Many years later as an active scuba diver I would dive a lot at this remote spot with friends from the University of the West Indies British Sub-Aqua Club. On one such expedition, I was at a hundred feet just above the coral pinnacle, and turned my head to the right to see a vast school of blackfin tuna filling the water column from my depth up to the surface. They had arrived suddenly and soundlessly and swam in a wide arc around the small group of bubble blowing divers.
They were awesome creatures with torpedo-shaped bodies, big bold black eyes, and a glowing bronze band along their flank highlighting the black back and pectoral fins spread wide. In those wild early days I was carrying a spear gun, not a camera, and my trigger finger was itching but the fish kept their distance, and behind the school followed a couple of hefty silky sharks. I knew if I was lucky enough to nail a tuna these big fast sharks would be all over the fish in a blur.
Years later my most memorable encounters with blackfins were off Belize while filming whale sharks. The mixed schools of blackfins, skipjack tuna and bonitos were corralling small sardines which in turn attracted the attention of young whale sharks. Snorkeling was the way to go, video camera in hand, and I got some superb footage of the combined effect of the tunas corralling the prey and the whale shark taking advantage of the bonanza. The sardines would swim into the open mouth of the whale shark at the surface to escape the bombardment by tunas. The ever-present silky sharks also joined in the food fest.
Blackfins are the most common small tuna around the Cayman Islands and can be caught year round along the deep drop-off, but tend to aggregate around the ends of the islands where the current hits the wall. They are plentiful at 12 Mile Bank, and are targeted by commercial and sports fishermen for use as bait.
Anglers use a small feather lure (pink works well), trolled at 4 – 8 knots to catch these scrappy fighters. They are used for live bait to catch bigger yellowfin tuna, wahoos or blue marlin. They are good food fare in their own right but hardly ever reach eight pounds in our waters.
The best way to see blackfins here is snorkel off the end of 12 Mile bank, either the northeast corner or the southwest corner in the deep water close to the edge. You can drift and get picked up by your boat to repeat the drift and see these speedsters cruising by. You are also likely to encounter other blue water species like rainbow runners, flying fish, wahoos and even the odd blue marlin.
For some reason the fully grown blackfins of 20-40 pounds do not frequent the waters of the central Caribbean. In Florida, the Gulf of Mexico and further south to Central America, they come jumbo-sized, averaging 25 pounds; yet off Jamaica and Cayman I have caught many in the half pound size range, which are less than a year old. This suggests that these juveniles migrate to the western and northern Caribbean as adults in search of better feeding opportunities. When and how they complete the cycle is not known as little migratory research has been done on this species. The known range for blackfins is from the northeast of the USA as far south as Brazil and they are limited to the western Atlantic, unlike many of their relatives like the yellowfin tuna and skipjack tuna that are cosmopolitan species.
In Florida blackfin tuna have an extended spawning season from April to October and from May to September in the Gulf of Mexico. It is likely they spawn year round in the Caribbean, as I have seen active gonads in blackfins caught here in every month.
Blackfins feed largely on pelagic crustaceans, larvae and juvenile crabs, shrimp and squid, as well as small fish and fish larvae. I have often seen them plunder schools of juvenile puffer fish and sardines, frequently clearing the surface in high jumps as they swim speeding from below to the prey above. They also feed on any juvenile fish that shelter beneath flotsam.
If frigate birds are around, they will swoop down and pick the Sargassum weed up in their bill, fly several feet, then drop the weed, exposing the small fish hiding beneath the weed to the tunas. I have yet to see how this benefits the frigate bird!
In turn, blackfin tuna are consumed by larger tunas, king mackerel, barracudas, wahoo and blue marlin, plus a variety of fast ocean-going sharks. The sight of a blue marlin chasing blackfins is amazing; the ocean drama of predator-prey interaction at its best.
If you see blackfin tuna on a menu in a local restaurant as sushi, seared or fried, give it a try as you will be happy with your choice. Blackfin are fished sustainably here in Cayman and elsewhere in their range.
It is our collective responsibility to conserve the marine environment and maintain the biodiversity of the planet.
Dive safely, fish responsibly.
Guy Harvey PhD.