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Shark Genetics and Breeding Biology

SharkPIs: Dr. Samuel H. Gruber, University of Miami*; Dr. Kevin Feldheim, Field Museum of Chicago; and Drs. Demian Chapman and Ellen Pikitch, Institute for Ocean Conservation Science.

*External grantee

Do Sharks Home to Their Birthplace to Breed?
Solving the Mysteries of Shark Mating
Definining Shark Stocks using Population Genetics
Conservation Genetics of the Endangered Sawfish

Do Sharks Home to Their Birthplace to Breed?

PIs: Dr. Samuel H. Gruber, University of Miami*; Dr. Kevin Feldheim, Field Museum of Chicago; and Drs. Demian Chapman and Ellen Pikitch, Institute for Ocean Conservation Science.

Although large sharks are highly mobile, there is growing evidence that population declines are often remarkably localized. We believe that this is because juvenile sharks tend to stay close to their birthplace (a hypothesis called “natal site-fidelity”), while adult females return back to their own birthplace to breed (a hypothesis called “natal homing”). This results in the establishment of a series of largely independent subpopulations that dwell around nursery areas. Definitive evidence of these behaviors could transform current approaches to shark monitoring, assessment and management. As a result, the U.S. National Marine Fisheries Service has listed this as a research priority.

In the first study of its kind for any shark species, the Institute for Ocean Conservation Science is collaborating to develop a comprehensive picture of how sharks are connected to their birthplace—known as their “natal area”-- throughout their life. This work is building upon a 20-plus-year investigation of lemon sharks (Negaprion brevirostris) led by Dr. Samuel H. Gruber at the Bimini Biological Field Station in the Bahamas. The lemon shark is a “model” shark for the larger carcharhinid group, which contains most of the species that have experienced recent population collapse. The development of population models for lemon sharks can provide insights into the monitoring, assessment and management of other imperiled carcharhinids.

Dr. Gruber, Professor Emeritus in Marine Biology and Fisheries at the University of Miami, has conducted an exhaustive sampling program of an important lemon shark nursery ground in Bimini each year since 1985. More than 2,000 lemon sharks have been tagged and DNA-sampled at this site, revealing two significant findings: that almost all juvenile lemon sharks stay within their natal area up until the age of three, and that the females will come back to Bimini as adults every two to three years to give birth.

We are now conducting genetics and satellite tracking research to answer important questions about the association between older juvenile lemon sharks (3- to 12-year-olds) and adult lemon sharks (12+-year-olds) and their natal areas. We are testing the “natal site-fidelity” hypothesis by looking at whether juvenile sharks living around Bimini can be traced back to a local nursery area by a tag or DNA profile. We already know that adult lemon sharks are migratory, but we do not know where they go after giving birth at Bimini. We are therefore fitting adult female lemon sharks off Bimini with satellite tracking tags to address this question. Lastly, we are testing the hypothesis that female lemon sharks home back to their birthplace. This will be achieved by determining whether females born in the mid-1990’s are the mothers of babies being born from 2008-2011. We are poised to provide definitive evidence of this critically important behavior for the first time in any shark.

Addressing these issues for the first time in any shark species will provide an important framework for shark conservation efforts. If sharks leave their natal areas early in life and never return, then they should form large, well-mixed populations in which individuals face threats across a broad geographic range. Their conservation would require management over wide areas and probably across multiple jurisdictions. In contrast, if juvenile sharks predominantly remain near their birthplace and adults return there to reproduce, they will likely form smaller, more independent populations that are primarily self-sustaining, as we suspect to be the case in Bimini. Their conservation will therefore require more rigorous local management and monitoring efforts.

See the abstract.

Solving the Mysteries of Shark Mating

PI: Dr. Demian Chapman, Institute for Ocean Conservation Science

Unlike most fishes, sharks fertilize their eggs internally rather than reproducing by releasing eggs and sperm into the water column. Mating is therefore a crucial component of sharks’ life cycle, yet it has hardly been studied. Though we do know that fishing can have drastic impacts on the size and density of shark populations, as well as on their gender ratios and body size distribution, we have only a rudimentary understanding of how these changes affect shark mating behavior. The Institute for Ocean Conservation Science is engaged in collaborative genetics research to understand how fishing may disrupt sharks’ mating behavior and affect their genetic diversity and productivity. Current studies are focused on the mating behavior of endangered shark species, including scalloped hammerhead and sand tiger sharks. Among the fascinating discoveries to emerge from this work so far is that female sharks can sometimes become pregnant and reproduce without first mating with a male. Dr. Demian Chapman in 2007 proved, using genetic analysis, that this “automictic parthenogenesis” occurs in sharks. This first-time finding captured global television, print and electronic media attention in over 600 different venues and was listed as one of the top stories of 2007 by Science News. In 2008, Dr. Chapman and colleagues genetically documented a second case of virgin birth in a shark, which was reported globally in over 500 different media venues including Time magazine.

Up until these discoveries, it was logically assumed that female sharks had to mate with a male to produce offspring – an interaction that is becoming increasingly challenging as shark populations plummet globally. But even though female sharks can reproduce on their own, this does not guarantee that shark stocks will remain stable. In parthenogenesis, only the female’s DNA is passed on, producing offspring with low genetic diversity. Populations with low genetic diversity are less able to adapt to different challenges and more vulnerable to unfavorable environmental conditions.

Definining Shark Stocks using Population Genetics

PI: Dr. Demian Chapman, Institute for Ocean Conservation Science

A fundamental requirement of effective fisheries management is to define “stocks,” the distinct units in which different fish populations are structured across the world’s oceans. For sharks, very little is known about this global population structure and few stocks have been defined, making it very difficult to create informed management plans.

Population structure can be determined by measuring genetic differences between sharks collected in different regions. If sharks freely move and reproductively mix between regions, then genetic differences will be slight or nonexistent, and we will know that sharks in these areas should be assessed, monitored and managed as one well-mixed population. In contrast, if movement and reproduction is restricted between sharks inhabiting different regions, then genetic differences should develop over time, telling us that they need to be assessed, monitored and managed separately.

Dr. Chapman is collaborating with scientists from around the world to define management stocks for a host of exploited and imperiled shark species. In particular, Dr. Chapman is developing genetic markers from critical immune system genes that may prove more accurate and precise in delineating stocks than traditionally used genetic markers. By studying these genes, we hope to develop a better understanding of how heavily exploited shark populations may become more vulnerable to disease and parasites.

Conservation Genetics of the Endangered Sawfish.

PI: Dr. Demian Chapman, Institute for Ocean Conservation Science

Sawfish are shark-like rays that are best known for their tooth-studded snouts that can grow to more than six feet long –a third of their body length. There are seven sawfish species worldwide, and all are listed as critically endangered by the World Conservation Union due to overfishing and the destruction of their nearshore wetland habitats.

Sawfish body parts (fins, meat and snouts, called “rostra”) are internationally traded for food, traditional medicines and curios, while live animals are captured for the aquarium trade. Smalltooth sawfish (Pristis pectinata) were also included on CITES Appendix I in 2007, prohibiting most international trade in this species. This is also the first marine fish listed under the U.S. Endangered Species Act (ESA).

These giant, critically-imperiled fish were once abundant from Texas to the Carolinas but have precipitously declined by around 95 percent in the last century due to incidental mortality in fisheries operations, and extensive development in their estuarine nursery areas. U.S. sawfish nursery areas now only occur in scattered locations off the coast of southwestern Florida. It remains unknown whether sawfish in these Florida nurseries are demographically and genetically related, or distinct. They may very well be distinct, considering that juveniles can display fidelity to a certain site, and adult sawfish are known to return home to their birthplace to reproduce. If the nursery sites are distinct, then each sawfish population will be relatively small and vulnerable to local extinction, and many important management and habitat conservation decisions will have to be made at the state or even county level to preserve them.

The Institute’s shark research team is heading a collaborative conservation genetics study of smalltooth sawfish with scientists from The Field Museum in Chicago, Mote Marine Laboratory in Florida, James Cook University in Australia, and the Florida Fish and Wildlife Conservation Commission. The team aims to determine the level of connectivity between these last remaining breeding sites of Florida’s smalltooth sawfish, and to measure the severity of the population decline in this region and its impact on sawfish genetic diversity. This research initiative was featured as the cover story in the August 2007 issue of Science News.

More Information:   Ellen Pikitch, PhD  |  Demian Chapman, PhD

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