Yellowfin (Thunnus albacares) is easily identified, and the only member of the more tropical yellowfin group present in New Zealand in any numbers.
Riding the Thermocline
Yellowfin occur at sea surface temperatures between 18°C and 31°C, usually in the top 250 m. They spend most of their time above the thermocline — the depth at which temperature drops sharply — but make frequent dives into the cooler water below it.
Here in New Zealand, we're at the cold edge of the yellowfin's range. An acoustic tagging study out of California — which marks the northern limit of that range — showed fish bouncing above and below the thermocline, and found that at the edges of their range, thermocline depth was an excellent predictor of where the fish would sit.
Fast Growing, Short Lived
Yellowfin have to spawn in the tropics — their eggs and larvae won't survive below 20°C. They're a fast-growing, short-lived, high-turnover fish, growing several times faster than a bluefin. That's a large part of why they've been able to support such intense commercial fishing pressure. A yellowfin generally reaches sexual maturity at around 70 cm and 3 years old.
Next in the series: the southern bluefin — the fish behind the recreational fishery that exploded off Waihau Bay in 2017.
← Previous: Albacore | Series hub | Next: Southern Bluefin →
Words by Lonny, marine biologist.