New Research Reveals Broad Spawning Distribution for Bluefin Tuna

Atlantic bluefin tuna migrate over long distances and spend much of their lives in the open ocean, making them notoriously challenging to study. A question persisted for decades: Where exactly do they spawn in the western Atlantic? Now, new research provides more clarity.

Diving Deep into Seven Decades of Bluefin Tuna Data

Working with partners, NOAA Fisheries scientists did a deep dive into bluefin tuna spawning patterns. They compiled a large dataset from fisheries surveys, archive and museum specimens, and research cruise reports going back to the 1950s. Their analysis included more than 35,000 plankton tows, and they examined nearly 5,000 individual tuna larvae. The results, published in Progress in Oceanography, indicate that bluefin tuna have a much broader spawning distribution than previously recognized. In addition to the known spawning ground in the Gulf of America, bluefin spawn in:

• Northwest Caribbean Sea
• North of the Bahamas
• Blake Plateau
• Off of the Carolinas shoreward of the Florida Current
• Western Slope Sea (an area off the Northeast U.S. continental shelf, between the shelf break and the Gulf Stream)

Of these areas, the northern Gulf in the late spring and the western Slope Sea in the early summer produce the most larvae. The results suggest that bluefin spawn in a continuous area during a prolonged spawning season. Spawning starts in April in the southernmost areas-the northwest Caribbean and southern Gulf of America-and ends in early August in the northernmost spawning area, the Slope Sea.

Research fish biologist Dave Richardson, the lead author of the study, explained, "Previous larval studies outside the Gulf of America were often based on a single year of sampling. When we compiled data from many surveys, the consistency was remarkable. When you sample the same area at the same time of year, you consistently find bluefin larvae. This confirmed the pattern we've seen in recent years has been going on for a long time. For example, bluefin larvae have been collected from the 1970s through the 2000s in both the Yucatan Channel in the south and the Slope Sea in the north."

Historically, bluefin tuna have been managed as two stocks-one that spawns in the Mediterranean Sea (the eastern) and the other that spawns in the Gulf of America (the western). Recent larval and reproductive sampling added the Slope Sea to the list of known spawning grounds. Previous research suggests that the populations may mix in the Slope Sea. The scientists conducting this study wanted to know where else bluefin tuna spawn. Mapping all of their spawning grounds is critical to provide a more realistic picture of the population structure. It could also show the extent to which there are unique groups of fish that primarily interbreed with one another.

How do scientists know where fish spawn?

This collaborative team set out to evaluate bluefin tuna spawning patterns in the western North Atlantic since the 1950s and uncover potentially undocumented spawning grounds. They also wanted to understand how long-term environmental change might be affecting where and when bluefin spawn.

The most reliable way to document where fish spawn is either to catch adult fish in spawning conditions or to collect larval fish-most are fewer than 10 days old- in plankton tows. These direct methods provide high confidence, but they require sampling the right place during the right season, which can be tricky. This is particularly a challenge for highly migratory fish like bluefin tuna. Sending a research vessel into the open ocean to search for larvae is expensive and labor intensive.

Researchers can also predict where fish spawn using indirect methods like habitat models and geographic data from electronically tagged adult fish. These methods can cover a larger area and provide clues that tip off researchers to previously undocumented spawning grounds. However, they still need larval or reproductive samples to be certain.

Both direct and indirect methods helped scientists pinpoint bluefin spawning grounds. They used sea surface temperature patterns and data from electronically tagged bluefin to identify areas and seasons that might support spawning. Previous research indicates that bluefin tuna spawn at temperatures from 73-82 degrees F (23-28 degrees C). They then relied on larval or reproductive sampling to confirm whether those areas were spawning grounds.

Bringing Dark Data into the Light by Exploring the Archives

For this comprehensive study, scientists analyzed larval and reproductive data for bluefin from:

• Research surveys
• Museum and archive samples dating back to 1972
• Cruise reports, published data, paper records, and field notes dating back to the 1950s

Most of the research surveys had other objectives that happened to bring them to bluefin spawning grounds at the right time to collect larvae.

The Southeast Area Monitoring and Assessment Program, a state-federal program that collects fishery-independent data in the southeast United States, contributed the most bluefin larvae to the analysis. Since 1982, the program has been consistently sampling the northern Gulf of America during the peak spawning times of April and May.

Plankton sampling on NOAA Fisheries Northeast protected species monitoring surveys also contributed larval samples, particularly the Atlantic Marine Assessment Program for Protected Species. Because these NOAA Fisheries surveys have used consistent larval sampling protocols since the 1970s, scientists were able to use them to compare larval abundance between areas.

Museum and archived samples were critical to this analysis. Scientists measured larval bluefin tuna abundance in previously unsorted plankton samples collected from 1982 through 2021. They also reanalyzed museum collection samples dating back to 1972. Many bluefin larvae were actually "hiding in plain sight"-not identified at the time because larval fish guides stated that bluefin larvae were only found in the Gulf.

"Larval fish look nothing like their adult selves. Fins, pigment, even mouth and eyes develop over the first few days or weeks after hatching," explained Katey Marancik, a research fishery biologist and co-author.

"We need to rule everything else out to confidently identify the species. This takes skill and experience. It comes down to minute details under a microscope, like specific and unique pigment patterns, that change as the larva develops. We did this several thousand times."

This re-analysis was possible because of academic and government institutions that preserve and store plankton samples:

• Harvard Museum of Comparative Zoology
• College of Charleston Grice Museum
• Smithsonian Museum of Natural History
• Virginia Institute of Marine Science Nunnally Ichthyology Collection
• North Carolina State Museum of Natural Science
• University of Rhode Island
• Oregon State Ichthyology Collection
• NOAA Fisheries larval fish archives at the Northeast and Southeast Fisheries Science Centers

"When we think of fisheries, we think of the adult fish that we catch and enjoy as seafood. This study shows the value of ichthyoplankton data-data on fish in their egg and larval stages. It also demonstrates how archives and museums allow researchers to go back and reexamine samples and data from the past," said Jon Hare, director of the Northeast Fisheries Science Center.

"It is exciting to see this careful and thorough analysis change what we know about one of the ocean's most iconic species."

Finally, scientists used reproductive data from fisheries survey reports, published data, paper records, and field notes from exploratory longline surveys going back to the 1950s. From the 1950s to the 1970s, scientists were evaluating the commercial viability of tuna fisheries with exploratory longline surveys. During these surveys they were also often documenting the reproductive status of the fish they caught. Later, during the 1980s and 1990s, scientists focused on sampling the well-documented spawning grounds in the Mediterranean Sea and Gulf of America. They assumed minimal spawning was happening outside of these areas. Based on new evidence, scientists today are looking more broadly again for bluefin spawning grounds.

Understanding Spawning is Key to Management

Documenting where and when bluefin tuna spawn is critical to understanding their population structure, which is necessary to effectively manage the species. Bluefin tuna are highly valued by both recreational and commercial fishermen. The species has a history of stock depletion from the 1970s to the early 2000s, but recent population trends have been positive.

A new genetic method called close-kin mark-recapture can help evaluate connectivity between spawning grounds. This method uses genetic data to identify half-siblings or parent-offspring pairs, and can resolve complex population structures.

"These new data tell us where bluefin tuna are spawning and when, which gives us an idea of where they are migrating and how much they are contributing to future generations," summarized Trika Gerard, co-author on the paper and deputy director at the Southeast Fisheries Science Center. "This, combined with information about genetics and population structure, helps managers decide the most appropriate way to sustainably fish for this prized species."

Timing, Not Location, of Ideal Spawning Conditions has Changed

This study mapped 42 years of sea surface temperatures and looked for changes in suitable areas and times for bluefin spawning. Between 1982 and 2023, spawning areas reached ideal spawning temperatures earlier in the year due to warming ocean temperatures. The Slope Sea in particular is warming faster than other spawning grounds. However, there is little indication that the location of bluefin spawning has changed over the time series.

"Our findings re-affirm that bluefin tuna spawning is related to biology and habitat qualities, rather than arbitrary geographic boundaries. Like most tuna species, they spawn when their internal cues for spawning, including their gonads and energetic stores, match up with external habitat cues, including water temperature," clarified co-author Christina Hernandez, an assistant professor of biological sciences at Old Dominion University. "Because of this, they may be more resilient to environmental change than if they only spawned in one area, for example, the Gulf."

The Windward Passage, between Cuba and Hispaniola, is the only area that was once a documented spawning ground that may no longer be. Longline surveys found fish in reproductive condition there in April from 1955 through 1961. However, a targeted survey in 1976 failed to catch bluefin. We have also rarely observed tagged bluefin migrating to this area in April over the past couple of decades.

A Mystery Left to Solve in the Northwest Sargasso Sea

While this study broadens the known spawning grounds for bluefin tuna, it also makes it apparent that sampling over the years has been uneven. Scientists often rely on finding larvae in plankton samples taken during surveys for other purposes. There could be additional bluefin spawning grounds yet to be discovered.

In particular, scientists are interested in sampling the Northwest Sargasso Sea-south of the Gulf Stream and east of the Florida Current-for bluefin larvae. Bluefin tuna adults occur there in June when the temperatures are right for spawning. Despite including more than 35,000 plankton tows in their analysis, none came from that area. It could be the missing piece to confirm whether bluefin have a continuous spawning distribution.

"We are keeping our eyes open for opportunities to sample in June in the northwest Sargasso Sea, a few hundred miles east of the Carolinas." explained Richardson.

"The plankton tows we use to collect bluefin larvae are quick, typically taking only 10-15 minutes. The challenge and expense is getting to these areas of the open ocean. 'Piggybacking' on existing research surveys can be a cost effective way to sample and ultimately answer longstanding questions about bluefin population structure."