the docks for fishermen’s scraps. However it has only been the last few years that the area has emerged into the main stream and has become recognised as the place to dive with aggregations of Great Hammerheads.
I was there with Dr Craig O’Connell founder of O’Seas Conservation foundation and his team (Guido Leurs founder of Oceaware, Marcella Uchoa and Rachel Jacobson), I had first met Craig on my initial visit to Bimini and having kept in touch and collaborated on a research paper together it was a fitting place to catch up in person! We has rented a boat and were observing and photographing animals in situ by freediving and SCUBA diving.
Put simply, my dives with the Hammerheads blew me away!! Once the bait had gone in the water we were inundated with nurse sharks typically after only a couple of minutes, on average we only had to wait around 20 minutes until the unmistakable outline and approach of the Hammers were spotted from the boat, a speedy kitting up and grabbing of camera equipment ensued (with a quick nod to the O’ring gods) and we descended down to the white sand with depths around 6m. Seeing these animals up close in the water was incredible, I certainly don’t possess the vocabulary or the literary skill to do them justice, they are truly humbling and awesome.
On each dive Hammers exhibited different behaviours and swimming patterns which allowed for some fantastic photography opportunities , unlike the more classic shark body shape the angles and form of the Great Hammerheads combined with the sunlight filtering through the water and the approach of the sharks created the potential for yet another different and interesting shot, the opportunities were endless!
Sphyrna mokarran is the largest species of hammerhead shark, capable of attaining a total length of just over 6m (20ft) with 4m being a more usual maximum length. They are unmistakable with the straightness of the front margin of their head combined with their enormous sickle shaped first dorsal fin, they are a coastal pelagic and semi-oceanic species, considered to be an opportunistic predator feeding on a variety of prey including; stingrays and other batoids, small shark species, groupers, toadfish, jacks, crabs and squid etc. Males reach maturity around 2.5m and females around 3m, females breed every 2 years and have an 11 month gestation period (they are viviparous – pups nourished with a yolk sac placenta), litter size ranges from 6-42.
S. mokarran is classified as endangered by the IUCN Red List with a declining population trend, animals are taken as by-catch and as a target species in longline, fixed bottom net, hook & line, and pelagic and bottom trawl fisheries. Their fins are highly prized for shark-fin soup. These fishing pressures coupled with the Great hammerheads size at maturity and low fecundity make it extremely vulnerable, global population declines range from 79-90% in the last 25 years. In 2014 S. mokarran was listed on CITES Appendix II, which includes a list of species that are not necessarily now threatened with extinction but that may become so unless trade is closely controlled. International trade in specimens of Appendix II species may be authorized by the granting of an export permit or re-export certificate. No import permit is necessary for these species under CITES (although a permit is needed in some countries that have taken stricter measures than CITES requires). Permits or certificates should only be granted if the relevant authorities are satisfied that certain conditions are met, above all that trade will not be detrimental to the survival of the species in the wild.
On the subject of protection I worry about the future of Bimini and its marine wildlife, since I first visited the island 9 years a lot has changed. A Malaysian consortium (work originally began by “Bimini Bay Resort” but is now “Resorts World Bimini”) was granted approval to build a resort, casino, marina complex designed to attract 500,000 tourists a year, as a result mangrove habitats have been destroyed (essential shark nursery areas) and dredging of channels for cruise ships has taken place with the resulting repercussions on the coral reefs and critical habitats for other species such as lobster, grouper, and conch not to mention the natural storm protection for the island itself.
Bimini is a place with the power to turn dreams into a reality, my sincere hope is that people in positions of power will recognise the true beauty of Bimini and conserve it so that these Islands will never be referred to in the past tense, as a paradise lost….
References and Further Reading;
The Ultimate Shark Conservation Experience!
-Sign up for the trip of a lifetime with Saltwater Life & Basking Shark Scotland Summer 2014.
– Learn about Shark Biology, Physiology, Ecology & Conservation
– Observe and swim in the wild with the gentle giants of the shark world – Basking Sharks
More than one in every six assessed shark species is classified by the IUCN (International Union for Conservation of Nature) as threatened with extinction.
Participation in the AWARE Shark Conservation Program taught by Shark Biologist Dr Lauren Smith will provide you with information about shark species, how and why they are being overexploited, current management practices in place, as well as advising you about what actions you can take to aid the understanding and protection of shark species worldwide.
The Basking Shark is the second largest fish in the world and the largest in the UK (reaching up to 12m in length!) and throughout the British summer can be seen cruising around our coastline, congregating in particular areas along the West Coast of Scotland. Here they can be seen “Basking” at the surface – filter feeding plankton out of the water.
During this time you have a unique opportunity to observe the sharks by boat and/or in the water, with Basking Shark Scotland and Dr Lauren Smith on hand to answer any questions you may have about this species. This is wildlife at its best, on their terms in their environment.
– This unique shark conservation experience is ideal for those considering a career in shark biology, and conservation management practices as well as for interested individuals looking to gain a greater understanding about global shark conservation alongside a unique and inspirational encounter with a local shark species (that just happens to be the second largest in the world – pretty awesome!!)
– The AWARE Shark Conservation Program (recognized by an official certification) and Basking Shark trip can be completed in 1 day and will be running on Saturday 16th August 2014 (total cost £250 per person, minimum number of participants for trip to run 6), with the option to extend your time on the water with Basking Shark Scotland.
– For further information about this course, or if you have any questions, please don’t hesitate to contact Lauren here; email: firstname.lastname@example.org website: www.saltwaterlife.co.uk