Reproductive Philopatry in the Bull Shark, by Andrew Penney

The bull shark (Carcharhinus leucas), known for its aggressive nature and adaptation that allows it to live in brackish and freshwater environments in addition to the normal marine environment, is a requiem shark found worldwide in shallow coastal waters and rivers. Philopatry is defined as a behavior where an animal remains in or returns to a specific location and reproductive philopatry specifically refers to when adults return to a specific location to either mate or give birth (Tillett et al., 2012). Reproductive philopatry is significant because it can have a large effect on species management and conservation. If a local population of animal that practices reproductive philopatry experiences decline, the possibility of it recovering in that specific area is reduced and needs to be managed as a unique population separate from other populations of the same species (Tillett et al., 2012). Strategies such as shark nursery areas, spatial management of shark fisheries, and stocking areas with local and genetically similar populations would be better for philopatric animals (Hueter et al., 2005). Although bull sharks and sharks in general are not traditionally seen as philopatric animals, there is genetic and statistical evidence that shows bull sharks exhibit reproductive philopatry and this can have implications on the management and conservation of the species.

Bull sharks spend most of their time in warm, shallow waters along the coast and in rivers and feed on a variety of fish, birds, sea turtles, and mammals. Ecologically, they help keep the populations of their wide variety of prey items in check. Bull sharks are not known to migrate long distances consistently (Encyclopedia of Life). They usually breed during the summer months and young are born roughly 10 to 11 months after mating occurs (Encyclopedia of Life). Bull sharks are viviparous which means they give birth to live young and can have liters of up to 13 pups. Sexual maturity in bull sharks is estimated to occur at the age of 8 to 10 years old (Encyclopedia of Life).

One study that was conducted and supported the existence of reproductive philopatry in bull sharks used mitochondrial and microsatellite DNA comparisons in juvenile bull sharks. The study was conducted across 13 river systems in northern Australia and targeted the NADH dehydrogenase subunit 4 gene in mitochondrial DNA and three microsatellite loci in nuclear DNA (Tillett et al., 2012). The study found that there were population structures in the mitochondrial DNA but not at the same scale in the microsatellite DNA (Tillett et al., 2012). This supported the hypothesis that the population structure is caused by at least the females’ movements and supported the existence of reproductive philopatry in bull sharks. The lack of population structure in the microsatellite DNA and absence of sexually mature males in the nurseries suggest that the male bull shark’s movements are different (Tillett et al., 2012). Using the comparisons, they were also able to rule out the possibility that geographic barriers were the cause of the genetic structures in the populations of bull sharks because they did not find any clusters of haplotypes that were closely related enough in geographically similar locations to be caused by geographic barriers.

Bull Shark   Source Wikimedia Commons

Another study that was conducted looked at the change in populations and average length of various shark species (one of them being the bull shark) off the coast of Florida. The study looked at the effects of recreational fishing that took place in the 1970s and how overfishing caused the population and size decline of shark species in the area. The study showed that sizes and populations of sharks (one of them being the bull shark) declined heavily one at a time in coastal sites where recreational shark fishing was introduced, not at the same time across all the fishing sites (Hueter, 1991). This supports the hypothesis of reproductive philopatry in bull sharks because the population and size declines occurred one at time and was limited to the coastal area where the fishing took place. If bull sharks did not practice reproductive philopatry, then the population and size decline would have been seen at the same time in areas that were close to each other.

In conclusion, there is genetic and statistical evidence that shows reproductive philopatry in bull sharks. The study done in northern Australia used genetic evidence to support this hypothesis by comparing mitochondrial and microsatellite DNA in juvenile bull sharks across 13 river systems. Although the microsatellite DNA did not show any population structure, the mitochondrial DNA did which supported reproductive philopatry in at least the female bull sharks. This study was also able to rule out the possibility of geographic barriers causing the genetic structures in populations of bull sharks due to the lack of clusters of haplotypes in geographically similar locations that were closely related enough. The population and size study done in the coastal regions of Florida where commercial shark fishing occurred supported reproductive philopatry by showing that the declining size and populations occurred one at a time and separately as recreational shark fishing appeared in those coastal areas. Although these studies support the hypothesis of reproductive philopatry in bull sharks, more studies in different aspects could be done to make the hypothesis more concrete. For example, a study that uses tagging or another form of tracking method to study the actual movements of a bull shark would help make reproductive philopatry in bull sharks a more concrete hypothesis. However, a study like this could possibly take a long time if researchers decide to study a shark from when it is a juvenile to mature adult because it takes 8 to 10 ten years for a bull shark to reach sexual maturity.

References

Encyclopedia of Life. Available from http://www.eol.org. Accessed 01 Apr 2017.

Hueter, R.E. 1991. Survey of the Florida recreational shark fishery utilizing shark tournament and selected longline data. Mote Mar. Lab. Tech. Rpt., 232A: 1–94.

Hueter, R. E., M. R. Heupel, E. J. Heist, and D. B. Keeney. 2005. Evidence of Philopatry in Sharks and Implications for the Management of Shark Fisheries. J. Northw. Atl. Fish. Sci.  35: 239-247.

Tillett, B. J., Meekan, M. G., Field, I. C., Thorburn, D. C. and Ovenden, J. R. 2012. Evidence for reproductive philopatry in the bull shark Carcharhinus leucas. Journal of Fish Biology  80: 2140–2158.