How to identify a tuna and estimate its weight from its length
Two questions come up on every boat: which tuna is it? and how much does it weigh? Both can be answered without scales and, if you are releasing it, without taking it out of the water. Here are the keys to recognising the species that bite in our waters and the official ICCAT formulas to turn centimetres into kilos.
Which tuna is in front of you
- Atlantic bluefin (Thunnus thynnus): very robust, almost cylindrical body; short pectoral fins that do not reach the start of the second dorsal; yellow finlets edged in black; a small eye for its size. It is the giant: over 2.5 m and 300 kg.
- Yellowfin (T. albacares): in adults, very long yellow second dorsal and anal fins, like two sickles; a golden band along the flank; bright yellow finlets. Pectorals that do reach the second dorsal.
- Bigeye (T. obesus): large eye, stockier head and body, long pectorals. Very common in the Canaries and the Azores, and easy to mistake for a young yellowfin.
- Albacore (T. alalunga): extremely long pectorals, reaching past the anal fin, and a white edge on the tail. Much smaller on average.
- Skipjack (Katsuwonus pelamis): four to six dark longitudinal stripes on the belly; rarely over a metre.
Quick rule: look at the pectorals and the eye first. Short with a small eye, bluefin; extremely long, albacore; big eye, bigeye; yellow sickles, yellowfin.
How to measure
The reference is the straight fork length (SFL): from the tip of the snout to the fork of the tail, in a straight line, with the fish parallel to the hull. If you can only run the tape over the body (curved fork length, CFL), ICCAT gives the conversion: SFL = 0.9596 × CFL + 2.1. Mark the tape or the gunwale every 10 cm and you can measure in the water while barely touching the fish.
Weight by length
ICCAT publishes length-weight relationships by area and season, because a tuna that has just spawned in June weighs less than the same fish in the Bay of Biscay in September. The two that matter most to us:
| Straight fork length | Atlantic, summer (Lino et al. 2021) | Mediterranean, May–June (Deguara et al. 2017) |
|---|---|---|
| 80 cm | 10 kg | 10 kg |
| 100 cm | 19 kg | 19 kg |
| 120 cm | 32 kg | 32 kg |
| 140 cm | 49 kg | 50 kg |
| 160 cm | 70 kg | 74 kg |
| 180 cm | 97 kg | 104 kg |
| 200 cm | 130 kg | 141 kg |
| 220 cm | 169 kg | 186 kg |
| 240 cm | 214 kg | 239 kg |
| 260 cm | 267 kg | 302 kg |
| 280 cm | 327 kg | 374 kg |
| 300 cm | 395 kg | 457 kg |
Formulas: Atlantic summer, weight = 6.116·10⁻⁵ × SFL^2.7494; Mediterranean spring, weight = 2.8684·10⁻⁵ × SFL^2.9076, with length in centimetres and round weight in kilos. For the Cantabrian Sea in summer (Luque et al. 2025) the values are almost identical to the Atlantic ones.
The girth rule
If you also have the girth at the pectorals, the old skippers' rule still works as a check: weight (kg) ≈ length (cm) × girth² (cm) / 28,900. It tends to underestimate very fat autumn fish, but gives an order of magnitude in seconds.
If you are releasing it
You do not need to weigh it. A photo with the fish parallel to the gunwale and a known reference (the length of the cockpit, a mark on the hull) lets you estimate the length later, calmly. That is what scientific tagging programmes do. How to do it without harming the fish: bluefin tuna catch and release.
Sources
- ICCAT. Bluefin tuna length-weight relationships (LWR), table of parameters by area and season (Lino et al. 2021; Luque et al. 2025; Deguara et al. 2017; Katavic et al. 2018). PDF at iccat.int
- ICCAT. Curved to straight fork length conversion: CV074062563.
- Collette, B. B. and Nauen, C. E. (1983). FAO Species Catalogue Vol. 2. Scombrids of the world. FAO Fish. Synop. 125(2).
