Friday, May 26, 2017

Reproductive Philopatry in Bull Sharks, by Rachel Wadsworth

The Bull Shark, Carcharhinus leucas, is an apex predator reaching lengths over 11 feet, and can be found prowling in shallow tropical and subtropical waters around the world (Tillett 2012). Adults have been tracked migrating long distances stretching over 2000 km, and are targeted by commercial and recreational fisheries throughout their range (Karl 2011, Lea 2015). Due to the large scale migration patterns observed of Carcharhinus leucas, it has been assumed there is no population genetic structure. However, further research challenges this assumption after discovering characteristics of female habitat use indicates philopatry. With declining population concerns observed by the fisheries, further understanding of how reproductive philopatry shapes the Bull Shark population structure is crucial in the development and implementation of an effective management plan (Karl 2011)

Reproductive philopatry is when an organism returns to a specific nursery to mate or give birth (Tillett 2012). Although marine, Bull Sharks are capable of move up streams into brackish and freshwater habitats. This behavior is observed during the reproductive cycle where females return to an estuarine nursery for parturition. This nursery is then utilized by juveniles for 4 years before migrating to ocean habitats (Tillett 2012). Studies have determined the nursery chosen by a female is not random, indicating reproductive philopatry. Males are rarely found in freshwater estuaries suggesting male-mediated dispersal due to mating events occur elsewhere, and only females showing reproductive philopatry (Tillett 2012). 

Female Bull Shark.  Source National Geographic
Research was conducted in Northern Australia to test reproductive philopatry in Carcharhinus leucas by determining if population structure exists between close nurseries and if the structure is shaped by sex-specific movements or geographic distance (Tillett 2012). The population structure and sex-specific movement patterns was analyzed by comparing the relatedness of juveniles within and between each nursery and genetic difference between msDNA and mtDNA. Microsatellite (ms) DNA was used to differentiate male genetics while mtDNA was used to identify female genetics. The study determined population structure among juveniles from different nurseries exists in mtDNA, but not in msDNA in closely located nurseries (Tillett 2012). Because the population genetic structure is absent in msDNA but present in mtDNA, the habitat and migration of males is different in females. Due to the lack of correlation between genetic and geographic distances, females straying to nearby rivers does not occur frequently enough for an increase in genetic similarities (Tillett 2012).

With an increasing human population, Bull Sharks are faced with increasing fishing pressures from commercial and recreational fisheries globally. Bull Sharks of all age classes are targeted, and very little records the catch data are kept (Karl 2011). Because of insufficient records, the current population trend is unknown. However, fisheries have reported noticing long term declines in Carcharhinus leucas populations. With little regulations and current management plans, it is believed this fishery is overharvested (Karl 2011).

Not only are Bull Sharks faced with increasing fishing pressure, but they are also faced with degrading habitats as a result of an increasing human activity and development along coasts. Anthropogenic activity near coasts alters crucial habitats both directly through modification and indirectly from pollution (Heupel 2010). The loss of estuaries is most concerning since these habitats are critical to the reproductive and juvenile stages of Bull Sharks. With new studies revealing female reproductive philopatry, the degradation of these estuaries is extremely alarming because the estuary chosen by a female is not random, shaping the population structure of bull sharks (Tillett 2012).

The degree of reproductive philopatry and sex based gene flow has a direct effect on population subdivisions and genetic structure (Karl 2011). New research studying genetic structure supports previous evidence of female reproductive philopatry and determined strong subdivisions with in the population over long and short distances (Karl 2011). The false assumption of the abundance and genetic connectivity for Carcharhinus leucas because these species are vastly distributed must be removed from conservation management strategies. Understanding population subdivisions and migration patterns as a result of sex biased reproductive philopatry is extremely important in the construction of conservation management strategies (Karl 2011). Failure to implement proper management strategies with the updated knowledge will cause overexploitation and local extirpation that will ultimately lead to global extinction (Karl 2011). The conservation of Carcharhinus leucas will require a global effort with management plans implemented at local, regional, and international levels.

The IUCN has listed Carcharhinus leucas as near threated as a result of heavy fishing pressures and the degradation of key estuaries (Lea 2015). The assumption of the abundance and genetic connectivity for Carcharhinus leucas due to the vast distribution is outdated with increasing evidence supporting female reproductive philopatry shaping population genetic structure (Tillett 2012). The importance of estuaries and sex based gene flow is a critical in the development of effective conservation management strategies (Karl 2011). The Bull Shark is an important apex predator, and conservation of this species is necessary for the health of marine ecosystems (Chapman 2015). Further research must be conducted on the Bull Shark to implement the most effective management strategy and ensure a prosperous future for this crucial species.


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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. doi:10.1111/j.1095-8649.2012.03228.x

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