LIHU‘E — Where do Hawai‘i’ʻs ahi, yellowfin tuna, go when they’re not found in island waters?
Do they stay near the shore, or swim into entirely different parts of the ocean?
Kaua‘i and Kona anglers are working with marine scientists to find out, using cutting-edge satellite tagging of mature tuna.
A new paper in the journal Frontiers in Marine Science (published March 19, 2020) reveals intriguing details about Hawai‘i-caught ahi. It is entitled “Complex dispersal of adult yellowfin tuna from the Main Hawaiian Islands.”
Some ahi appear to stay close to home, while others range far, far afield, according to the tagging research. They seem to be around the Hawaiian Islands when water temperatures are most appropriate for spawning.
Local ahi stocks appear healthy, although some tunas elsewhere have been overfished — notably Pacific bluefin and juvenile bigeye in the Western Pacific.
Scientists and anglers are anxious to be sure island-associated yellowfin remain a healthy resource for years to come, said Kevin De Silva, a Kaua‘i fisherman active in the tagging research.
“Generations of Hawai‘i residents have depended on the ahi fishery, and we want to be certain that our grandchildren and their grandchildren can continue to enjoy fresh, locally caught fish for their poke, sashimi and ahi steaks,” De Silva said.
New research conducted off Kaua‘i is one of the first pieces in a larger inquiry designed to determine whether ahi are strictly a local population or part of a Pacific-wide population, as is currently assumed.
The tagging data also bares the perilous life of tuna. They often do high-speed dives to water nearly a mile deep to escape predators like false killer whales. Sometimes it works and they escape. Sometimes not.
“Fisheries scientists have been calling on ways to elucidate the complex lifestyles and migrations of long-lived tuna and billfish in order to improve population assessments, and to assure fisheries sustainability against changing oceans, fleet dynamics, regulations and seafood markets,” write the authors, Chi-Hin Lam, Clayward Tam, Donald Kobayashi and Molly Lutcavage, who lives in Lihu‘e.
Lam and Lutcavage are associated with the Large Pelagics Research
Center at the University of Massachusetts and the Hawai‘i-based Pacific Islands Fisheries Group. Kailua resident Tam is with Pacific Islands Fisheries Group and Kobayashi with the National Oceanic and Atmospheric Administration’s Pacific Islands Fisheries Science Center in Honolulu.
Local anglers who assisted in the research include De Silva, Ryan Koga, Eric Hadama, Curtis Matsumura, Mark Oyama, Nathan Abe, Mike Abe, Gary Shirakata, Kevin Awa, Marvin Lum, John Kauhaihao, Craig Koga and Cory Nakamura, as well as Basil Oshiro, Gavin De Silva, Bryan Hayashi, Alan Horikawa, Joe Koerte, Ryan Kusunoki, Justin Pasamonte, Lance Gilbert and the late Reid Shigeki Takayama.
They were responsible for everything from “fabricating tagging equipment to recommendations on how to catch and handle the big ahi to ensure optimal survival,” said Lutcavage.
They conducted tagging research from 2014 to 2016, collecting samples from dozens of harvested tuna and attaching satellite tags to 19 ahi weighing between 90 and 200 pounds. Those fish were then released.
The satellite tags record where the fish swims, how deep, how fast and at what water temperature they are found. The tags are programmed to automatically release from the fish at nine to 12 months, at which time they float to the surface and begin reporting the data to receivers on satellites. Sometimes the tags release prematurely, but even shorter tracks reveal many surprises.
“Adult yellowfin tuna frequenting the Main Hawaiian Islands have more complex movements than previously assumed,” the authors said. Some fish seemed to stay around the Hawaiian Islands. Others went off on long swims as much as 1,280 miles from the islands in different directions in less than 60 days.
One of the fish appeared to have been eaten by a warm-bodied predator, likely a false killer whale or short-finned pilot whale, Lutcavage said. That was determined by the tag’s records of the fish’s final attempts to escape, by the fact that the tag suddenly stopped sensing light, and by temperature information that suggested the tag had passed into the digestive tract of a warm-bodied animal — an animal with a body temperature consistent with a false killer or pilot whale.
In the daytime, ahi normally stay within the top 600 feet or so. Whales do, too, but separate whale-tagging studies show they also sometimes do fast, deep dives.
“Rapid descents to deeper depths by tagged yellowfin mirrored depth profiles of potential predators, especially pilot or false killer whales: the dash to depth may be an effort by a yellowfin to outrun its predator,” the paper said.
Several of the tagged tuna showed indication of high-speed deep dives. One of the tuna dove from 415 feet deep to 4,650 feet in five minutes, and managed to escape. Four more engaged in deep, fast dives right before their tags came off — so their fate is not known. The one tuna was clearly eaten and its tag spent some time in a whale’s body before popping to the surface to report its data.
In addition to the tagging studies, Lutcavage, Tam and local fishermen collected samples from harvested fish, and were able to show that many are in spawning condition when around Hawai‘i. Ahi spawn multiple times annually, and generally when water temperatures are between 75 and 82 degrees. Stomach contents confirm that the fish are generalists, eating small fish, squid and crustaceans.
Lutcavage said her team plans additional tagging work off Kaua‘i and across the state, to continue to build an understanding of ahi biology and behavior. Ahi are so important to local culture, economy, food security and diet that the information is critical to their sustainability and to continued access to the fishery, she said.
Here is a link to the paper: doi.org/10.3389/fmars.2020.00138
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Jan TenBruggencate is a longtime Kaua‘i science writer and author.
