When I visited the Galapagos in 2010, I remember going for a run along the beach with the family I was traveling with. It was the beginning of the sun setting where there was just enough light to run. We stumbled upon a white pickup truck near the beach, lights off, and these four men watched us until we were far enough that they didn't think we saw them get up off their cooler that had a dark color dripping down the sides. We kept our heads down and kept running but I had a sinking feeling in my stomach that something was inherently wrong about that.
Rumors around the town swirled the next day that a pickup truck was caught with a hammerhead in its cooler, the men trying to hack off the fins to illegally sell. I was sickened- how could that happen in the Galapagos?! I never got to confirm that species it was, but I have a pretty good idea it was today's critter.
Before reading the bio of this animal, you may want to brush up on your Sphyrnidae family knowledge.
Done? Fintastic. TFUI wants you to meet one of the most famous and common hammerheads: the Scalloped Hammerhead (Sphyrna lewini). This shark is also known as the bronze, kidney-headed, or southern hammerhead. As a coastal and semi-oceanic animal, they are found globally in coastal warm temperate and tropical seas. Observed all throughout the water column (from the surface and intertidal to at least 275 m deep), you can't confused these animals with any other shark... well, maybe others in the Sphyrnidae family. With a hammer-shaped head and grey color that fades into a snowy white underbelly... not many sharks fit that description! Specifically the head part.
A true cosmopolitan species, there is genetic evidence that there are multiple sub-populations of scalloped hammerheads. Recent studies indicate that the Northwest Atlantic, Caribbean Sea and Southwest Atlantic populations are genetically distinct from each other, and from the Eastern Central Atlantic and Indo-Pacific populations! By the way, the "Caribbean Sea" population includes Belize and Panama. And the "U.S. Gulf Of Mexico" covers from Texas to south-western Florida - so the "boundary" zone is between Texas and Northern Belize.
Many divers and dive guides in the Galapagos have seen a severe decrease in shark numbers and schools of hammerhead sharks. When I went diving in 2010 during the "high season" of seeing these famous schools, we were lucky to see two and from far away. It was a sobering first-hand experience and I discussed with the dive guides about how illegal fishing around the famous islands is not only practiced by local fishermen, but also by international industrial and artisanal fleets. What are these illegal fisheries after? Sharks... and their fins.
#Finfact: Hammerhead shark species S. zygaena and S. lewini were found to represent at least 4-5% of the fins auctioned in Hong Kong, the world's largest shark fin trading center (Clarke et al. 2006a). In fact, it's estimated that between 1.3 and 2.7 million S. zygaena or S. lewini are represented in the shark fin trade each year or, in biomass, 49,000 to 90,000 mt (Clarke et al. 2006b). While there is no species specific data for these illegal fisheries, the scalloped hammerhead is a common species found in the Galapagos and may be a target of illegal finning activities.
Scalloped hammerheads segregate by sex; females migrate offshore earlier and at smaller sizes than males. In the Gulf of Mexico and northern Australia, it was observed that males less than 1 m long were more abundant over the continental shelf, but females bigger than 1.5 m dominated areas near the edge of the shelf. Males are observed to stay deeper than female sharks in general. Adults spend most of the time offshore, and then females migrate to the coastal areas to have their pups.
Speaking of pups, this species is viviparous with a yolk-sac placenta. #Finfact: only their right ovary is functional! The number of oocytes can be as many as 40-50 per female (Chen et al. 1988). The gestation period is around 9-12 months, with pups being born in the spring and summer months. The average number of embryos in the uterus ranges from 12-41 and females pup every year. Pups tend to die quite early, often becoming the snack of other larger sharks (including other scalloped hammerheads!).
How big do they get? Scalloped hammerhead males on average measure 1.5 to 1.8 m (4.9 to 5.9 ft) and weigh about 29 kg (64 lb) when they get sexually mature. Females measure 2.5 m (8.2 ft) and weigh about 80 kg (180 lb) at sexual maturity. They like to eat a varied diet of cephalopods (squid and octopus) and fish- including sometimes other sharks!
The Scalloped Hammerhead is taken as both a target and bycatch by trawls, purse-seines, gillnets, fixed bottom longlines, pelagic longlines and inshore artisanal fisheries. The IUCN has assessed these animals as Endangered (EN).
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TFUI Founder Melissa C. Marquez is author of all animal bios and "Behind the Fins" segments.
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