Ocean News

How do you sample an unstressed shark?

2nd February 2010

One of the biggest problems facing anyone interested in the physiology fish is how to generate a baseline level of blood chemistry.  What does the blood chemistry of a fish look like if it hasn’t been captured, handled, poked and prodded all in the name of science?  What are the normal levels of lactate, glucose, carbon dioxide etc., to which we can compare our "stressed" samples to?

For small fish this is relatively easy.  Take the recent work on bonefish by the Flats Ecology and Conservation Program here at Cape Eleuthera Institute.  Bonefish were housed in darkened holding chambers with a steady supply of fresh seawater for 36 hours upon which they were rapidly removed and blood sampled before the blood chemistry could change.  However, it is a tricky proposition to try and apply this technique to a 6ft Caribbean reef shark!

To try and generate a baseline level of blood chemistry for stress physiology work described in the previous post, the shark research team travelled from Eleuthera to Stuart Cove, a well known dive operator in Nassau.  Stuart Cove have been conducting regular shark dives for the last thirty years and the Caribbean reef sharks they deal with are very used to human presence in the water.  The more experienced handlers can gently halt the motion of the shark through the water by gently rubbing a chain mail shrouded hand on the nose of the shark where ultra sensitive electro-reception pores are situated.  This action appears to initiate a response similar to tonic immobility, a reversible coma-like stasis, which is usually triggered by inverting the shark.  The response initiated by the handlers is not as strong as true tonic immobility, but it was hoped that it was strong enough for a diver to quickly draw some blood, thus gathering a sample from a shark that had not been captured or interfered with in any way – a baseline.

For the most part it worked – taking blood in full SCUBA gear, laying on you back under the tail of a shark with your hands clad in chain mail was a tricky thing to do.  And although the sharks were in a mild form of tonic immobility they did not appreciate a clumsy human prodding them with needles, so unless the needle was put in the right place at the first try, they tended to swim off.  Two and a half days of diving provided us with numerous dulled and bent needles, frustrations and thankfully three blood samples.  The blood chemistry values derived from the three samples were vastly different to even the shortest longline hooking durations and represent the first true baseline blood samples taken from a large free swimming shark.

Special thanks must got to Stuart Cove himself whose generosity with his staff and boats were unparalleled.