Wednesday, November 5, 2014

Tagging sharks in St. Croix

This post is one of a series on projects supported by the Aquarium’s Marine Conservation Action Fund (MCAF). Through MCAF, the Aquarium supports researchers, conservationists and grassroots organizations all around the world as they work to address the most challenging problems facing the ocean.  

In this post, Bryan DeAngelis, working on behalf of the Atlantic White Shark Conservancy, writes about a shark tagging project in St. Croix, U.S. Virgin Islands, that he and Dr. Greg Skomal led.

For scientists who track animals using passive acoustic technology, data downloads are like Christmas morning. This is especially true when the data that comes back is from the sharks you tagged many months earlier. After all the hard work invested to get the tags on the sharks, you don’t know if you have just given a fish a very expensive piece of body art that will never be heard from again or if you’ll provide valuable scientific information that will ultimately improve our ability to

conserve and protect these beautiful animals.  My friends and family who see me off to the Caribbean for these tagging trips regularly ask me:  “What hard work?  You’re off to fish for sharks in the Caribbean!” My response is always the same: “It’s not as glamorous as it sounds.” It typically involves hour after hour on a small boat, getting fried in the sun like a raisin, fishing hook after hook, constantly cutting bait…and smelling like it.  And there is the physical punishment.  Working over the gunnel of a boat in 4-foot seas to surgically implant a small tag in a big fish will leave you with some very large bruises across your sternum. But truth is, I love it.  And one of the reasons I love it is because of the data downloads. 

Passive acoustic telemetry has been a game-changing technology for biologists. It’s a method of tracking animal movement over long periods of time using sound. It works by placing a series of sound receivers in the area of study. Each receiver can “hear” within a range of a few meters to hundreds of meters, depending on the surrounding conditions. The animals to be tracked are tagged with an acoustic transmitter, which periodically sends a unique sound signal. When that sound signal is picked up by one of the receivers, it records the transmitter identification number as well as the date and time it was heard. The battery life on each transmitter is from months to years, essentially giving biologists the ability to track marine animals for very long periods of time. But here’s the catch:  If the animals don’t ever swim within the range of your receivers, you will never hear from them.  And when it comes to sharks, an animal that is typically classified as a highly migratory species, you keep your fingers crossed very, very tightly every time you tag and release a fish with a transmitter.  

A small incision is made to implant the transmitter, and then stitched, prior to release.

Through the generous support of the Aquarium’s Marine Conservation Action Fund (MCAF), my colleague Dr. Greg Skomal and I have recently completed our second year of a pilot study to track sharks around Buck Island Reef National Monument in St. Croix, U.S. Virgin Islands. The Buck Island Monument, which was established in 1961 and expanded in 2001, is one of only a few fully marine protected areas in the US National Park System. The 176-acre island and surrounding coral reef ecosystem support a variety of native flora and fauna, including several endangered and threatened species. Recently, a collaboration of multiple researchers was formed to initiate an extensive study on the ecology of multiple marine species over all trophic levels within the Buck Island Monument. These researchers from various organizations are using passive acoustic technology to study a number of species, ranging from demersal fishes and invertebrates to sea turtles and sharks. To streamline efforts, reduce costs and broaden spatial coverage, each team of researchers contributed acoustic receivers to build a massive array (Figure 1), which allows us all to track the movements of these animals over a broader spatial scale. Such an effort would typically cost hundreds of thousands of dollars, but the participation of multiple researchers greatly reduces those costs. Our work ensures, and will continue to ensure, that sharks are incorporated into this ongoing multi-species coral reef study, which is one of the first of its kind.

A shark returns to the reef immediately after being tagged

The value of apex predators to coral reef communities is becoming more apparent. Scientists increasingly understand that apex predators, like sharks, exhibit tremendous top-down effects on our coral reef ecosystems. For example, the absence of sharks may actually cause coral-dominated reefs to convert to algae-dominated reefs. As a result, major initiatives to protect and conserve sharks through the implementation of sanctuaries are being considered in some parts of the world. However, there is very little understanding about how to effectively implement such strategies for the greatest conservation impact. For example, many of the ecologically important Caribbean shark species utilize entirely different habitats during different stages of their life, and therefore will be exposed to different threats with different rates of vulnerability. Also, there is very little understanding of residency time, home range and site fidelity for sharks in this region. Without a better understanding of these relationships between habitat use, species and life stage, implementing conservation strategies such as sanctuaries and Marine Protected Areas may be limited in effectiveness.   

In 2013 and 2014, we acoustically tagged 20 sharks representing four species (Figures 2, 3). We have had two data downloads spanning more than a year since then … and we did not get coal in our stockings.  The results have been really exciting! Of the 20 sharks tagged, only one was never heard from again. In total, we have recorded more than 37,000 detections. While it’s still very early and a lot more tagging and analysis has to happen, we are very encouraged by the preliminary data.  Most sharks exhibited strong site fidelity to the Buck Island monument (Figure 4). 

Detection history plot of two juvenile lemon sharks tagged in 2013.  The data demonstrate strong site fidelity and habitat preferences within the Monument.  These two sharks have used this marine protected area exclusively for over a year.

Also, the data are revealing valuable insights into habitat use within the monument by the different species and age classes of sharks, and the rates of movement outside the protected waters of the monument.  Ultimately, we want to provide resource managers and communities with science-based information that will help them to implement the most effective conservation strategies for sharks.  While doing our part to conserve and protect sharks is what keeps Greg and me involved in this work, I would be lying if I didn’t admit that I also get the excitement and anticipation of a new Christmas present whenever a new data file shows up in my Dropbox.   

Bryan DeAngelis (foreground), working on behalf of the Atlantic White Shark Conservancy, and Dr. Greg Skomal, of Massachusetts Marine Fisheries, led the acoustic tagging study of sharks in St. Croix, which was supported in part by MCAF.

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