Since the early 1990’s, biologist Samuel Gruber and his colleagues have suspected that some coastal shark species may return to their birthplace to breed – a behavior termed “natal philopatry”. They suspected this because they had observed some sharks making seasonal migrations to discrete coastal areas to give birth. In particular, in the Bimini Islands, Bahamas, there were strong clues that lemon sharks there returned to their birthplace to breed. They noticed the same female lemon sharks returning to Bimini over and over to give birth. And they noticed that young lemon sharks in Bimini remain in the same nursery area they were born in until about age 3.
But how does one show or prove this? Well you have to track the animal from birth until it breeds. Sharks do not sexually mature and breed until late in life, so you have to track them for a long, long time. So in 1995, Samuel Gruber and several other scientists (including Blue Ocean Fellow Dr. Demian Chapman) decided to undertake a long-term monitoring study of the lemon sharks at Bimini, Bahamas, to find out if they do indeed return to their birthplace to breed.
The scientists tracked lemon sharks in Bimini from 1995 until 2012. Each year, the scientists deployed nets to sample the lemon sharks. To keep track of each shark they caught, they tagged and measured them. And they took a small sample from their fin to use to genetically id each shark. Afterwards, they released all the sharks back to the water alive.
Newborn female lemon sharks caught in the early years of the study would reach sexual maturity and potentially return to breed at Bimini in the later years of the study (2008-2012). The scientists used two different methods to try to determine if any of the newborn lemon sharks caught in the earlier years of the study had returned to breed. The first was to physically recapture pregnant female lemon sharks they had tagged in Bimini when they were a newborn. But, as one might imagine, capturing a large, adult shark is not that easy. The second method was to determine if any of the young sharks they captured in the late years of the study were offspring of any of the females born in Bimini that were captured many years earlier. To do this, they would try to match offspring to their mothers using genetic identification.
Well, after 17 years of tracking the lemon sharks, the scientists found evidence that at least 6 female lemon sharks returned to their birthplace in Bimini to give birth when they were between the ages of 14 and 17. In two of the cases, the scientists know the females returned because they recaptured them when they were pregnant. In the other four cases, the scientists did not physically capture the pregnant females, but they identified newborn sharks whose genetic profile matched them to mothers born in Bimini in the early years of the study. In some cases, offspring of mothers born in Bimini were captured just a few kilometers from where their mother was captured many years earlier!
Observations of just 6 sharks returning to their birthplace in Bimini to give birth may not seem like a lot. But when you consider that many sharks do not survive to breed and the challenges with trying to actually observe this behavior in these large predators, 6 sharks is actually quite a high number! Additionally, during their 17 year study, the scientists observed many other female lemon sharks that repeatedly returned to the exact same nursery area in Bimini to breed (either North Bimini or South Bimini). Though the scientists can’t prove all these sharks were born there [because the sharks were born before their study began], the scientists believe it is likely quite common for lemon sharks to return to their place of birth in Bimini to breed.
The scientists’ results provide the first evidence of a shark returning to its birthplace to breed! [Other marine species that do this include salmon and sea turtles.] How do species navigate their way back to their birthplace? One way is species might imprint on the geomagnetic field at their birthplace and use this information to return to this site when they are ready to breed.
It is likely that there are other coastal shark species that return to their birthplace to breed too. For instance, scientists have observed sibling blacktip reef sharks giving birth in the same estuary in French Polynesia, suggesting this species may return to their birthplace to breed.
The discovery that some sharks return to their birthplace to breed has very important implications. Generally, we consider sharks to have large, regional populations. But if some sharks are returning to their birthplace over and over to breed, it may be that some shark species actually have much smaller and more structured populations than we think! This means that fishery managers would need to assess the abundance of sharks and manage shark fisheries on a much smaller scale than they do now. It also means that local shark conservation efforts could play an important role in maintaining their populations. For example, threatened shark species, like the lemon shark, could benefit from the protection of important coastal nursery habitats.
You can read about the full lemon shark study at Bimini, Bahamas in the December issue of the Journal of Molecular Ecology.
For more cool information on sharks, check out these blogs:
Elizabeth Brown is a research scientist at Blue Ocean Institute.