Conservation & Science

Dispatch from the Sea of Japan: Tuna tagging 101

The Conservation & Science team at the Monterey Bay Aquarium has worked for more than two decades to understand and recover bluefin tuna – particularly Pacific bluefin, whose population has declined historically due to overfishing. A key piece of our efforts is tagging bluefin in the wild so we can document their migrations across ocean basins. Much of our work takes place in the Eastern Pacific, but this summer we’re partnering with Japanese colleagues to tag bluefin tuna in the Sea of Japan. Tuna Research and Conservation Center Research Technician Ethan Estess, working with Program Manager Chuck Farwell, is chronicling his experience in the field. This the second dispatch in his series; you can read the first here.

A Pacific bluefin tuna swims in the Sea of Japan.

Before we go further into our bluefin tagging expedition in Japan, I want to share a bit of background on this fascinating and politically-charged fish we study: Thunnus orientalis, the Pacific bluefin tuna.

You may have read that bluefin are in decline due to overfishing. The challenge is to sort the headlines from the science. Scientists don’t always have perfect answers, but they do use the best data available to make educated guesses.

Here’s a primer on bluefin tuna stock assessment and conservation:

  • Bluefin tunas (there are three species globally) have historically been overfished. The good news is that they are capable of recovery. There are early indications that some bluefin tunas, particularly Atlantic bluefin in the Mediterranean Sea, are making a recovery.
  • Sorting out the “right” population size and appropriate path to recovery is no easy task, and it is hotly debated. Different stock assessment models predict current and future populations based on critical assumptions about the species. Scientists are constantly seeking to fill these knowledge gaps to improve bluefin population estimates and inform international fisheries negotiations.
  • In the presence of such uncertainty, it is probably best to err on the side of caution, using conservative population estimates and scientific best practices to set sustainable fishing levels. Personally, I do my small part by avoiding bluefin tuna at sushi restaurants. (Try some Japanese troll- or pole-caught albacore instead.)


Chuck Farwell, Monterey Bay Aquarium’s tuna research and conservation program manager, gets ready to release a tagged bluefin.

The main goal of our collaboration with scientists from Japan’s National Research Institute of Far Seas Fisheries is to generate new data on a poorly understood aspect of the stock assessment models: the spawning cycles of Pacific bluefin tuna.

Key pieces of data

Understanding spawning is important because it gives us a better estimate of just how many bluefin are out there. Fisheries managers are  interested in what is called “spawning stock biomass”: the combined weight of all reproductively capable fish in a population. Generating a good estimate of spawning stock biomass is key to determining sustainable levels of bluefin fishing across the Pacific.

Counting bluefin in the open ocean is tricky, since they swim from Japan to California and back. Catching them in the act of spawning is even harder. Fisheries scientists use a variety of techniques—genetics, stable isotope chemistry, mark-recapture studies, and more recently, electronic tags—to try and get a good estimate of fish populations. While we’re in Japan, we and our colleagues are focused on tagging Pacific bluefin.

There are a wide variety of electronic tags used to study animals, but the ones used on highly migratory ocean species typically have these characteristics:

  • They record sunlight levels to estimate latitude and longitude—an old sailor’s trick—because GPS doesn’t work underwater.
  • They use a pressure sensor to measure the animal’s swimming depth.
  • They record changes in water temperature.
  • They store information on an internal memory chip.


The tags we use to track Pacific bluefin tuna pop off and transmit their data by satellite.

Each of these tags is worth about six new iPhones. When you attach one to a fish and throw it in the ocean, you really hope you’ll see it again. Many tagged tuna are caught by commercial fishermen who receive cash rewards for mailing the data-filled tags back to the lab.

From sea to space

The satellite tags we’re hoping to deploy on bluefin in the Sea of Japan, however, bypass the fishermen and instead transmit their stored data directly to the lab via satellite. These “pop-off” tags, so named because they automatically pop off the animal at a pre-programmed date, are very useful for answering questions about the migrations of far-ranging animals.

If all goes well, a pop-off tag we deploy on our trip will ride on a tuna’s back for 8 months, recording data the whole time. Then a corrosive metal pin will dissolve, allowing the tag to detach and float to the surface. An ARGOS satellite  will pick up the tag’s signal and upload its stored memory to the researchers. The secret to bluefin spawning behavior will be hidden in more than a million lines of data, and we’ll rely on statistical methods to extract movement and behavior patterns.

Seagulls scan for leftovers from the Sado Island fishing cooperative.

Spawning secrets – revealed

Careful analysis of these data will help researchers pinpoint the spawning areas for Pacific bluefin—results that could inform management decisions. For example, data obtained by tagging a large Atlantic bluefin (which can tip the scales at more than 1,000 pounds) has identified important spawning areas in the Gulf of Mexico. Government agencies responded by closing specific areas to commercial fishing during the spawning season.

When it comes to tuna numbers, there are still so many unknowns. The best way to fill the knowledge gaps is through transparency and collaboration—industry, government and scientific institutions across the world, working together to generate, share and apply the best available science.

Now back to the Sea of Japan—let’s tag some tuna!

Photos by Monterey Bay Aquarium/Ethan Estess.

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