Fishing blues

It was our first day tagging. I was nervous and excited.

Seven- to ten-foot oceanic whitetips dwarf the baby lemon sharks and bonnetheads I tagged last year. Tagging these larger, far more dangerous, animals requires a lot of skill and knowhow. Good thing the experienced SEA Collaborative (Shark Ecosystem Assessment or SEAC) researchers were running the show! Most of these scientists have been friends since graduate school and have been tagging oceanics for over five years, with many years of planning beforehand. This well-earned trust is relied upon when handling these big sharks: team members know each others’ movements; thoughts — and tagging a shark takes on the air of a well-choreographed dance.  I am learning from some of the best in the field.

Within two minutes of chumming, a ghostly grey shadow moved languidly: a broken, unsure shape glimpsed through the choppy blue surface of the water. A telltale dorsal fin broke the surface. Shark.

Delight burst through me. I jumped up to take pictures and chase the 8 ½ foot shark along the side of the boat. I took over 50 pictures in a minute.  I felt like I was 5 years old again, seeing a pile of perfectly wrapped presents on my birthday.  I thrilled with excitement, awe, wonder. But, as I looked around excitement waned and there was a quiet disappointment seeping from the team. Even from afar, they could tell this wasn’t the one we were looking for, not the infamous oceanic whitetip, just a blue shark.

Blue sharks are commonplace in the waters off New York where I am from. What was it doing all the way in the Caribbean?

As it turns out, blue sharks have the widest range of any shark species. Usually they prefer cooler water and frequent greater numbers in higher latitudes. Whitetips rule the ocean down here.

The shark was a she. On male sharks there are two short appendages that look like white asparagus stems used for mating attached their bottom (pelvic) fin called claspers. Clear evidence of mating scarred her side: a round ring of gashes in her skin made by the teeth of an amorous male blue shark no more than two weeks ago.

Mating scars on a blue shark. Photo credit: Andy Mann.

Female sharks are actually born with thicker skin than males because they need to withstand the vicious pleasantries of mating. Males bite down hard, curl around the female and rotate her on her back, which puts her in an almost hypnotic state called tonic (a handy method we also employed when working on sharks). They then use their claspers to fertilize her.  Usually the scars heal up in about a month.

Blue sharks and oceanics are very different. But, unfortunately, they share two things in common: they are both targeted in the shark fin trade and often mistakenly killed on hooks and lines meant for other fish.

But why is the oceanic whitetip endangered, and the blue shark not?

Fundamentally the blue shark is much hardier. Many sharks die when accidentally caught on hooks of longlines set out over miles of ocean to target such fish as tuna. Called “bycatch,” this unnecessary killing happens when one species ends up dying from fishing that targets another species. Blue sharks have a much higher survival rate as bycatch when they are caught on the longline. They also reach sexual maturity around 6-8 years old, which is young compared to other sharks.  Perhaps more importantly, blue sharks have lots of babies (called pups!): around 20-50 each year compared to 2-14 for an oceanic whitetip.

Even though they make up about 40% of the shark fin trade, blue sharks are technically sustainable, teetering right on the edge of maximum sustainable yield. There are just enough blues born into the population each year to replenish the ones that die.

After a recent trip to Hong Kong, where 50% of the shark fin trade takes place, Debbie Abercrombie, one of the shark scientists, exclaimed “I can’t even believe there are still blue sharks swimming around in the ocean anymore!” She described four two-story malls filled from top to bottom dried seafood products, “…with bags upon bags of shark fins from mostly blues and some oceanics!”

Blue shark being tagged.  Photo credit: Andy Mann
Blue shark being tagged. Photo credit: Andy Mann

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