Connectivity in scalloped hammerheads
Scalloped hammerheads (Sphyrna lewini) are large sharks that live in tropical and warm-temperate oceans worldwide. In Australia, they occur from Geographe Bay (33°S) on the west coast, across northern Australia to Sydney (34°S) on the east coast. They feed on fish, squid and other marine animals.
Females give birth to litters of live young close to shore, possibly returning to their place of birth. Each litter can be made up of both full and half siblings as a result of multiple paternity. Young sharks remain on the continental shelf but adults move offshore, travelling unknown distances to unknown places. Adults occur to depths of at least 275 metres.
The species is listed on the IUCN Red List as critically endangered. Globally, their numbers are declining because all life stages (juveniles and adults) are vulnerable to fishing activities.
Connected or separate?
Connectivity studies reveal the geographical boundaries of animal populations. For fished species, the results are important for identifying and managing stocks. Results can provide useful context to inform conservation decisions for endangered species.
Madi Green is a forensic fisheries ecologist at CSIRO. She studied connectivity among scalloped hammerheads as part of her PhD at CSIRO and the University of Tasmania.
“Information about population connectivity is useful to neighbouring nations when they manage their biodiversity,” Madi said.
“I’m interested in using genetics to provide evidence about connectivity.”
During her study, Madi looked at genetic markers throughout the genomes of 541 scalloped hammerheads from 12 different locations across the Indo–Pacific. She used samples collected over many years in different circumstances. They included specimens collected by scientists under permits, fins sampled in markets and sharks caught during commercial fishing.
Answers in the DNA of scalloped hammerheads
Madi’s work built on previous studies of genetic connectivity in scalloped hammerheads. Her lab work took around a year, followed by another six months of analysis. She used a combined approach using different genetic techniques. They included sequencing mitochondrial DNA, using genetic markers known as microsatellites and looking at single nucleotide polymorphisms (SNPs) across the scalloped hammerhead genome.
“Using newer genomic methods was really helpful for answering our research questions because they gave more resolution for understanding shark connectivity and stock structure,” Madi said.
“The DNA story tells us populations of scalloped hammerheads are separated by oceanic basins, but when there is connected continental shelf, we see genetic connectivity.”
Globally, Madi found that scalloped hammerhead populations divide into four key regions, separated by ocean basins. These regions are the Western Indian, the central Indo–Pacific, Central Pacific and the tropical East Pacific.
“Big oceans barriers cause big genetic barriers for these sharks. But the population in Australia is genetically similar to the populations in Papua New Guinea and Indonesia.”
Madi noted that her study isn’t the final word on scalloped hammerhead populations.
“Genetics measures connectivity over fairly long timescales. During their lifetimes, scalloped hammerheads in Australia, Papua New Guinea and Indonesia may be more resident. Therefore, it would be good to find out how far individual scalloped hammerheads travel between breeding seasons,” she said.
You can find the research paper Updated connectivity assessment for the scalloped hammerhead (Sphyrna lewini) in Pacific and Indian Oceans using a multi-marker genetic approach in Science Direct. The authors thank the Australian Centre for International Agricultural Research and the Australian Government’s National Environmental Science Program through the Marine Biodiversity Hub for providing funding for their research.