Know Your Target: Pacific Bluefin Tuna

Anatomy and physiology

Tunas in general, and especially bluefin, are champion swimmers. Every part of their inner and outer anatomy is fine-tuned for fast and efficient swimming, from their streamlined bullet shape and dragreducing finlets to oversized bands of aerobic red muscle (the “blood line”) for sustained distance swimming. All that swimming generates heat from muscle contractions, and tuna have special counter-current exchangers that trap the heat and focus it in their core and head. This type of partial “warm bloodedness” makes their muscles more efficient and powerful, boosts metabolism, and even improves their vision for feeding down deep in cold water and low-light conditions 1.

What this means for targeting bluefin: Stay mobile. Just because bluefin were in spot one day doesn’t mean they’ll be there the next week, or even the next day. They have no problem swimming dozens or even hundreds of miles in just a day or two.

Be patient if they totally disappear. They could be in the same area and just down deep for a day or two, especially if the bait also sinks out. Since they can keep their body temps warmer than the surrounding water, it’s easy for them to feed deep in the low-light zone where bait is trying to hide.

Make your gear invisible. Bluefin have excellent vision and are notoriously line-shy. For fishing live bait, use light line, a fluorocarbon leader, and the smallest hook you can get away with for the size of the bait. The tradeoff is that the bluefin’s hyper-efficient metabolism means you’ll be in for a long, hard battle. You can get away with using heavier gear at dawn or dusk when the low light makes your line harder to see. And of course, fly a kite! It keeps your line out of the water where bluefin have a harder time seeing it, meaning you can get away with heavier line and stand a better chance of actually landing the fish.

Chill out. All that heat in the bluefin’s core rapidly degrades the quality of the meat after the catch, and it increases the rate of bacterial growth. Getting your fish cold quickly will dramatically improve the end product you put on the table. Bleed your catch as soon as it hits the deck to remove a major source of heat from the core of the fish. Spike the brain to stop muscle contraction and prevent additional heat generation. Gut your fish to remove warm organs, and stuff the gut cavity with ice. A little seawater in your ice helps create a slurry to chill the fish even more. Plan to carry at least ¾-1 pound of ice per pound of bluefin you hope to catch per day, plus a quality insulated kill bag or cooler big enough to fit it all.

Diet

Bluefin eat a wide variety of fishes and invertebrates based on what’s available. They’ve been known to eat mackerel, saury, juvenile rockfish, squid, red crab, and even krill 2 , and they’re locally famous for busting up on big spots of anchovy and chasing down flying fish. This wide variety of prey types spans a big range in how easy each prey is to catch, and how nutritious it is. If there’s a ton of easy prey around, like big balls of anchovy, red crab, or krill, there isn’t much reason for a bluefin to spend more energy chasing down a healthy flying fish.

What this means for targeting them: Match the hatch. Have a variety of baits and lures at your disposal and try to match what the fish are feeding on at that time. When you find a spot of bluefin foaming up on anchovy, try a small chrome jig like a Colt Sniper. If the tuna are holding deep, drop a vertical jig like a Flat Fall to imitate squid. Poppers, sticks baits, and surface iron do a great job of imitating injured or panicked baitfish trying to escape hungry predators. These bigger lures will sometimes work when the tuna are on tiny anchovy, either by triggering a reaction strike or by the simple luck of having a tuna swim into your jig with its mouth open. And of course, nothing beats a real flying fish or yummy flyer under a kite when the tuna are eating flying fish.

Basic demographics: growth, reproduction, and migration

Age & Growth: Pacific bluefin grow fast and big. They can reach 50-inches long and almost 80 pounds in their first 3 years, and even the biggest “cows” (200+ lbs.) we’ve seen the last few seasons probably aren’t much more than 6-7 years old. They typically reach reproductive maturity by age 5 and can live for more than 15 years and weigh 1,200 lbs. 3,4

Reproduction: Pacific bluefin spawn on the other side of the Pacific, in the Sea of Japan and East China Sea4,5. They’re broadcast spawners, which means females release millions of eggs into open water for nearby males to fertilize. Fertilized eggs hatch into larvae, which then develop into juveniles and grow into adults. Most of these fertilized eggs and larvae don’t make it past the first month or two – they either starve to death or get eaten by predators. The number of juveniles that do survive each year varies a lot, making it very hard to predict how the population is going to change from year to year 5.

Migration: Some of those surviving juveniles then migrate to our side of the Pacific in their first year, and the rest stay behind to grow up in the western Pacific. We don’t know what percentage migrate to our side, but it’s probably very different each year. After 3-5 years of feeding and growing up, the bluefin then migrate back across to the spawning grounds4. Some bluefin stay here longer than others, and lately they seem to be staying here longer than normal, up to 6 or 7 years.

Population Trends: All that year-to-year change in reproductive success and migration means bluefin populations fluctuate a lot. Even this boom cycle is at a low point in their total biomass across the entire Pacific – spawning stock has been estimated at just 2-4% of what it could be without fishing5. So how long is this cycle going to last? Nobody really knows for sure. At its core, it’s basically just the right combination of good reproduction, good numbers migrating, good numbers staying longer than normal, and good oceanographic conditions bringing them in to the Bight, all happening at the same time. One clue to watch is how many smaller 5-20-pound bluefin show up each year – that’s a marker for how many make the migration. After that, it’s up to local oceanographic conditions to determine where on this side of the Pacific they will hang out, and for how long.

What this means for targeting them: Be prepared. With so many different age groups hanging out on our side of the Pacific for longer than normal, you never know if the next spot of bluefin you run across is going to be the firstyear 10-20-pound grade or the older 200+ lb. cows. Bring the full arsenal – you don’t want to miss your fish of

a lifetime just because you left the heavy gear at home.

Enjoy responsibly. Nothing lasts forever, and when you add it all up, the local bluefin “boom” is going to “bust” at some point. Enjoy it while it’s here but remember that the bluefin we catch in SoCal haven’t spawned yet. We need to be careful to make sure enough fish make it back to the spawning grounds and have multiple chances to reproduce, especially since the number of successful babies each year is so variable and prone to neartotal failure. This means multi-national cooperation and responsible harvest limits at each point in the life cycle on both sides of the Pacific – including when they’re here in our local waters.

Help out!

There’s a lot we still don’t know about the bluefin life cycle and migration patterns, but you can help researchers gather vital data to find out. Support collaborative fisheries research efforts like the Tag-a-Giant program, and donate your bluefin heads and organs to the Highly Migratory Species research team at NOAA Fisheries in La Jolla.

1 Block, B., et al. 1993. Science 260: 210-214.

2 Snodgrass, O., pers. comm. Southwest Fisheries Science Center, La Jolla, CA.

3 Shimose, T., et al. 2009. Fisheries Research 100(2): 134-139.

4 Tag A Giant. https://tagagiant.org/science/pacific-ocean

5 2016 Pacific Bluefin Tuna Stock Assessment: Report of the Pacific Bluefin Tuna Working Group. International Scientific Committee for Tuna and Tuna-Like Species in the North Pacific Ocean. Available online at http://isc.fra.go.jp/pdf/ISC16/ISC16_Annex_09_2016_Pacific_

Bluefin_Tuna_Stock_Assessment.pdf Tuna illustration by www.studio-abachar.myshopify.com