Over the last four years I have been fortunate enough to crew on a Bluefin Tuna charter boat which has been very exciting as seeing them hit a stinger is an awesome sight and a visual demonstration of their raw power.
Trolling for Bluefin also attracts Thresher Shark which is an equally exciting and powerful fish. The Thresher sees the plastic squids on the bar as a small shoal of baitfish and thrashes them with its tail in an attempt to stun them.
You can almost see the look of confusion on the Threshers face when after thrashing the squids with its powerful tail they just carry on regardless.
Although this is exciting to watch it is also quite dangerous for Thresher Sharks, as having thrashed the squids, the stinger which is set a short distance from the squids, can foul hook it in the tail or elsewhere on its body. Since Thresher Sharks are ram ventilators hooking them anywhere other than in the mouth has the potential to drag the shark backwards which restricts the flow of water through their gills which in turn can result in a life changing injury and even death.
Although there is nothing we can do to avoid a Thresher attempting to stun a set of plastic squids on Bluefin bars, take a trolled lure/dead bait, with our without bars, or a live bait, we can and should in my view not purposely set out to catch Thresher Shark using Bluefin gear.
Shark gear uses wire traces whereas Bluefin gear use either monofilament or fluorocarbon leaders which significantly increases the chance of the Thresher Shark cutting through the line, leaving gear attached to it.
The Thresher Shark (Common) is listed by the IUCN as Endangered so we should be mindful that this could attract the attention off those who oppose our sport which always has the potential to lead to some form of restriction which is never good.
Quite apart from that of course we should always work to limit the chances of compromising any fish through a bad hook up situation.
Interestingly there was a study on post release survivorship of Thresher Sharks captured in a Californian recreational fishery. The study assessed post release survival for Thresher caught on what we recognise in the UK as Bluefin trolling gear versus those caught using mouth based angling techniques , ie live bait, drifted dead baits and trolled deadbeats without the attractor bar sets.
Pop-up satellite archival tags (PSATs) were used to assess post release survivorship. The PSATs for the threshers caught on trolling gear were set at 90 day and the ones caught on mouth based techniques set at 10 days.
Of the nine sharks caught on trailing gear, 6 died within 5 days and one after 81 days giving a survival rate of only 22%.
The 7 sharks caught using mouth based angling techniques survived the entire 10 day monitoring period which gave a 100% survival rate.
I have no idea why the scientists didn’t set the PSATs deployment times the same for both groups as this didn’t allow a fair comparison in terms of the monitoring period, however the findings, to my mind, confirm that post release survivorship for Threshers, caught using Bluefin style trolling gear due to the risk of it being hooked in the caudal fin, is at best not good.
There is a school off thought which believes that saying you are fishing for Thresher Sharks when your target is actually Bluefin means the authorities cant prove otherwise and to some extent that’s true. However those who oppose our sport will see it quite differently and will point out that in their opinion there is an unacceptable risk for Thresher Sharks, especially since its IUCN status is Endangered, when using Bluefin trolling gear which has the potential to lead to restrictions in the Thresher Shark and Bluefin fisheries.
Update 19th November 2024
Although the article above has increased awarness in a number of areas it has also led to some readers to suggest a ban on using trolling bars. This was not my intention and neither do I consider it necessary.
Whilst the study shows that Thresher survival rate with gear attached to them is poor, it important that we keep a perspective on this from a UK position.
Firstly the population of Thresher Sharks in the UK is significantly less than in California and despite the many thousands of hours which bars have been used to target Bluefin, both commercially and recreationally, only a very small number have actually been hooked. Although we don't have any means of knowing if they suffered any other injury, I don't know of any that died boat side.
Secondly, trolling bars is the most effective way to catch Bluefin. It is also the method favoured by the scientific community as it reduces fight time and in my opinion is an acceptable method when targeting Bluefin.
The purpose of the article was twofold.
The first was to highlight the issue so we can apply ourselves to finding ways to reduce the risk of a Thresher Shark hook up when using bars. This could be as simple as pulling the gear when one is in the spread and of course not purposely targeting Thresher Shark using bars.
The second reason was that we must always be mindful that there are many who disagree with our sport and would prefer we didn't fish for Thresher Shark or Bluefin. I don't know anyone who purposely targets Thresher Shark using bars but I do know people who claim they are targeting Thresher Shark while tolling bars for Bluefin. It would therefore be quite easy for those who disagree with our sport to combine the Thresher Shark post release study with the social media posts where people are claiming to be fishing for Thresher Shark, when they are actually fishing for Bluefin. They could then use that as a platform to make their case which is something we should work to avoid.
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