Shark survival tale

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A silky shark. [Photo: Alex Chernikh]

As part of a Pacific-wide study, NIWA is measuring the survival rate of sharks returned to the sea by commercial tuna fishers.

The Western and Central Pacific Fisheries Commission (WCPFC) is trying to determine whether the way sharks are caught and released makes a difference to their survival.

The study began in New Zealand waters in 2017–18, then spread to Fiji, and is about to expand further into the Pacific. Mako and silky sharks are the two species selected for the study.

The first stage in New Zealand found that only one of 34 mako sharks released from tuna longlines by commercial fishers died, indicating that they are relatively hardy. Fishers bring sharks smaller than 1m long onto their boats to remove the hooks. Larger sharks are brought alongside the boat and the hook is removed or the trace is cut while it is still in the water.

Warrick Lyon, a NIWA fisheries technician, says the study is heading to the Pacific to test survival rates for sharks treated differently – such as those where hooks are not removed and where long pieces of monofilament fishing line are left attached.

“We want to assess whether the manner of catching and releasing sharks, in addition to age and sex, makes a difference to their chances of survival,” Lyon says.

 

Warrick Lyon, NIWA fisheries technician. [Photo: Dave Allen]

A mako shark is tagged and released. [Photo: Matt Saunders]

 

“The fishers will bring the sharks to the side of their boat to measure, sex and tag. Other than that, they will handle and release the sharks as they usually do.”

The tag measures the animal’s depth over the following two months. A steel attachment pin corrodes after 60 days, allowing the tag to float to the surface. Once there it starts broadcasting its data to the Argos satellite system.

Mako and silky sharks usually move actively up and down in the water column. If the depth of the tag is constant for several days, the shark is almost certainly dead on the seabed.

NIWA scientists, in association with scientists from the South Pacific Commission (SPC), are training fishery observers in New Caledonia and the Marshall Islands to deploy tags on an initial 30 mako sharks and 30 silky sharks caught by tuna longliners.

The aim is to eventually tag 100 each of mako sharks and silky sharks. The data transmitted back by the tags will be collated and the analysis will be completed early in 2019.

Research subject: Sharks