The Lifespan of a Lemon: Shark Science in Bimini

Juvenile lemon shark in the mangroves. PHOTO CREDIT: Jillian Morris-Brake.
Juvenile lemon shark in the mangroves. PHOTO CREDIT: Jillian Morris-Brake.

[dropcap]A[/dropcap]s the saying goes, age is just a number, but when fisheries managers are building their shark population models, it can be a very important number. In Bimini, scientists have discovered lemon sharks as old as 37 years, which is almost double the previously estimated life span.

Every June for 25 years the Bimini Biological Field Station has been able to capture juvenile lemon sharks within six weeks of their birth. These sharks give birth to live young, leaving an umbilical scar (belly button), which closes up at approximately 6 weeks of age. Each shark is measured, a DNA sample is taken (small fin clip) and a barcode- PIT (Passive Integrated Transponder) tag is inserted under the skin. These tags are just like the microchip your pet at home might have. The DNA is sent to a lab in Chicago to be analyzed and becomes part of a shark family tree, meaning the lab knows mom, dad, brothers and sisters. The little sharks are caught again and again over the first 6-8 years of the life, providing data about their initial growth rates and habitat use.

Sharks born in 1990 are still alive and still giving birth. The mothers do not need to be captured because the DNA indicates which neonate (new born) sharks are theirs each year. One shark, affectionately named Lucy, seems to live by the motto, “It’s Better in the Bahamas,” as she has been coming to Bimini for 32 years. Another female, ‘B-Female-12’ subscribes to the same motto, having been to Bimini to pup (give birth) since 1989, making her approximately 37 years old. Lucy was originally caught in 1997 and gave birth that same year. Her pups were caught and tagged subsequently in 1999, 2001, 2003 and she was caught again in 2005, as were her pups. Her pups were tagged every other year from 2005 to 2013. In 2014 she was caught again and was covered in fresh mating wounds, her capture was actually filmed by my husband and I for the BBC’s Shark documentary.

Sharklab biologist Rachael Cashman inserting an acoustic transmitter. Photo courtesy of Jillian Morris-Brake.
Sharklab biologist Rachael Cashman inserting an acoustic transmitter. Photo courtesy of Jillian Morris-Brake.

Lucy was equipped with an acoustic transmitter, a coded tag that pings her own identity code every three minutes for the next nine years. Now her pups can be identified through DNA, but her movements around the Bahamas and the USA can be tracked using the acoustic receiver array. These underwater receivers detect these tags and record date, time and tag number, which gives scientists another glimpse into the life of these sharks.

All of this data is important because it allows scientists to do fairly accurate estimates of populations by knowing how long sharks live, the age at which they start pupping (14-17 years for lemon sharks) and how many pups they give birth to. They can estimate natural mortality rates and the time required for local populations to rebound. If any of this information is wrong when setting harvesting limits, this could lead to collapses in local fish populations, which has happened in

the past. Having such accurate life history data is important for the sustainable management of any fish species, especially sharks, like lemons, which are highly susceptible to overharvest because of their life history characteristics (slow growth, late maturity and few offspring).

Lemon sharks are protected in the Bahamian Shark Sanctuary, but are not protected in the Federal waters off the coast of the USA, so having this population data is invaluable for conservation management of the species.

To view a video of a lemon shark birth, visit https://www.youtube.com/watch?v=hdQoBbGLGZ8

Juvenile lemon sharks part of the Sharklab PIT Project. PHOTO CREDIT: Jillian Morris-Brake.
Juvenile lemon sharks part of the Sharklab PIT Project. PHOTO CREDIT: Jillian Morris-Brake.
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