The long yellow sickles on a large yellowfin tuna usually get the attention. Look farther back, toward the narrow tail base, and another feature appears: two rows of much smaller fins, one above the body and one below it.
Those little fins are finlets. They move with the swimming fish and influence the surrounding flow, but calling them extra propellers oversimplifies what researchers have found. Their story is more interesting than a simple claim that more fins mean more speed.
For anglers, understanding the distinction helps separate fish biology from tackle decisions. A tuna's swimming anatomy does not prescribe a stickbait retrieve, a reel ratio or a drag setting.
Where are the finlets on a yellowfin tuna?
Yellowfin tuna, Thunnus albacares, have small, separate finlets behind the second dorsal and anal fins, continuing toward the tail. They are different structures from the large, elongated second dorsal and anal fins that give many adult yellowfin their dramatic outline.
The NOAA Pacific yellowfin profile describes the torpedo-shaped body, metallic dark-blue back and bright yellow dorsal and anal fins and finlets. The conspicuous long fins of an adult should not be confused with the series of little fins behind them.
Think of the rear of the fish as a sequence: larger median fins, rows of small finlets, the narrowing tail base, then the crescent-shaped tail. A quick side view may conceal the lower row. Not seeing a feature clearly is not evidence that it is absent.
Are finlets unique to yellowfin tuna?
No. Finlets occur in other members of the scombrid group, including other tunas, bonitos and mackerels. Their presence alone cannot establish that a fish is a yellowfin.
Color is not enough either. Light, viewing angle and the condition of the fish can affect what a photograph shows. Use several features and a suitable regional identification guide rather than assigning a species from one yellow detail.
For a different comparison, our yellowtail vs yellowfin guide explains why California yellowtail are jacks, not tuna. Here, the question is not which fish has finlets, but what the finlets actually do.
They are small fins, not rigid decorations
In Tuna locomotion: a computational hydrodynamic analysis of finlet function, researchers combined high-speed video of swimming yellowfin with computer simulations based on the observed movements.
The finlets both heaved and pitched. In plain language, they moved from side to side with the rear body and changed their angle. Their motions were also staggered along the row rather than occurring as one perfectly synchronized rigid panel.
That matters because a fin interacts with moving water through its position, orientation and timing. Counting fins in a still photograph cannot tell you the forces they create during a swimming cycle.
The researchers examined the flow structures around multiple finlets, not just an isolated triangular shape. Neighboring fins can interact through the water moving between and behind them.
Do finlets produce extra thrust?
Not in the simple way the word propeller suggests. In that study's simulations, the finlets produced drag rather than net thrust. At the same time, interactions between them reduced their combined drag, and pitching affected their energetic cost and the forces associated with rear-body movement.
These findings are compatible. A component can resist forward motion while an arrangement or movement reduces that resistance relative to another arrangement. Reducing a cost is not the same as generating forward thrust.
The study also found redirected cross-flow and organized wake structures. This supports a hydrodynamic role without turning every small fin into a miniature engine.
Why “21.5% less finlet drag” does not mean “21.5% faster tuna”
The paper reported that interactions among finlets reduced total finlet drag by 21.5% in its modeled comparison. The word finlet is essential.
That is not a measured 21.5% reduction in the drag of the entire fish. It is not a 21.5% increase in swimming speed, and it does not predict how long a hooked tuna will run.
The model also had defined conditions and limitations, including a reduced Reynolds number and a specified range of steady-swimming motions. The authors cautioned against extending the conclusions automatically to other speeds, maneuvers, accelerations or species with different finlet shapes.
A percentage becomes useful only when you know what was compared. Finlet drag, whole-animal energy use, forward speed and fighting behavior are different outcomes.
What about experiments on steady swimming?
A separate study of finlets and yellowfin steady-swimming performance reported no statistically significant treatment effect on swimming speed or tail-beat frequency in its experimental comparison.
That does not make finlets functionless. It means those measurements, under those conditions, did not establish the proposed performance difference. Local flow effects, routine swimming measurements and performance during rapid changes are not interchangeable tests.
The professional reading is therefore neither “finlets guarantee extreme speed” nor “finlets do nothing.” They are part of a complex swimming system, and the answer depends on the movement, measurement and comparison.
What should an angler do with this information?
First, enjoy noticing a detail that is easy to overlook. Observe swimming fish from an appropriate distance without pursuing them for a closer anatomical view or extending handling to inspect fins.
Second, keep the biology separate from the presentation. A steady-swimming finlet model does not show that a yellowfin wants the fastest possible lure, a fixed pause or a lure with matching yellow trim.
Watch whether your actual lure tracks, recovers and remains clear of its leader. A clean presentation is something you can assess. An assumed relationship between a tuna's finlets and your handle speed is not.
Our yellowfin stickbait retrieve guide addresses that separate practical task. Use it to diagnose lure behavior, not to infer a fish's hydrodynamics from a missed strike.
Match yellowfin casting tackle to the complete lure
For a surface-casting setup, compare the actual lure assembly and intended method before choosing rod power. The GOOFISH Purple Diamond popping rod lists 280cm, two-piece spinning construction, MH power, MF action and a 40–100g lure range.
The GOOFISH Titan Surge saltwater popping rod provides a different reference: approximately 270cm, two-piece spinning construction, H power, MF action and a 60–200g lure range.
These are different working ranges, not rankings of how effectively each rod can oppose a fast-swimming fish. Count the complete lure, including hooks and rings, and assess available casting space. Neither a lure rating nor a line rating is permission to lift fish with the rod.
Reel gearing does not set the lure's speed by itself
The GOOFISH YB Series saltwater spinning reels offer YB50, YB60, YB70 and YB80 models with listed 5.9:1 gearing. Compare model-specific capacities against actual line diameter, along with physical fit and overall handling.
Gear ratio does not give one fixed line pickup per handle turn. Spool fill and winding cadence matter, while rod input and line geometry affect how that pickup reaches the lure. Maximum drag is not a normal fishing setting or a substitute for assessing the complete outfit with the skipper.
Plan the mainline and connections together
With GOOFISH Max-8 braided fishing line, verify the selected option before planning spool fill, leader and knots. Do not infer diameter, breaking strength or supplied length from the product name.
These products are individual specification references, not a tested matched kit. Expected fish size, working drag, usable line capacity and the landing plan still need an appropriate assessment. A streamlined fish does not make one outfit suitable for every encounter.
Yellowfin tuna finlets: quick answers
Are the long yellow sickles the finlets?
No. The elongated second dorsal and anal fins are larger structures. Finlets are the small separate fins behind them toward the tail.
Are finlets found only on yellowfin?
No. Other tunas, bonitos and mackerels also have them. Use multiple features for species identification.
Do finlets act like little propellers?
That description is too simple. The cited simulation found drag-producing finlets whose movements and interactions changed flow and reduced some modeled costs.
Should I retrieve faster because tuna are built for swimming?
Not automatically. Work the actual lure within its intended behavior and the conditions; swimming anatomy does not prescribe a universal retrieve.
Small fins deserve a careful explanation, not a bigger speed claim. Notice the anatomy, keep research results within their scope, and choose fishing tackle by the presentation you can control.
