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
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
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!  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 
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.
Williams P, Terawasi P. 2012. Overview of Tuna
Fisheries in the Western and Central Pacific Ocean, Including Economic
Conditions – 2011. WCPFC-SC8-2012/GN WP-1. WCPFC.
 Sibert J,
Senina I, Lehodey P, Hampton J 2012. Shifting from marine reserves to maritime
zoning for conservation of Pacific bigeye tuna (Thunnus obesus). PNAS. 109(44):
Click to display Randi's posts. Dr. Randi Rotjan is a research scientist at the Aquarium, with expertise in coral reefs, symbiosis, and climate change. She coordinates the Aquarium’s research partnership with Kiribati on the Phoenix Islands Protected Area (PIPA) and co-chairs the PIPA Science Advisory Committee. She is the Chief Scientist for the current expedition to the PIPA, coordinating the expedition by satellite.
Sangeeta Mangubhai, PhD
Click to display Sangeeta's posts. Dr. Sangeeta Mangubhai is an adjunct scientist at the Aquarium. She has been working with the Aquarium since 2000, during the first trip to the Phoenix Islands. This is her fifth trip to PIPA. She is the Chief Scientist onboard the expedition, working with 15 others onboard and Rotjan remotely to study the current El Nino and the impacts on PIPA marine life.
Simon Thorrold, PhD
Click to display Simon's posts. Dr. Simon Thorrold is the Director of the Ocean Life Institute and a Senior Scientist in the Biology Department at the Woods Hole Oceanographic Institution (WHOI). He serves on the Science Advisory Committee for the Phoenix Islands Protected Area. He is a co-organizer of the 2015 PIPA Expedition, working closely with Rotjan and Mangubhai to ensure a successful voyage.