Dr Kim Holland from the Institute of Marine Biology, University of Hawaii recently held a shark tagging workshop during his visit to Seychelles under the MADE (Mitigating Adverse Ecological impacts of open ocean fisheries) programme being implemented in Seychelles by IRD and local partners.
Within Seychelles, the objective was to place pop-up satellite relayed tags (PATs) on a number of pelagic shark species found on the offshore fishing grounds, as had been done on tuna, to help define their movement and behaviour patterns. It was also planned to deploy a number of the new ‘mini-PATs’ developed by Wildlife Computers especially for smaller marine animals.
Unfortunately previous studies had encountered high failure rates of tag attachment and so prior to the deployment it was felt appropriate to hold a practical tagging workshop to test how well different tag-anchors held in real conditions, using a number of dead sharks previously caught in local fishing activities.
When attaching PATs to small sharks (below 3 m in length) the usual method has been to place the anchor dart through the base of the dorsal fin so that it passes between the cartilaginous ‘fin-rays’ and then uses them like a picket fence for the anchor to wedge behind. On larger sharks the tags are often placed into the muscle or dermal layers, as on whale sharks, and so the anchors require a somewhat greater holding ability.
The first day of the workshop was an introduction to the theory of anchor design where Dr. Holland passed on information from veterinarians regarding how tissue deals with foreign objects such as tag-anchors or darts… generally, rounded surfaces without sharp edges were better at promoting tissue adhesion, while sharp edges would tends to cut through tissue adhesions if the anchor or dart moved in any way.
A variety of tag darts and anchors were available for testing the following day; this included the titanium PAT or 'M-dart used by many research teams, a new plastic toggle anchor under development by Wildlife Computers, a modified nylon ‘slip-tip’ dart with two large wings from Dr. Mike Musyl (NOAA), an umbrella dart designed by Dr. Mike Domier (Pfleger Institute of Environmental Research ) and a new design being developed by Dr. Holland himself.
The three good muscle/tissue anchors, from the top the new Wildlife Computers toggle design on its stainless steel applicator, middle is the titanium PAT or M dart, and at the bottom the nylon 'slip-tip' design from Mike Musyl.
The first three of these tags did well in pull tests using ballistic foam, a good substitute for muscle or skin tissue, but the last two darts did poorly in this medium. In experiments on the shark carcasses to test how well each tag performed in being inserted between the fin rays and locking behind them, the latter two tags, the Domier umbrella and the ‘Holland’ dart worked well if the anchor was inserted fully between the fin-rays.
Of the three other attachments, the new Wildlife Computers design faired well but was found to be harder to push through the fin rays while the Titanium anchor dart tended to cut through the fin rays and thus weaken their supporting ability to help retain the tag; the big winged anchor was too large to test on smaller sharks.
The two effective intra-fin-ray tags: top is the 'Holland' anchor with the tether attached at right angles to the shaft, below is the umbrella dart from Mike Domier on its applicator tool.
The ‘Holland’ dart was very unusual as the tag was attached at right angles to the shaft of the tag, a design that it was hoped would reduce movement of the anchor itself.
After the practise session it was out to sea for a week for the tagging team to try to put out the first batch of tags…. But first you have to catch your sharks... here's Kim hard at work trying to do just that!
We wait to see how they got on and look forward to the coming whale shark season to try out some of the new anchors...