//
you're reading...

Sharkbites Saturday

The Great Migration: Blacktip Sharks

Paper: : Kajiura SM, Tellman SL (2016) Quantification of Massive Seasonal Aggregations of Blacktip Sharks (Carcharhinus limbatus) in Southeast Florida. PLoS ONE 11(3): e0150911. doi:10.1371/ journal.pone.0150911

The Greatest of Migrations

Nature boasts of many cool migrating species: monarch butterflies, penguins and salmon to name a few. But, perhaps one of the most unique and awe-inspiring migrations of them all occurs off the Southeastern coast of Florida. Every winter, tens of thousands of blacktip sharks gather along the coast of Palm Beach County as they all reach the southernmost destination of their seasonal migration, as seen in this stunning aerial photo.

Figure 1. Picture from aerial survey of the Palm Beach county coastline during the Blacktip aggregation. Source: Kajiura and Tellman, 2016

What is a blacktip shark?

The blacktip shark (Carcharhinus limbatus), not to be confused with the blacktip reef shark (Carcharhinus melanopterus), is a medium-sized coastal shark found along the coast of the Southeastern United States. They are called blacktip sharks because of the characteristic dark edges along their fins (Figure 2). Although sometimes found offshore, they are most commonly found on or near insular (island) and continental shelves. They can get up to 2 meters long and feed on bony fishes, small elasmobranchs (sharks, skates and rays) and invertebrates. In addition to their impressive migration, blacktips also exhibit another unique behavior, parthenogenesis, which is a form of asexual reproduction in which an embryo grows from an unfertilized egg!

Figure 2. Photo of a blacktip shark. Source: By Albert Kok -Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=45447550

You’re on candid camera!

Dr. Stephen Kajiura of Florida Atlantic University and his team have been studying and quantifying the yearly aggregation of these impressive creatures since 2011. They use aerial biweekly surveys to take videos as they fly along the coast of Palm Beach County. Using this high-resolution video, they can count the number of sharks in the camera’s field of view to determine the abundance of blacktips in the area.

In their most recent paper, which analyzed data collected between 2011 and 2014, they found that the abundance of blacktips was highest between January and March. The highest number of blacktips counted in a survey was 12,128 sharks, which is approximately 803 sharks per square kilometer! Most of the sharks can be found between Boynton Inlet and Jupiter Inlet. The migrating blacktips are usually gone by April or May once the water starts to warm up, but while they are all there, it is a sight to behold (Figure 1).

 

Why Palm Beach?

Kajiura and Tellman propose that the blacktips aggregate so close to shore of Palm Beach because the continental shelf narrows significantly, which creates a natural bottleneck for the sharks that utilizes the shelf. The Gulf Stream also comes close to shore in this area, so the sharks would have to expend more energy swimming against the strong offshore current.

There are 2 smaller peaks in shark abundance along the East coast of central Florida, which suggests that the sharks are traveling from Northern Florida to Palm Beach County and back, using central Florida as a migration corridor.  Kajiura and Tellman present several factors that may lead to this migration and large aggregation: feeding, thermoregulation (maintenance of appropriate body temperature), predator avoidance, or a combination of various factors. There is evidence to support all these factors: the migration aligns with the spawning of blacktip prey species, blacktip predators (larger sharks) move offshore, which could force blacktips inshore, and the blacktips begin moving into the area once the water temperature is below 25 degrees Celsius, which could be a result of the need to thermoregulate.  However, there are some sharks that stay in the area year-round, which indicates that temperature is perhaps not the only factor.

 

Which sharks are migrating?

The fact that there are resident blacktips in the area year-round presents an interesting question: “Why don’t all blacktips migrate?” Kajiura and Tellman propose that the resident sharks may be non-mating females, for example females that are outside of reproductive ages, or females that are in the ‘resting’ phase and not mating that year. Alternatively, they suggest that the resident sharks could be a subpopulation of blacktips with a larger thermal tolerance that are better adapted to handle more extreme fluctuations in temperature and therefore don’t need to move to regulate their internal temperature. The average size of the sharks in the aggregation is 173.4 cm, which suggests that most sharks migrating are mature adults.

 

Blacktips and Humans

As you can imagine, this large aggregation of sharks gets a lot of media attention every year. Despite what the movie Jaws has led us to believe, sharks are not man-eating machines, and the blacktips pose minimal risk to humans. Many beaches close during the time of the aggregation, however, in an abundance of caution. But, if anyone is at risk, it’s the blacktips. blacktips are harvested in commercial and recreational fisheries for their meat. In fact, blacktips are the second most harvested shark in Florida. Regulations have been put in place to protect the sharks during this vulnerable time. During the months of the mass aggregation, harvesting of the species is limited to 1 shark per person or 2 sharks per vessel to prevent the species from being over-exploited.

In addition to fishing pressures, the Blacktips must contend with warming oceans, which will likely force a shift in migration time or aggregation location. This could have cascading effects on the ecosystem off the coast of Palm Beach County.  There has already been preliminary evidence that the aggregation is decreasing in size, suggesting that Kajiura and his team will have more interesting data to gather as they continue to monitor the blacktips’ response to the changing climate. We can only hope the species will be able to adapt, but for now we can appreciate the greatest of migrations.

Discussion

No comments yet.

Post a Comment

Instagram

  • by oceanbites 2 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 3 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 4 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 5 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 5 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 7 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 8 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 8 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 9 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 9 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 10 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 11 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 11 months ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com