Sunday, July 19, 2009

The Basking Shark

First off, thanks to Markus Bühler of Bestiarium for inspiring this post by sending me this:


A juvenile basking shark 2.6 m in length from Izawa and Shibata (1993). I was completely unaware of this morphological feature - possible because only a handful of smaller specimens are known. Apparently the elongated snout functions in a similar manner as the cephalic fins of Manta rays.


As the above photo hopefully illustrates, many aspects of basking shark biology remain poorly known. I have a blatant bias towards covering large animals, but given the vulnerability and emerging literature on the animal, I think I can justify this post.



Tracking Basking Sharks

The most interesting aspect of basking shark biology which is now receiving long overdue attention is data on their range and movements. For several decades it was widely assumed (based on very circumstantial evidence) that basking sharks shed their gill rakers and "hibernated" in deep water for the winter! Basking sharks are observed year-round in Monterey Bay, although this population is apparently atypical (Francis and Duffy 2002). Francis and Duffy (2002) analyzed 203 basking shark records (consisting of sightings, captures, and strandings) from New Zealand and determined that basking sharks were swimming in midwater during the winter and did not lose their gillrakers. Sims et al. (2003) used "pop-up" satellite transmitters to gather tracking data for the first time in nearly two decades and directly demonstrated that hibernation does not occur (this study occured off the UK); the study also demonstrated that the sharks traveled extensively vertically and horizontally to productive areas. Skomal et al. (2004) demonstrated that the sharks do not hibernate off the West Atlantic and I think that old hibernation hypothesis can be regarded as completely dead.

Of course, these recent studies have done more than refute the bizarrely widespread hibernation hypothesis. Francis and Duffy (2002) provided unique records of basking sharks 4 km into the brackish Lake Ellesmere - made all the more impressive by (much more common) records of sharks off the continental shelf in water at least 904 m deep. Mancusi et al. (2005) provided the first data on basking sharks from the Eastern Mediterranean and Sandoval-Castillo et al. (2005) provided the first definitive records from Mexico (Pacific coast) - which leads up nicely to Skomal et al. (2009). Skomal et al. (2009) discovered, amazingly, that basking sharks do not have an antitropical distribution as widely assumed previously, but do live in the tropics after all! Sharks tagged off Cape Cod, Massachusetts and found tags in the Sargasso Sea, the Bahamas, Puerto Rican Trench, Caribbean Sea, Guyana, and Brazil - the latter record (a 6480 km trip) is the first transequatorial movement recorded via tagging from a "fish". It seems incredible that a species which was once heavily exploited completely avoided human detection over a considerable portion of its range - it goes to show that basic facts about megafauna are still being discerned. Mitochondrial DNA evidence from Hoelzel et al. (2006) demonstrates that basking sharks have low genetic diversity, a very low worldwide population (effective population ~10,000), and suggested a recent population bottleneck and worldwide panmixia; Skomal et al. (2009) suggested that if basking shark stocks are to recover there will have to be a worldwide effort.



Size & Growth

Since asymptotic size is used for growth calculations, this gives me an excuse to discuss basking shark size. The famous Stronsay carcass was described as a 16.8 m "beast" with all sorts of fanciful features - however its vertebrae were found to correspond exactly in morphology and size with a 9.3 m basking shark (then) recently described by Sir Everard Home (Wood 1982). Wood (1982) mentioned another case where a shark was lashed to the size of a boat and estimated via pacing at ~10.4 m - it was found to be 7.36 m when measured out of the water! Apparently other large figures have been obtained by measuring around the curve of the body. Such is the veracity of basking shark size claims - if a carcass hasn't been scientifically measured in recent decades the data should automatically be categorized as apocryphal.

This by all means doesn't mean that basking sharks aren't huge animals. Francis and Duffy (2002) looked at clasper length in assorted male basking sharks from New Zealand and determined that they reached sexual maturity at ~7.5 m in length; if this is accurate (and it may be somewhat conservative) it means that males in the East Coast region (<>

Anyways, Pauly (2002) used 10 m and ~7.5 tonnes* as an asymptotic size for a von Bertalanffy equation and determined (contra previous studies) that basking sharks do in fact grow very slowly, so slow in fact that they may have a gestation period of 2.6 years. The slow growth also implies considerable longevity (~50 years) and presumably a considerable age at maturity (well over 10 years judging by Figure 4) (Pauly 2002). Estimates of the mortality rate due to the fishery indicates it was impossible for any fish species to withstand for long, particularly one as extremely vulnerable as the basking shark (Pauly 2002). Compagno (2002) notes that in some regions basking sharks numbers have shown no signs of recovery even after decades. It's such a shame that some people are just catching on to this whole "responsibility" thing - hopefully those continuing to hunt the fish in East Asia will figure this out before yet another collapse occurs.

* Wood (1982) cites a 2.99 tonne/7 m individual which scales up to ~8.7 tonnes for 10 m. His 4.65 tonne/7.9 average figure scales up to ~9.4 tonnes for 10 m. I'm guessing that the weight estimation in Pauly (2002) is probably too conservative, but it would be nice to have more data on the subject.



That's about all I want to say about basking sharks. It certainly is remarkable that the biology of a species so heavily exploited could still be a largely emerging picture. Things aren't entirely bleak for the species and fortunately it is getting some good publicity.



References:

Compagno, Leonard J. V. 2002. Sharks of the World. Available (in part)

Francis, M. P. and Duffy, C. 2002. Distribution, seasonal abundance and bycatch of basking sharks (Cetorhinus maximus) in New Zealand, with observations on their winter habitat. Marine Biology 140, 831-842

Hoelzel, A. R. et al. 2006. Low worldwide genetic diversity in the basking shark (Cetorhinus maximus). Biol. Lett. 2, 639–642. Available

Izawa, Kunihiko and Shibata, Terukazu. 1993. A Young Basking Shark, Cetorhinus maximus, from Japan. Japan J. Ichthyol. 40, 237-245. Available

Mancusi, Cecilia et al. 2005. On the presence of basking shark (Cetorhinus maximus) in the Mediterranean Sea. Cybium 29, 399-405.

Pauly, D. 2002. Growth and mortality of the Basking Shark Cetorhinus maximus and their Implications for Management of Whale Sharks Rhincodon typus. In: Elasmobranch biodiversity: Conservation and Management. Available

Sandoval-Castillo, J. et al. 2005. First record of basking shark (Cetorhinus maximus) in Mexico? JMBA2 - Biodiversity Records

Sims, David W. et al. 2003. Seasonal movements and behaviour of basking sharks from archival tagging: no evidence of winter hibernation. Mar. Ecol. Prog. Ser. 248, 187-196. Available

Skomal, Gregory B. et al. 2004. Archival tagging of a basking shark, Cetorhinus maximus in the western North Atlantic. J. Mar. Biol. Ass. U.K. 84, 1-6.

Skomal, Gregory B. et al. 2009. Transequatorial Migrations by Basking Sharks in the Western Atlantic Ocean. Current Biology 19, 1–4.

Wood, Gerald. 1982. The Guinness Book of Animal Facts and Feats.

3 comments:

bill said...

Great Post! I never believed the shark hibernation theory. I learned a real lot about basking sharks! Nice research,very informative. Thanks for the read.

bill;www.wildramblings.com

Ben Speers-Roesch said...

Hi Cameron, nice post and great photo of the "hooked" rostrum of the juvenile basking shark. I've previously (in unpublished online ramblings, I believe) drawn a comparison between this feature and the "horse-like head" of the Naden Harbor carcass, which I think was a basking shark.

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