Abundance & Distribution Discovery

New findings by scientists studying the Mid-Atlantic Ridge ecosystem suggest that over 70% of the world oceans are devoid of sharks

Deep ocean exploration over the last 20 years and in-depth study of records dating back 150 years have convinced scientists from the Mid-Atlantic Ridge Ecosystem project (MAR-ECO) that sharks only exist in 30% of the world's ocean (by volume). Scientists from MAR-ECO, a field project of the Census of Marine Life program, hoped that as exploration of the world's deep oceans and abyssal areas progressed, new species of sharks would be discovered. However, based on the research to date, it appears that the deep ocean regions are largely shark-free. The reason for this may be attributed to the relative lack of food available in the deep ocean for large animals such as sharks.

Beginning in the late 19th Century, scientists began to notice indications that sharks were absent from the abyssal depths. Scientists often chalked this up to the primitive sampling methods that were used in those times. However, as the trend continued, the question was raised of whether the absence of sharks in the deep ocean was an actual condition and not just and artifact of primitive sampling methods. Utilizing data collected on a 2004 MAR-ECO research cruise in the North Atlantic between the Azores and Iceland, as well as two decades of research conducted by the University of Aberdeen's (Scotland) Oceanlab and analysis of 150 years of historical records, scientists have concluded that it is unlikely that any more shark species will be discovered in the deep ocean. The deepest confirmed report of a shark sighting was at 3,700 m depth. MAR-ECO scientists believe that there is no hidden population of sharks in the deep sea. The fact that this study includes data spanning multiple sampling and census methods has caused these researchers to conclude that the apparent lack of sharks in the deep ocean is not due to shortcomings in our ability to detect them and that they are simply not present at abyssal depths.

Because such a fact places all the world's shark populations within reach of human-fishing activities, this discovery has serious environmental implications. Sharks are already threatened worldwide by the fishing industry and this new finding suggests that sharks may be more vulnerable to over-exploitation than was previously thought. Another issue raised by this finding is the implications that such a fact could have for conservation or management of demersal species that inhabit the area surrounding isolated islands or seamounts. Such populations may prove to be more isolated, both geographically and genetically, than previously suspected.

Another interesting question raised by this finding is one of the evolutionary ecology of sharks and their relatives. The fact that bony fish have successfully colonized the abyssal depths while sharks, which have a longer evolutionary history, have not is the opposite of scientists' expectations. Seeking answers to the question of why this is may lead to new research and discoveries in the fields of shark ecology and evolution.

What: No new shark species discovered as exploration of the deep ocean becomes more complete
Who: Imants Priede, Rainer Froese, David Bailey, Odd Aksel Bergstad, Martin Collins, Jan Erik Dyb, Camila Henriques, Emma Jones, and Nicola King
When: July 2004
Where: Mid-Atlantic Ridge between Iceland and Azores, North of Hawaii, South Atlantic off the Falkland Islands, North West African Slopes off Angola, North East Atlantic Ocean west of Ireland, and five research cruises in the North East Atlantic
How: ROV observations
Reference: Priede I.G., Froese R., Bailey D.M., Bergstad O.A., Collins M.A., Dyb J.E., Henriques C., Jones E.G. & King N. 2006. The absence of sharks from abyssal regions of the world's oceans. Proceedings Of The Royal Society B: Biological Sciences. Vol. 273: 1435-1441.

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