Applying a transmitter to a shark first requires its capture. Several different capture techniques were used over the course of the thesis, but shark handling procedures were consistent.
Sharks were generally caught using baited hooks, set on scientific longlines, rod-and-reel, drumlines or handlines. Circle hooks, as opposed to traditional J-hooks, were used to minimise the incidence of animals being hooked in the gut – circle hooks are designed to hook fish in the corner of the mouth (Cooke & Suski 2004). This should improve post-release survival as it is much easier to remove a hook from the mouth than the gut. All hooks were also de-barbed to facilitate their removal.
Scientific longlines were predominantly used in shallow habitats (<6 m), where a 100 m line was anchored to the substrate at both ends, with 10 hooks set along it at regular intervals (Figure 5). Floats were placed along the line to keep it off the bottom. Each hook was on a 2 m
‘gangion’: this is where the hook is attached to some metal trace to mitigate bites, the trace is then attached via a swivel to some monofilament to make the line harder to see for the fish, the monofilament is then attached (again via a swivel) to a short piece of line, making the gangion easy to handle. Each gangion is attached to the main longline using a tuna clip, allowing it to be separated from the main line when a shark is caught. To avoid any captured sharks waiting on the line, small floats were attached to each gangion, which were pulled under whenever a shark took the bait. Once a longline was set, it would be left to soak for one hour, with the research vessel waiting nearby to watch the floats and deal with any caught sharks. In the event of capture (signalled by a submersed float), the research vessel would approach the line and an extension line (with a tuna clip) would be clipped onto the gangion.
The gangion could then be removed from the longline, allowing the boat to drift away with the shark, without interfering with the longline. The boat would then be anchored when clear of the line and the shark drawn in to be worked up.
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Figure 5: Image shows a longline set in the lagoon of St Joseph Atoll, Seychelles.
Alternative capture methods included and-reel, drumlines or handlines. Traditional rod-and-reel from the research vessel was used to capture some sharks, using metal trace attached to the end of the monofilament to prevent the line being bitten through. When a shark was caught it would be gradually reeled in to the boat, allowing the workup to commence.
Handlining was performed in a similar fashion: a baited hook with metal trace was attached to the end of a line and lowered into the water from the research vessel. When a shark took the bait, it could simply be pulled in by hand to allow for the workup. Drumlining was used for larger sharks, and involved suspending the baited hook and line from a large float or drum – any shark caught would then initially tire fighting the large float, allowing the research vessel to then pick up the drumline and pull the shark in to be worked up.
Where possible, another capture method was used that did not involve hooking the sharks:
manually lassoing them by the tail whilst SCUBA diving. Ground bait was used to chum the
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water and bring the sharks close enough to capture: a premade noose carried by a diver would be placed over the caudal fin of the shark and tightened (Figure 6). This noose was also attached to the research vessel, allowing the shark to be drawn up to the boat and worked up.
Figure 6: Image shows a tawny nurse shark being captured on SCUBA using a lasso. Photograph by Kyle Gordon.
The workup and shark handling techniques were universal for all capture methods and species.
Once the captured shark had been brought up alongside the research vessel, it would be restrained by tying a rope around the base of its caudal fin (or pectoral fin, if caught on SCUBA). This provided control over both the head and tail of the shark, allowing it to be positioned appropriately alongside the research vessel for the workup. The head would always be orientated towards the bow of the boat so that the current maintained water flow over the gills. If the boat was not anchored (e.g. water too deep), a salt water pump was placed into the shark’s mouth to maintain water flow over the gills. Once in position, the lines would be tied off and the shark rolled over such that its ventral surface faced up. In this position the shark
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goes into a state called tonic immobility, where it relaxes and stops responding to most stimuli (Watsky & Gruber 1990; Brooks et al. 2011), making the subsequent workup significantly easier, quicker, and less stressful.
If the shark were to receive an acoustic transmitter, this was usually done first while the shark was upside down and in tonic immobility. Acoustic transmitters (either V13 180 s nominal delay or V16 120 s nominal delay, Vemco Ltd, Bedford, Canada) were surgically implanted into the shark’s abdominal cavity, via a small incision (2.5 cm) made through the abdominal wall (Figure 7). The small incision was closed with three sutures (Ethibond Excel 4 x 75 cm non-absorbable coated, Ethicon Inc., Somerville, USA). A small tissue sample (1 cm) would then be taken from the tip of the shark’s anal fin for genetic analysis by collaborators. This also served as an indicator of previous surgery if a tagged shark was recaptured, as the incisions healed so well – a shark recaptured nine days after tagging was almost fully healed. While upside down the shark’s sex was noted, as well as notable features that may indicate sexual maturity (e.g.
calcified claspers for males, mating scars/abdominal movement for females).
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Figure 7: Image showing implantation of an acoustic tag into a shark’s abdominal cavity. Photograph by Rainer von Brandis.
The shark would then be rolled over so its dorsal fin faced upwards again. This allowed several length measurements to be taken, all to the nearest centimetre and starting from the tip of the nose. The first was the precaudal length, measured to the precaudal peduncle at the base of the tail. This provides a universal measure of body length, independent of any damage there may be to the caudal fin (e.g. bites). The second is the fork length, measured to where the tail forks, with the third being total length, measured to the tip of the upper caudal. If the shark were to receive a satellite tag, it would be performed at this point due to the easy access to
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the dorsal fin. Satellite tags (SPOT5, Wildlife computers, Redmond, Washington, USA) were attached near the tip of the first dorsal fin - using the nylon bolts, steel washers and steel nuts provided by the manufacturer - to maximise chance of signal transmission to overpassing satellites when the animal was near the surface (Figure 8). A drill and template was used to create the holes in the appropriate places (shark fins are predominantly cartilage with very little innervation (Compagno 2001)). The shark would then also be tagged with a small Floy Tag (T-bar anchor, Floy Tag, Seattle, Washington, USA), which was anchored with a small barb under the skin and contained contact details in the event of recapture by someone else.
Figure 8: Image shows a SPOT5 bolted to the first dorsal fin of a tiger shark. Photograph by Daniel Beecham.
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With the workup complete, the shark would then be released. First, while the shark was held by the dorsal fin, the hook would be removed using a pair of pliers or bolt-cutters. Then the tail rope would be removed, but the dorsal still held to make sure the shark was swimming before being released. Once swimming, the dorsal would be released, allowing the shark to swim away and be tracked by the corresponding method.