Investigating the population connectivity of silky sharks around the world

Words by: Andrea Bernard

Recently, I co-authored a paper entitled ‘Global mitochondrial DNA phylogeography and population structure of the silky shark Carcharhinus falciformis’, which has just been published online in the journal Marine Biology. In general terms, the paper describes a study that I, together with several Save Our Seas Shark Research Center (SOSSRC) colleagues as well as international collaborators, undertook to learn more about the genetic connectivity and diversity of silky sharks in a large portion of their global distribution.

Silky sharks, which grow to a length of up to three metres and usually occur in the warm pelagic waters of the world’s oceans, get their common name from their smooth skin that is ‘silky’ compared to the skin of other shark species.

Unfortunately, silky sharks are subjected to intense fishing pressure across large parts of their distribution. They are caught in pelagic commercial and artisanal fisheries, and also as by-catch by long-lining vessels that target mostly tunas. Few details about their population status are currently available, but it is widely believed that their global numbers are on the decline. Because of this, silky sharks are classified on the IUCN Red List of Threatened Species™ as Near Threatened globally, and in some areas as Vulnerable to extinction. Their precarious conservation status makes it essential to understand how different populations around the globe are connected, both genetically and demographically. This knowledge is needed to guide management efforts aimed at ensuring the continued existence and genetic diversity of silky sharks.

This is where our work at the SOSSRC comes in. With the help of many collaborators and dedicated fishers, we obtained samples from silky sharks from across their global distribution in order to characterise the genetic variability of the species and determine the extent of connectivity between populations. By sequencing their mitochondrial DNA, we found that even though these sharks are able to swim long distances, they generally show a great deal of genetic population structure – or in other words, very little connectivity across large expanses of ocean. Firstly, like many other shark species, silky sharks in the Western Atlantic are very different genetically from silky sharks in the Indo-Pacific. Unexpectedly, however, silky sharks from various regions within the Indo-Pacific also showed little connectivity, which suggests that they are demographically independent.

What does this mean for the management or conservation of silky sharks?
Basically, it seems that silky sharks from different regions are independent population units. This means that they need to be managed (or conserved) at a regional scale. Thus, regional governments must work to conserve the silky sharks in their own backyard. It also means that if silky sharks are lost from parts of their distribution, they are unlikely to recover via immigration from other distant areas.