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Posted by Lauren Smith on

In Search of a Plankton Feeder

In Search of a Plankton Feeder

A Basking Shark feeding on plankton. Photo Credit Richard Aspinall

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.

A large female Basking Shark (Cetorhinus maximus) with possible mating scars visible on her pectoral and pelvic fins. Photo credit: Richard Aspinall.

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.


Posted by Lauren Smith on

The Giants of the Ocean April 2010

The Giants of the Ocean

LOL – Lauren on Location – Lauren gambles with the ATM and gets to swim with whale sharks.

After a good nights sleep in Manila I was back in the Airport this time flying domestic with Cebu Pacific (terminal 3), to Legaspi in South Luzon, here I was hopefully meeting up with a representative from Donsol EcoTour / PADI Dive Asia who I had been corresponding with for some months and then travelling to Donsol where I would be staying and (fingers and toes crossed) going out to see the whale sharks.
A mild panic at the airport ensued when I decided to get some more cash out just incase (I had previously exchanged travellers cheques and some US currency at Manila airport and for ease I would recommend other travellers to do the same), I had put my card in the ATM (Country Bank) and it had accepted my PIN but froze when about to dispense the cash, I waited a little while and then thought I should probably press cancel, when my card was still not spat out I became slightly worried and just as my mind started to go into action plan mode of phoning the bank etc, my card re-appeared – phew! I later learnt that it is best to use BDO ATM’s and I would urge others to use these where-ever possible (note the limit for some ATMS is 10,000 pesos per day, however at times certain ATMs will not dispense the 10,000 – instead giving you less, and in other cases it is possible to put your card in several times extracting 5,000 each time – almost like a game…. almost).


The flight across with Cebu Pacific was like no other I have ever experienced! Mid flight the air hostesses play a game where they shout an item out and the first to wave it in the air wins an airline travel pouch, believe me I have never seen so much energy, excitement and enthusiasm on a flight before! Excellent!


On the flight I was seated next to a couple from Arizona, G & Ray, we started chatting and fuelling each others excitement at the prospect of seeing whale sharks the following day. When we landed I felt like what I used to refer to as a “Champagne Backpacker” (someone who claims to be travelling but insists on staying in hotels and who refuses to use the public / cattle class transport), this feeling however did not prevent me from climbing gratefully into the air conditioned pickup that had been provided for me by my Donsol EcoTour contact.


Legaspi is quite striking with Mt Mayon the volcano as the backdrop to the town, as we made our way through the city the concentration of shops and people became less and the landscape gave way to coconut palms, banana trees and rice paddies, the journey through to Donsol took about 1hour and 10 minutes, upon arrival I went into the PADI Dive Asia shop and registered to go out on a boat the following morning; at this point I must pause to thank Jessica Noelle Wong with whom I had been corresponding before my trip and also Ruby Lita who took care of me on-site, their help and generosity enabled me to go out on 2 trips free of charge, the value of such actions are immense in a place where money provided by the whale shark ecotourism venture is vital to its continued success and existence.


That evening I watched the sunset on the beach which gave way to a brilliant red sky, at which point I had already begun praying to the shark god for a whale shark sighting the following day (red sky at night, Lauren’s delight?). I headed to bed full of hope…


I should never have been worried … over the 2 days I was spoilt with 10 whale shark sightings!!!!! All of which I was able to swim / freedive with, the largest shark was estimated at around 12m, the longest interaction time I had was about 20 minutes, and I am pretty sure that one of the females was pregnant (as I dove down her lower abdomen was extremely swollen). The buzz I got out of being in the water with these animals was phenomenal, their grace, their patterns and colouration and of course their immense size.
Here I was cruising along with the biggest fish in the sea – I say cruising, they were cruising, 1 slight change in direction for them meant some hard fin work for me! Just incredible sharks and I felt honoured to be in the water next to them, the length of time with the animals also allowed my mind to register the fact that here I was in their presence, this led to much “YES WHALE SHARKS !!!!” being shouted in my head, plenty of fist clenching and grinning (which led inadvertently to me joining in with the whale sharks diet of plankton, thanks to me flooding my snorkel on more than 1 occasion).
The huge female (12m) which hung around for 20 minutes was exceptional, she gave me eye contact on more than 1 occasion, but didn’t appear to be bothered by me presence in the slightest; after all I probably qualified as an oversized remora with me measuring in at (1.5m). When it came time to let this female be on her way I dropped back and followed behind her for a little while from an appropriate distance, which was immense – I mean how many times do you get to ride a whale sharks wake?!


Way too amazing!


On the evening of the 9th I met up with Jackie Ziegler, a whale shark researcher investigating the effects of whale shark ecotourism around the world for her Masters thesis. I had spoken to her earlier in the day about ongoing whale shark research and sharks / elasmobranchs in general and was eager to discuss more. I headed to where she was staying which turned out to be the Mayors house in Donsol (Mayor Alcantera’s), there I enjoyed rather too much local hospitality and was spoilt with home cooking (had my first encounter with pigs intestine!) but primarily with (what felt like gallons) of Red Horse Beer, G & Ray also joined us which was great but after 5 hours Jackie got more than she bargained for when I had to stay with her for the night having had too much Red Horse to go anywhere!
I left Donsol the following day with a slight headache, but most importantly feeling like I had a connection with the place despite having spent less than 4 days there in total, mind-blowing sharks and incredible people an experience never to be forgotten.


LOL – Lauren on Location