Yellowfin tuna, aptly named for their bright yellow dorsal and anal fins, are quite striking visually, much more so than the other tropical tunas that inhabit the Phoenix Islands. This is the tuna commonly referred to as “ahi” on sushi menus, though ahi can sometimes refer to bigeye tuna as well.

Credit: NOAA Fishwatch via

The metallic blue/green of their backs are separated from their silvery white bellies by a thick, yellow stripe starting at the upper jaw and ending near the tail. Also notable are the bright yellow finlets outlined in black, which run from the second dorsal and anal fins to the tail. As they age, the long second dorsal and anal fins become prominent and may reach lengths greater than 20 percent of the fish’s total length, but as juveniles, yellowfin look quite similar to both skipjack and bigeye tuna, and the three species often school together when they are young.

Like skipjack tuna, yellowfin are batch spawners, capable of spawning every few days once they’ve reached maturity at about 3 feet (100 cm) in length. Because they mature at a relatively young age (1–2 years) and spawn frequently, they are fairly resistant to fishing pressure. However, the segment of the population that inhabits the tropical Pacific, where 95 percent of landings occur has been heavily impacted by the explosive growth of the purse seine fleet and the use of FADs. Though conservation measures have been enacted to keep the stock from becoming overexploited, proper monitoring and compliance by fishing fleets must be ensured to maintain the population at healthy levels.

Yellowfin tuna is often called "ahi" from the Hawaiian word for tuna ʻahi.

In the waters of Kiribati, yellowfin are caught by both purse seine vessels and longline vessels. Kiribati has a small domestic longline fleet, but most yellowfin caught within its territorial waters are landed by vessels from distant water fishing nations (DWFNs). An average of 35,000 metric tons (more than 77-million pounds) of yellowfin are landed annually by all gear types (purse seine, longline, other artisanal methods), with a value of around $28.4 million USD [1]. Though tuna catches contribute only minimally to the Kiribati GDP (because very little tuna is landed or processed in Kiribati), licensing fees paid by DWFNs to fish for yellowfin and other tuna species in Kiribati account for greater than 40 percent of government revenues. Obviously, maintaining healthy populations of yellowfin and other tuna species will be essential for the economy and welfare of the Kiribati people.

Currently the only legal commercial activity going on inside this marine protected area—an area the size of California—is tuna fishing by Distant Water Fishing Nations (DWFN) under bilateral and multilateral treaties. Purse seine and longline vessels target yellowfin, big eye and skipjack tunas destined for, among other countries, markets and restaurants here in the United States.

Climate change, which is projected to affect the abundance and distribution of yellowfin and many other important commercial species in the Pacific, will be another added stressor that could have negative long-term impact on revenue from tuna fisheries. Current model projections are optimistic for Kiribati, indicating that increasing water temperatures and other factors that go hand-in-hand with climate change are likely to increase the abundance of skipjack and yellowfin in the region.

Credit National Undersea Research Program via

However, there is substantial uncertainty around these projections due to the complexity of interactions among climate change variables, so they must be viewed with caution. Closing PIPA to tuna fishing is an important part of Kiribati’s commitment to maintain sustainable tuna fisheries within its territorial waters in the face of inevitable changes in the near future. Implementing the closure, along with other measures to control fishing effort and FAD use, can offset fishing excesses in other areas and some of the changes likely to occur with an increasingly warm ocean.

Guest blogger Jennifer Goldstein has worked in several different departments at the New England Aquarium, including the Edgerton Research Lab and the Conservation Department's Sustainable Seafood Programs. Her career as a researcher and analyst has focused on large pelagics, primarily tuna, including research into the reproductive biology and foraging energetics of bluefin tuna. More recently, she's been researching aspects of sustainability of various tuna fisheries.

Jennifer recently left the Aquarium to focus her career on science and environmental writing. Here she presents a series of posts for us on the species of tuna that can be found in the Phoenix Islands Protected Area of Kiribati.

Bigeye tuna (Thunnus obesus) are another tuna species common in PIPA. They share some behavioral and life history characteristics with skipjack, but differ in important ways as well. Like skipjack, bigeye are highly migratory, traveling across international boundaries and are found in the temperate and tropical regions of all oceans (except for the Mediterranean Sea). Bigeye are also sequential batch spawners capable of spawning one batch of ripe eggs, and then as another "batch" of eggs matures, are able to spawn again a few days later. They also share the attraction to floating objects and often school beneath them, particularly as juveniles.

Bigeye grow to be much larger than skipjack, and can reach lengths of six and a half feet (200 cm) or more and weights over 350 lbs (160 kg). They mature later (at around four years of age) and live longer (to approximately 12 years of age) than skipjack. They are quite similar in appearance to the bluefin tunas (Pacific, Northern and Southern), but do not grow as large. Bigeye also have remarkable diving abilities, and are capable of reaching depths greater than 1600 feet (500 m) while searching for food. At these depths, the ambient water temperature can be as much as 20° C lower than the surface temperature, and there is very little light penetration. Bigeye, aptly named for their relatively large eyes, have a tapetum lucidum, or light reflecting layer, behind the lens that helps them to see in low light conditions. Skipjack (and yellowfin tuna as well) can’t tolerate such extreme temperature gradients, and tend to remain closer to the surface to avoid depths that would expose them to water temperature differences greater than 8° C. This diving ability and eyes that are adapted to see at very low light levels, allow bigeye to exploit prey resources not available to other tuna species. 

Bigeye tune close-up via Wikimedia Commons

Bigeye are under heavy fishing pressure in the western Pacific. A longline fishery for mature bigeye has been operating in the western Pacific for decades, and in 2011, the longline catch of adult bigeye was 67,700 metric tons—almost 150-MILLION pounds of fish. However, juvenile bigeye are caught incidentally in the purse seine fishery for skipjack as well, because the two species school together, particularly around FADs. In 2011, the incidental catch of juvenile bigeye tuna in the purse seine fishery for skipjack was 77,100 metric tons—that is 169,976,404 pounds and almost 10,000 metric tons more than the directed catch of adult bigeye in the longline fishery! [1]  The figure below shows the increase in purse seine catches of bigeye compared with other gear types since 1990. Though the international body that manages the tuna fisheries in the western Pacific, the Western and Central Pacific Fisheries Commission (WCPFC), has adopted restrictions on FAD use and closed areas to halt the overfishing of bigeye, those measures have proved insufficient to date.

      

 

Catches of bigeye tuna in the Western and Central Pacific from 1950 to 2011.Note the sharp increase in landings by purse seine vessels starting in the early 1990s. Source: ISSF Status of the World Fisheries for Tuna, 2013: http://iss-foundation.org/

So what affect will the closure of PIPA to tuna fishing have on bigeye populations? At this point, it’s difficult to say, however, a recent study by John Sibert and colleagues [2] suggests reasons to be hopeful. Sibert’s study modeled a suite of different closure and effort control measures in the western and central Pacific management area using catch data from 1990 to 2003 (the time period during which the purse seine fishery and FAD use exploded). The idea was to determine what different closure and effort scenarios would have on bigeye population trends compared with what we already know occurred during this time period in the absence of these adjustments. Results indicated that closure of the high seas areas between 10° N and 10° S latitude and 170 ° E to 150° W to longline fishing would be beneficial to the bigeye population. This is because this region encompasses the western edge of a known spawning area for bigeye, so by protecting mature adults, more bigeye will be able to spawn and add recruits to the next generation. Since the 408,250 km² of PIPA fall within the boundaries of this wider spawning area, one would expect to see an added benefit from increased protection of spawning size adults when PIPA becomes off limits to tuna longlining and other modes of capture.