1. What Do Scientists Attach to a Fish, and What Does Each Tag Record?
You cannot follow a fish through the open sea by eye. So scientists put an identification tag on it, release it, and collect reports when someone catches it again. Linking the release spot to the catch spot shows part of the fish's route. This method is called tagging and release (or mark and recapture).
The Mie Prefecture fisheries research institute in Japan, which has run yellowtail tagging for years, uses two kinds of tag together. One is a dart tag, shaped like an arrowhead, with two attached on the fish's back. Each carries a number that links the fish to its release record. The other is an archival tag, a small device with a sensor that sticks out on the belly side. It records water temperature, depth, and the amount of light, and scientists use those records to estimate where the fish was and which way it traveled [1].
The two tags give different kinds of information. A dart tag gives two dots: where the fish was released and where it was caught. An archival tag fills in what happened between the dots, but only if the device itself is recovered, because the records can be read only after someone gets it back.
So how many fish actually come back?
2. Where Were Nine Tagged Yellowtail Caught Again?
On February 28, 2011, researchers released nine yellowtail off Muroto, on the southeastern tip of Shikoku, one of Japan's four main islands. The fish measured 79 to 88 cm from snout to the fork of the tail. Six were caught again [2].
| Days after release | Where it was caught | How |
|---|---|---|
| 354 | Owase, Mie Prefecture | Fixed net |
| 356 | Muroto, Kochi Prefecture (the release area) | Fixed net |
| 417 | Tosashimizu, Kochi Prefecture | Fixed net |
| 466 | Ito, Shizuoka Prefecture | Fixed net |
| 822 | Shikinejima, an island south of Tokyo | Fishing line |
| 1,097 | Nachikatsuura, Wakayama Prefecture | Fixed net |
The catches are spread widely: to the east as far as Ito and Shikinejima, to the west at Tosashimizu, and to the northeast at Owase and Nachikatsuura. One fish was caught back near Muroto after about a year. A tag also records growth. The fish caught after 1,097 days had grown from 79 cm to 90.1 cm, and the Muroto fish had grown from 84 cm to 86.7 cm [2].
It is tempting to draw a line from the release spot to each catch spot and call it the route. But the table gives only two dots per fish. It does not say how the fish got there, how fast it swam, or whether it took a straight path. The institute's record does not state travel distances, so this article does not either. Six fish caught in six places also suggests that fish released at the same spot do not all behave the same way, but nine fish are too few to say which movement is typical.
How can anyone learn about the part in between?
3. How Can an Archival Tag Show the Path Between Release and Catch?
An archival tag measures temperature, depth, and light, not position. Position is worked out afterward. In daylight, the light level shows roughly when the sun rose and set around the fish, and that can be turned into a rough position. Scientists often compare the tag's temperature record with satellite maps of sea-surface temperature to improve the estimate. Studies that tested this approach against known positions exist, and they treat the results as estimates with an error, not as exact locations [6].
The Mie institute used this method on yellowtail in Kumano Sea, an area of the Pacific off the Kii Peninsula. In March 2004 it released 10 fish, and in February and March 2005 another 22, all with archival tags and each measuring roughly 73 to 93 cm. Of the 32 fish, 25 were recovered, which is a high return rate. All 10 from 2004 came back within 725 days. The catches were west of the release point, between Kumano Sea and the coast of Shikoku, and none was caught east of Enshu Sea. The light records suggested that the fish moved between the area around the Izu Peninsula and the area around Shikoku, and a few may have gone as far as the sea south of Kyushu [3].
The two studies cannot simply be merged. They released fish at different places, in different years, in different numbers, and with different tags. The Muroto fish turned up to the east, at Ito and Shikinejima. The Kumano Sea fish were never found east of Enshu Sea. Both records can be true, and together they hint that yellowtail along the Pacific coast of Japan do not all follow one route. This is the writer's reading, not a conclusion in the institute's reports.
A return rate of 25 out of 32 sounds like proof that tagging works well. But not every tagging program gets so many fish back.
4. Why Do Some Tagged Fish Never Come Back?
The same institute also tags Japanese Spanish mackerel (sawara) in Ise Bay, using orange dart tags. In fall 2023 it released 69 fish in three batches. By December 2025, not one had been reported. In fall 2025 it released 48 more, and one was reported: a fish let go on September 25 was caught 35 days later, on October 30, in Ise Bay off Matsusaka [4].
A missing tag has many possible explanations. The fish may have swum far away, died, been caught without anyone reporting it, or lost its tag. The institute's page does not say which, and nothing in the record can settle it. What it does suggest is this: the places where tagged fish are reported are not simply the places where fish live. They are the places where someone was fishing, found the tag, and told the researchers. A map of recaptures can be thin in an area that the fish visited but nobody fished. That caution is the writer's reading of the data, not a statement by the institute.
The single 2025 recapture was inside the same bay as the release, so it cannot show whether these fish stay in the bay or leave it. Still, the example shows why a tagging program depends on getting reports back.
5. Who Makes Tagging Work After the Fish Is Released?
The people who find the tags. The Mie institute asks anyone who catches a tagged fish to report the date, the place, the tag number, the type of fishing gear, the length and weight of the fish, and their own name and contact details. If the fish carries an archival tag, they ask finders to keep the cable uncut and to keep the fish cold if they can. The institute buys the fish to study its age, sex, and maturity, and the person who reports a tagged yellowtail receives a small souvenir [1][4]. This is the procedure for the Mie program. Other programs use different steps.
The same idea works elsewhere. In the United States, NOAA's Cooperative Tagging Center relies on volunteer anglers and captains to tag fish and report recaptures. In November 2021, two Atlantic sailfish tagged off southern Florida were recaptured: one about 120 miles north after 189 days, and another about 60 miles away after only 14 days. NOAA scientists said the recaptures also show that a fish is still alive, and they can help reveal how far fish travel and how long they live [5].
A tag number alone means nothing. It becomes a fact about a fish only when a finder reports it and a researcher matches it to the release record.
6. Try It Yourself: Can You Map the Yellowtail's Catches?
You can do this at a table with a map of Japan and a pencil. Mark Muroto, Owase, Tosashimizu, Ito, Shikinejima, and Nachikatsuura. Draw a line from Muroto to each of the other five places and write the number of days next to it. How spread out are the catches? Then ask what the lines do not tell you: the real path of each fish, and what happened to the three fish that were never reported.
Your questions are the ones researchers ask next. To read the original records, search for the Mie Prefecture fisheries research institute's pages on yellowtail tagging, listed below, or look up your own country's fisheries agency to see whether it runs a tagging program and how finders can report a tag.