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!
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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.
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