Pelagic Fisheries Research Program

 

One of the greatest perks of being a fisherman is the time it allows us to quietly think while waiting for hana pa’a. All of us seem to wonder about the lives and times of the fish we’re pursuing while out on the ocean.  “Where do “our” tuna come from?  Where do they go?  How long do they hang around the FADs?  Do they ever leave Hawaii? How fast do they grow and how old do they get?  Has the shibi I just put in the box had a chance to spawn yet?  What do they eat?”…well, it is more than just coincidence, but fisheries managers and biologists often have a similar list of questions.  The answers to these and other questions are the basic tools that they need in order to provide sound advice and information necessary for the sustainable management of our ocean resources. That said, one of the greatest challenges facing fisheries managers in Hawaii and elsewhere is the scarcity of sound, accurate and regionally specific data and information about the species they are supposed to manage. For example, a study on the reproductive biology of red weke that was done in Australia may not be accurate or have little relevance to the same species in Hawaii. In fact, using that information in Hawaii could lead us astray. The same is true for wider ranging fish like tuna and marlin.

To address this shortcoming of information, the Pelagic Fisheries Research Program (PFRP) was created in 1992 with funding from the National Marine Fisheries Service. The PFRP provides information on pelagic fish and fisheries to the Western Pacific Regional Fisheries Management Council (WESPAC) from many disciplines, including biology, oceanography, statistics and modeling, bycatch, ecology, sociology, fisheries anthropology, and economics. The term ‘pelagic’ usually refers to the bluewater fish of the open sea like ahi, aku, mahimahi, billfish and the deep-sea sharks.

While many of the PFRP’s projects are well known and highly regarded in the scientific community, you may be more familiar with some of the tagging studies that PFRP has funded over the years. 

The Hawaii Tuna Tagging Project (HTTP) was conducted in Hawaii and surrounding areas, with study objectives to investigate and describe the movement, catch rates and stock structure of tuna in the Hawaiian EEZ and surrounding areas as well as the fisheries that compete for these resources. To accomplish this, around 18,000 plastic   “dart” or “spaghetti” tags were attached to roughly equal numbers of yellowfin and bigeye tuna throughout the Hawaii EEZ all the way to Kure Atoll. 

Over 12.6% of these tagged tuna were later recaptured and reported to the PFRP researchers thanks mainly to Hawaiian fishermen. During the project, recaptures ranged from tuna that were recaptured on the same day as that they were released to a bigeye tuna that was at liberty for over 7.5 years! Tagged tuna were recaptured from as far away as Okinawa and Mexico, but the majority of recaptures were recovered around the Main Hawaiian Islands, on the Cross Seamount where many of the bigeye tuna were originally tagged and on anchored FADs surrounding the main Hawaiian Islands (MHI).  In fact, 83.1% of all the MHI recaptures were caught at FADs! 

Almost all of the recaptures seen in the entire study were taken on seamounts, FADs, koas or banks, which demonstrates how influential the aggregation effect is to the behavior and “catchability” of tuna. Another interesting aspect of the HTTP data was that almost every yellowfin recaptured during the entire project was recaptured within the Hawaii EEZ, suggesting that the majority of yellowfin that settle in this region remain in the Hawaii region. Bigeye recaptures ranged out further with time and were taken north, south, east and west of Hawaii by pelagic longline vessels.  A fundamental difference in life history between the two species may provide at least one reason for these observed differences. Yellowfin are known to engage in a prolonged spawning season close to the Hawaiian Islands while bigeye must move well south of Hawaii before they enter their spawning grounds south of Johnston Atoll. In other words, if you have plenty of food, can grow old and spawn in a favorable environment, why leave?

You can draw your own conclusion on what this means in the broader sense, especially in regards to the potential paradigm shift in the future of tuna management in Hawaii and the broader central and western Pacific. 

Tagging data from the HTTP clearly highlighted the incredible influence that natural and man-made structure, such as seamounts, FADs and banks have on the behavior of bigeye and yellowfin tuna. For example, over 81% of all tuna tag recaptures reported from the main Hawaiian Islands came from fish caught on FADs with 12% being taken on ledges or koas close to shore.

The next series of studies focused on the influence of FADs on tuna behavior and their exposure to fisheries. Bigeye and yellowfin were implanted with specialized transmitter tags and all FADs around Oahu have been equipped with underwater sonic receivers since August 2002. The arrival time, departure time and continuous residence of tuna equipped with a sonic transmitter is recorded on these FAD receivers whenever it swims within about 3/4 of a nautical mile of any Oahu FAD.

All fish in these studies were tagged at a receiver equipped FAD around Oahu and most were only detected at the FAD where they were originally released.  Fish that moved to other FADs usually only visited one or two other FADs before they were not heard from again and apparently left Oahu for good. This may be due to the fact that the original FAD had favorable conditions and when those conditions changed an attractive FAD could not be located. When tuna did visit several FADs they tended to move clockwise around the island or moved within a FAD cluster, such as between S-R-CO or LL-U-MM.  One yellowfin made 14 movements between FADs over 133 days starting at CO on the west side of Oahu and was last reported at MM off Kaneohe. On average, yellowfin remained in the Oahu FAD network for 29 days while bigeye remained only 6 days; but 35% of tagged yellowfin were recorded in the Oahu network for more than one month. The majority of tagged tuna on a FAD left that buoy at the same time or within a few days of each other but a few remained. However, it seems when a FAD lost its appeal most of the fish would leave the FAD, visit one other FAD and then were no longer recorded. Presumably they left Oahu and some tagged tuna were later recaptured on Kauai and the Big Island.  What makes a FAD appealing to tuna remains a mystery that further work may reveal.

Ongoing acoustic studies use special tags that transmit the depth of the tagged fish every minute to the FAD mounted receivers. This data is critical to our understanding of how FADs and floating objects (like logs and cargo nets) influence the depth and behavior of tuna. The influence of FADs on tuna behavior by size and species is necessary to help design ways to avoid small tuna and bigeye tuna in particular by the large purse seine fisheries in other regions. The use of drifting FADs by purse seiners has become the main way tuna is captured worldwide with negative impacts on tuna resources and juvenile bigeye in particular. In this way, the PFRP projects are using our local FADs as important experimental grounds to provide much needed information for the management of the large-scale fisheries that land the majority of tuna worldwide. Along the way, though, we are learning much about how local tuna stocks are using FADs, their migrations, and other parts of their ecology, which is important for regional and local tuna management.

The next questions arose:  where do these fish roam to when they leave Oahu and how long do they remain in the “Hawaiian” region? Do they leave Hawaiian waters and get swept up by purse seiners working south or east of us or do they remain in the central Pacific as the HTTP dart tag data suggested? These questions and others are being addressed by the Hawaii Tuna Tagging Project  2 (HTTP2) was funded in 2008 which  also includes skipjack tuna in its analysis.

HTTP2 is mainly a conventional dart tag study to update information on yellowfin and bigeye tuna and provide the first ever movement and mortality data for Hawaiian aku. The study will also target non FAD associated tuna, which may provide some insight on FADs’ affects on these free ranging schools.  While the large scale conventional tagging for HTTP2 has not begun, archival and acoustic tagging is currently being conducted by PFRP scientists.  These sensitive archival tags, surgically implanted in the belly cavity of the fish, record water temperature, depth, and location using an external light-sensing stalk. These tags must be recaptured and returned to download the data from the tag but a large cash reward is given to cooperating fishermen.

Another useful, but expensive, tool is the satellite pop-up tag.  These devices record location, temperature and depth for long periods and then detach or “pop” off of the animal and float to the sea surface. Data collected over the time it was on the animal is transmitted to satellite, and on to the owner of the tag for analysis. The great thing about this kind of tag is that the fish does not have to be captured to be useful. However, a more complete data record can be downloaded directly from a tag, so please return PAT tags if found. Preliminary data from the HTTP2 are being compiled but more recaptures and returns of archival tags are needed before meaningful analyses can proceed. The over-arching fact remains that if you stay on a FAD, your chances of getting caught are pretty good. We have received quite a few archival tag recoveries but many of these came within one week of when we tagged the fish so provided only limited and questionable data. We believe it takes a few days for a fish to resume “normal” behavior after being implanted with a computer tag so data after one week may be more important. However, some of the longer-term archival tag recoveries and depth transmitting sonic tags have provided us with some nice diving behavior plots for both yellowfin and bigeye tuna.

The Pelagic Fisheries Research Program and HTTP2 will certainly have a busy year ahead of them, with goals to deploy large numbers of conventional and electronic tags around the Main Hawaiian Islands, as well as other projects in fisheries. Mahalo to all the fishermen that have recovered and reported tag recaptures; without your help none of this work would be possible. For all of you out there on the water, closely inspect your tuna for an orange or yellow tag in the back, or a plastic sensor stalk coming out of the gut area of the fish!  When you do catch a tagged fish, please inspect it thoroughly (especially the gut cavity), as there may be up to 3 individual tags in the fish.  When you do catch a tagged fish, please record the following information:  1) Date and position or physical location of catch, 2) Fork length and weight of the fish, 3) species, and any other notes about the catch.  Each tag has our tagging hotline number printed on it, (800) 588-8066. Please leave your name, tag number and your phone number and we will get back to you as soon as possible. This is not only valuable to the HTTP2 project, but you may receive a handsome reward for the return of the tags.

If you’d like more information or have questions about the program, please visit www.soest.hawaii.edu/pfrp/  or
www.soest.hawaii.edu/pfrp/biology/holland_http2.html or contact us directly by email to David Itano dgi@hawaii.edu (PFRP),

Dr. Kevin Weng, kweng@hawaii.edu (PFRP), Jeff Muir jmuir@hawaii.edu (PFRP), and Dr. Kim Holland kholland@hawaii.edu

(Hawaii Institute of Marine Biology/UH)