A Caribbean reef shark in a mild form of tonic immobility whilst being blood sampled by the CEI shark team.

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.

A Caribbean reef shark blood sampled underwater.

Stuart Cove Shark Dive

Drawing Blood from a mature male Caribbean reef shark.

Small scale longline surveys are the predominant method for investigating shark populations, and when longlines are implemented on a much larger scale, are responsible for the widespread commercial harvest of sharks all over the world. Any capture event, including longline capture, unleashes series of physiological and physical disturbances, the issue is that very little is know about how this physiological stress impacts the behaviour of an animal post release, or if indeed the animal survives.

This year’s project took a two stage approach to begin to investigate the effects of longline capture on the Caribbean reef shark (Carcharhinus perezi).  Firstly, blood samples were taken from sharks that were captured during our longline surveys, using hook timers to accurately determine the amount of time the shark had been on the line.  Blood was taken from the shark and portable blood analysers were used to quantify various blood chemistry parameters which in turn indicate the level of physiological stress the shark was under for a given duration of hooking.  Secondly, a subset of fifteen sharks were equipped with acoustic transmitters which emit an ultrasonic series of pings every 45 seconds which can be detected by an array of underwater hydrophones.  These transmitters had a three-dimensional accelerometer incorporated into the tag which measured the activity level of the shark every 20 seconds post release, the data for which was in turn transmitted and stored on the seabed hydrophones.  The hydrophone array itself consisted of 32 receivers covering approximately 14 square kilometres of seabed in prime reef shark habitat.  The use of these transmitters allowed us to quantify the activity level, depth association and movement patterns of the Caribbean reef sharks post release and begin to understand how capture events might impact their behaviour.

The project officially drew to a close in November 2009 after taking blood from over forty Caribbean reef sharks and collecting approximately 33,500 detections  from the accelerometers attached to them.  The shark research team is currently collating the results which will be presented at the annual American Elasmobranch Society meeting in Rhode Island in July 2010.  The team will take part in a special symposium entitled The Physiological Stress Responses in Elasmobranch Fishes organised by Dr. Greg Skomal of Massachusetts Division of Marine Fisheries, and Dr. John Mandelman of the New England Aquarium.

An accelerometer attached to the dorsal fin of a Caribbean reef shark

An accelerometer equipped Caribbean reef shark just prior to release.

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