By Lina Sorg
Depending on the context, transport and mixing by oceanic currents can help or hinder fish larvae from reaching their nursery areas or other intended destinations. This is true of the Atlantic bluefin tuna (ABT, Thunnus thynnus), whose peculiar spawning habits and consequential larval transport have recently captured the attention of scientists. During a minisymposium presentation at the 2019 SIAM Conference on Applications of Dynamical Systems, currently taking place in Snowbird, Utah, Irina Rypina of Woods Hole Oceanographic Institution investigated the water temperatures and advection patterns necessary for successful ABT spawning in the Slope Sea.
The ABT is one of three species of bluefin tuna. They can live up to 40 years, grow to three meters in length, and weigh as much as 600 pounds. These endangered predators are strong swimmers and divers, reaching speeds of 70 kilometers/hour and depths of up to one kilometer. ABT follow the same three-phase migration pattern from year to year, and travel between coastal feeding areas in warm weather, deep water in the winter, and spawning grounds in the spring. Females produce millions of eggs at once. The eggs hatch after four days, yielding three-millimeter larvae that immediately begin to grow. Two weeks later, these larvae are swimming. They reach their full swimming capacity at four weeks, which marks the start of the juvenile state. Larvae at this stage look and behave like adult fish but cannot reproduce. They also prefer warmer water temperatures (between 23 and 28 degrees Celsius) and typically swim close to the surface.
ABT—which fall into two different spawning stocks—are loyal to their spawning grounds, and fish born into a certain stock will return to their respective spawning grounds to mature. The western stock (which matures at nine years) spawn in May/June in the Gulf of Mexico, while the eastern stock (which matures at four years) spawn in June/July in the Mediterranean Sea (see Figure 1). However, Rypina pointed out a peculiar incongruity in this maturation discrepancy; since both stocks are of the same species, those fish that spawn in the Gulf of Mexico can technically do so at four years of age. Yet it takes them another five years to return to the gulf. “What does this fish do between the ages of four and nine?” Rypina asked.
Researchers had wondered for years whether the western stock of ABT neglect to spawn for the missing five-year period, or simply choose to spawn somewhere else. Although they had observed tagged fish in spawning conditions several times in the Slope Sea—bounded by the U.S. continental shelf to the north and west, and by the Gulf Stream in the south—such observations only served as indirect supposition. That changed in 2013, when very young larvae (estimated to be six days old, based on their size) were discovered in the Slope Sea. “There’s no chance that these larvae can go from either the Gulf of Mexico or the Mediterranean Sea to the Slope Sea,” Rypina said. This discovery was the first piece of concrete evidence proving that ABT do spawn in the Slope Sea.
Motivated by this finding, Rypina evaluated the Slope Sea’s suitability for ABT spawning based on the following four questions:
Before using MABGOM2 to simulate ABT larvae, Rypina ensured that the model accurately represents the ocean’s conditions by plotting the observed temperature against the model temperature. “This is just to confirm that the model does what it’s supposed to,” she said. “Our model is not perfect. But if you look near the surface, which is where the larvae are, the mismatch is fairly small.”
Next, Rypina backtracked the larvae’s trajectories (based on their age) to confirm that they did in fact originate in the Slope Sea. This was unsurprising, as the small, young larvae realistically could not have travelled far over the course of a few days. The fact that the larvae did not backtrack to the Gulf Stream suggests that the spawning strategy is likely different from a biological organism targeting the Gulf Stream for transport.
To determine which larvae found success in the Slope Sea, Rypina proposed the subsequent three criteria for effective spawning:
Rypina then displayed four statistical probability maps for successful spawning locations based on the aforementioned criteria. Favorable spawning temperatures are more prevalent in the warmer southern/southwestern parts of the Slope Sea, while the retentive advection patterns prefer northern areas away from the Gulf Stream’s influence. “The bottom line is that the combination of temperature and retention creates hot spots close—but not too close—to the Gulf Stream,” she said. Figure 3 offers a graphical representation of successful spawning times. The black line accounts for all three criteria—spawning and larval temperature, in addition to retention—and is thus the most relevant. According to the chart, ABT spawning begins in July, peaks in early August, and declines rapidly in September. This is later than spawning in both the Gulf of Mexico and the Mediterranean Sea. However, the Slope Sea’s more northern location means that it takes longer for the water to warm to a suitable temperature.
Rypina’s high-resolution MABGOM2 ultimately demonstrates that ABT spawning in the Slope Sea is most prominent when it occurs in close(ish) proximity to the Gulf Stream. In particular, a robust hot spot with a 50 percent time-averaged probability for successful spawning is located near the northwestern bight of the sea. Rypina acknowledges that the larval distributions are filamentary, and that a single survey of larval distribution may not be statistically robust. Nevertheless, her work reveals valuable information about advection patterns within oceanic currents and provides concrete evidence that ABT of the western stock spawn in the Slope Sea from ages four to nine.