TAGGING ALONG
Saving one of ocean’s finest fish
TINNED, pan-fried or sliced into fresh sushi steaks, tuna is one of the world’s most popular table fishes. But with nearly six million tons of tuna removed from the world’s oceans every year for human consumption, there is no guarantee there will always be plenty of these tasty fish in the sea for our children.
That’s one of the reasons why local anglers have been asked to keep an eye out for any tuna species with bright red or yellow plastic tags projecting from their backs.
As part of a global scientific research project to monitor the health of tuna and tuna-related fish species in the Atlantic Ocean, marker tags are being inserted into 120 000 of these fish, some of which will also carry miniaturised data-collection gizmos.
So far, nearly 67 500 fish have been fitted with the special plastic tags in a vast sea area stretching from South Africa to Brazil and from West Africa to the island of St Helena in the South Atlantic Ocean.
More than 130 yellowfin tuna and skipjacks were tagged last year about 64km west of Cape Town and six have been recaptured over the last 15 months, helping to throw more light on how fast they grow, where they go to and other data that could help scientists to understand more about the sustainability of tuna stocks.
Stewart Norman, a Cape Town-based fisheries consultant and observer who helped to tag the fish off the Western Cape coast early last year, said the project was funded by the European Union and more than 50 member states of the International Commission for the Conservation of Atlantic Tunas (ICCAT).
The commission was set up in 1969 to conserve these fish populations.
“Tuna are highly migratory and are known to swim large distances each year in search of favourable feeding and spawning conditions. These migrations may even extend across entire oceans. Some initial indications suggest the yellowfin tuna tagged off the Cape are swimming up the east coast in the direction of the Mozambique Channel,” said Norman.
“There have recently been good signs of yellowfin on the east coast and we are therefore asking all anglers fishing for tuna off KwaZulu-Natal to be extra vigilant and to report any tags that are found.” Norman said yellowfin tuna were also seasonal migrants to the Cape coast each year.
“So far the only fish recovered in South African waters are those that were also tagged here,” he said, “but that is not to say that we shouldn’t be expecting migratory fish from the tropics to appear in our waters – in fact the likelihood of this is high!”
Most of the marked fish will carry yellow tags, while about 10% will have red tags.
Those with red tags have been injected with oxytetracycline (OTC), a chemical marker that, once recovered, allows scientists to precisely calculate how fast the fish grew (between tagging and recapture) by looking at the amount of calcium carbonate deposited on their bones.
Norman said red-tagged fish also have electronic archival tags surgically implanted in their body cavity. These tags can record up to 600 days of data such as depth, temperature and salinity preferences of tuna to help understand how tuna survive in the vast pelagic environment.
The tag recovery team from CapMarine and the Department of Agriculture, Forestry and Fisheries will focus efforts from South Africa’s main ports and landing sites, as well as catch reports from commercial, recreational and charter vessels.
According to an ICCAT newsletter, more than 12 000 of the 67 500 tags fitted in the Atlantic since 2016 have been recovered (a recovery rate of nearly 18%).
Norman said the tagging data recovered so far suggests that bigeye tuna travelled over twice the distance per month than previously found, and skipjack almost three times the distance previously recorded, averaging approximately 1 200 and 1 500 nautical miles per month, respectively.
Yellowfin tuna did not travel as extensively compared to previous findings, averaging 800 nautical miles per month.