Manta Malady

I wasn't sure what I was seeing in this observation taken by Kate Malcolm (Dive! Tutukaka) until Clinton Duffy explained.
The image shows a large Manta Ray, Mobula birostris and six Yellowtail Kingfish, Seriola lalandi. Clinton pointed out the hook that wraps over the upper jaw and the trailing fishing line that runs along the fish's back. He noticed that there are several large clumps of encrusting organisms on the fishing line. These and the line are abrading the skin in a couple of places leaving long white scars.
While this is an unfortunate occurrence, it was presumably accidental and the hook should corrode away in time. Hopefully the fish won't be affected too badly in the meantime.
Both Mobula birostris and Mobula japanica are protected in New Zealand waters, where Kate's observation was made. Kate stated, "We see a load of life at the Poor knights - more mantas than ever this summer, and more turtles than ever the winter just gone."
Kate has now uploaded a video that clearly shows the fishing line and the injury it is causing.
Posted by markmcg markmcg, February 01, 2018 01:52


Remember birostris is a Mobula now too...

Posted by gmoo over 4 years ago (Flag)

You are spot on @gmoo! Thank you. Hold fire... update about to happen. M

Posted by markmcg over 4 years ago (Flag)

Done. Thanks again @gmoo. I was so concerned about getting the post out I overlooked this. I'll look at having it changed across iNaturalist.

Posted by markmcg over 4 years ago (Flag)


Posted by gmoo over 4 years ago (Flag)

What an amazing shot! And not being a fishy person, I've had lots of questions about this photo...
One is, how does a manta ray accidentally get hooked up like that...just as it is swimming along, and the hook was floating in its path? It seems like very bad luck... :-(

Also, has anyone got an idea on how long it takes for a hook like that to rust away enough to 'fall off' the poor manta?

Posted by alexbluemountains over 4 years ago (Flag)

Thanks Alex. Great questions. Mantas are generally regarded to eat plankton. I'm not sure if they would take a bait. Perhaps someone in the community knows. @julianpepperell any ideas? I'll email Kathy Townsend. @clinton @julianpepperell Any ideas on the issue of how long it would take for the hook to rust away?

Posted by markmcg over 4 years ago (Flag)

I had no idea!

Posted by henrick over 4 years ago (Flag)

Manta and devil rays are actually quite a common bycatch in tuna longline fisheries. Some are no doubt hooked accidentally after blundering into the line but others probably actively take the bait. Several of the devil rays actively hunt baitfish and I know of several that have been hooked in New Zealand waters after taking lures or trolled live-baits.

As for how long it would take the hook to corrode out that depends on what the hook is made from. I was able to follow a large great white at Stewart Island, nicknamed 'Tracey' because of the hook and trace hanging out of the corner of her jaw, for four years. The hook and trace were stainless steel. They appeared new when we first saw her. After a year the trace had halved in length, after two years the trace was gone, and after four years the eye of the hook and half of the shank had corroded away. As stainless steel is supposed to be resistant to corrosion I assume that hooks and tackle made from other types of metal will corrode at a faster rate, as will tackle made of combinations of different metals (this enhances electrolysis). Temperature is also a factor.

Posted by clinton over 4 years ago (Flag)

A video of this ray can be seen here:

Posted by clinton over 4 years ago (Flag)

Fascinating stuff, especially about mantas hunting baitfish and taking lures. What's the association with the kingfish about? I was out at the Poor Knights (and beyond) a couple of weeks ago on a seabird-watching pelagic trip and we saw what at first we thought was a manta not far from the Sugarloafs. As we approached we saw that it was a sunfish, lying on its side, with several kingfish around it. As we approached it righted itself and swam away strongly, but a few minutes later was back at the surface, not sure if the kingfish were still with it. As an aside, the sunfish was strongly marked with many bright silvery spots - would it have been the new species, Mola tecta, or can M. mola also have a pattern like this?

Posted by number8dave over 4 years ago (Flag)

The kingfish are just being a nuisance to the ray (and the sunfish), they use them as 'rubbing posts' to remove parasites. Kingfish, trevally and other jacks will also do this to sharks.

Your sunfish could have been Mola tecta, its the most common sunfish in NZ waters, but it could also have been Mola alexandrini (was Mola ramsayi). Mola mola is thought to be rare around NZ.

Posted by clinton over 4 years ago (Flag)

Agree with @clinton on all points. Have often seen devil/mobula rays at domestic long-lining landing docks in places like Taiwan, Sri Lanka and India, and yes, they are a by catch in our own longline fishery, but at low levels and all would be cut off, preferably with line-cutters close to the hook. Some would presumably break off though. Regarding sport fishing, I have heard anecdotes of very occasional hooking of 'manta' rays which definitely took a whole fish bait (from memory, small baitfish such as yellowtail (Trachurus novaezelandiae).

Posted by julianpepperell over 4 years ago (Flag)

Some recreational fishers actively seek out mantas in order to target the kingfish and cobia that commonly accompany them. So maybe not such an "accidental" hooking.

Posted by allanlugg over 4 years ago (Flag)

@clinton @julianpepperell @number8dave This is a fascinating exchange. Thanks guys, I've learned heaps. :)

Posted by markmcg over 4 years ago (Flag)

Hi all,
Thanks for your amazing posts! I can certainly say I've learnt a lot about manta rays, and fish now (well, more than I ever have before)...
Very interesting!
:-) Alex

Posted by alexbluemountains over 4 years ago (Flag)

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