A decade of genetic research in Fiji has uncovered extensive bull shark family networks—and evidence that some males and females repeatedly reproduce together over many years.
Particular male and female bull sharks apparently reproduce together repeatedly.
Bull sharks may have considerably more structured family lives than previously recognised. A decade-long genetic study of sharks in Fiji has revealed extensive kinship networks, females repeatedly using familiar nursery areas and, most strikingly, evidence that the same breeding pairs can produce offspring together years apart.
Researchers analysed 296 bull sharks sampled at Fiji’s Shark Reef Marine Reserve and in the nearby Rewa, Navua and Sigatoka rivers. By examining thousands of genetic markers, they reconstructed relationships spanning different generations, locations and years.
The results reveal a population connected not simply by geography, but by persistent reproductive relationships.
Families across reefs and rivers
Adult bull sharks regularly aggregate at Shark Reef Marine Reserve, off Viti Levu, while pregnant females move into nearby river systems to give birth.
The genetic analysis identified 375 relationships among the sampled sharks, including 146 full-sibling and 229 half-sibling pairs. Some young sharks in rivers were connected to adults previously sampled at Shark Reef Marine Reserve.
The findings supply genetic evidence that the reef and surrounding rivers function as interlinked parts of the sharks’ life cycle.
They bolster previous evidence of reproductive philopatry: female bull sharks tend to return to particular areas to give birth. Earlier genetic research has shown that this behaviour can produce distinct regional populations, even though bull sharks can travel considerable distances.
Familiar partners
More surprising was evidence,that particular males and females apparently reproduced together repeatedly.
Researchers identified 64 sharks from 20 family groups that shared the same inferred mother and father despite being sampled in different years. In some cases, offspring attributed to the same parental pair were born as much as a decade apart.
The researchers describe this as the first evidence of repeated pairings across years between the same male and female bull sharks.
That does not necessarily mean bull sharks form permanent pairs or remain together between breeding events. The genetic evidence shows repeated reproduction by the same parental combinations, rather than the stable social partnerships familiar from some birds and mammals.
Nevertheless, it suggests that bull shark reproduction may be considerably less random than once assumed.
Conservation implications
The findings also carry practical implications.
If females repeatedly return to particular rivers to give birth, damage to those nursery habitats might disproportionally influence individual family lines and ultimately the wider population. Bull sharks’ ability to travel widely therefore does not necessarily protect them from local ecological stresses.
The study also produced a relatively low estimate of effective population size—the number of breeding individuals contributing genetically to subsequent generations. The researchers caution that estimates are affected strongly by the presence of related animals in samples, but the results nevertheless underline the importance of understanding kinship when assessing shark populations.
For divers accustomed to seeing bull sharks gathering at predictable sites, the research adds another dimension to those aggregations. Individuals sharing the same reef may also be linked via generations of family relationships extending between the open sea and Fiji’s rivers.
