Capture Methods

Long-lining is a globally recognised commercial fishing technique often used to target swordfish, tuna, halibut and many other fish species. It is also an effective tool for catching sharks and allows scientists to monitor their abundance and distribution as well as population demographics. It uses a long-line, called the main line, with baited hooks attached at intervals by means of branch lines called gangions. A gangion is a short length of line, attached to the main line using a clip, with the hook at the other end. Long-lines are classified mainly by where they are placed in the water column i.e. at the surface or on the bottom. Lines can be set through use of anchors or left to drift. Hundreds or even thousands of baited hooks can hang from a single line.
 
At the sharklab
The marine environment at Bimini is diverse with habitats ranging from semi-enclosed shallow flats bordered by mangroves, to seagrass beds and coral reefs sloping to the deep pelagic zone. The plankton-rich waters that flow in daily with the tides from the Gulf Stream generate pristine waters and environments for a diversity of marine animals. BBFSF use two longline techniques that allow us to survey shark population demographics, movement and growth around the Bimini Islands both in shallow and deep water habitats.
 
Shallow water:
BBFSF has been long-lining since 1990. Currently we set five lines monthly, four run north to south down the east side of the main lagoon and the fifth, “wild card” line, is a roaming line that is deployed in a new location each set. Our lines are designed for the shallow sandy bottom waters in Bimini, being 500m in length with 15 gangions positioned at 30m intervals, secured with anchors and marked with floats at each end. Circle hooks are used to avoid foul hooking and reduce by-catch i.e. capture of non-shark species. Lines are typically set in the early afternoon, soak for 24hrs and checked every 3-5 hrs. 
 
Photo Credit: Matthew D Potenski
 
Deep water:
In 2005 BBFSF began setting a deep line during its University courses to sample the elasmobranch fauna in the deep waters off Bimini. The main line is 300 or 400m in length (depending on depth set at) with between 4 and 30 hooks, positioned at regular intervals in the last 30m. The line is designed so that the bottom section rests on the seabed with concrete block and anchor, rigged with a break-away mechanism in case of entanglement. This is then attached to two large polyballs at the surface, allowing us to recover the line.
 
 
When targeting large tiger sharks we set early in the morning and haul just after lunch using 4 large baits on 20/0 circle hooks. We are also able to target smaller deep water species, such as bigeye sixgill (Hexanchus nakamurai). During these sets we include 30 smaller baits on 8/0 circle hooks and deploy over night. An electric pot hauler attached to a boom and pulley is used to pull the lines, taking ~30 minutes to haul the main line back onboard the boat before reaching the baits.
 
Once captured sharks are secured to our research vessel through tail and or pectoral fin ropes and processed for biological information including: species, sex, size (pre-caudal, fork and total lengths) and fin samples for genetic and stable isotope analysis. Sharks > 130 cm (total length) are also tagged with a Casey dart from the National Marine Fisheries Service (see Identification tags). Lemon sharks (Negaprion brevirostris) are further marked with a Passive Integrated Transponder (PIT, RFID) microchip tag adding to our long term data set for this species (Research Techniques - Identification Tags). Additional comments about the shark (i.e. hook location, mating scars etc) and environmental conditions (i.e. depth, temperature) are also recorded. Finally the hook is removed and the shark released. For results from our longline program please see current and past research.
Gillnetting
Photo Credit: Matthew D Potenski
 
Gillnetting is a common fishing method used by commercial and artisanal fisherman around the globe, in fresh and marine water. It is also an effective tool for catching sharks and allows scientists to monitor their abundance and distribution as well as population demographics. Gillnets are a series of panels of meshes with weights along the bottom, and a headline, to which floats are attached. They can be used in a variety of environments in shallow and deep water by altering the mesh material, floats, weight and net length. In commercial fisheries, the meshes of a gillnet are uniform in size and shape. Fish smaller than the mesh of the net pass through unhindered, while those too large to push their heads through the meshes as far as their gills are not retained. This gives gillnets the ability to target a specific size of fish, unlike other net gears such as trawls, in which smaller fish pass through the meshes and all larger fish are captured in the net.
 
At the sharklab
Since 1995 BBFS have used gillnets as the primary means to capture juvenile lemon sharks (< 100 cm total length). Nets are 180 m in length, 1.5 m high with 20 lb monofilament diamond 10 cm stretch-mesh. Small white floats are threaded along the headline of the net, with lead weights along the bottom line keeping the net vertical in the water. On deployment nets are secured at the shoreline to mangrove roots or rebar then run perpendicular to shore manually or by boat. Each net is further split into four sections by floats set at 45 m intervals then finally pulled tight and attached to an anchor and rebar. Nets can be set for as long as the crew can stand it, in Bimini we typically set over night for 12 hrs!
 
Nets are checked every 15 min via walk, snorkel or boat. Once a shark has been captured it is gently, quickly and carefully removed from the net and placed in a holding container on the boat. It is then checked for a PIT tag (Research Techniques - Identification Tags) and the time and location of capture (net sector) recorded.

All sharks are transported to a ‘tagging’ boat or ‘work up pen’ where they are processed for biological information including: species, sex, size (weight and pre-caudal, fork and total lengths) and fin samples for DNA and stable isotope analysis (Research Techniques - Diet Analysis). Each shark is scanned again for a PIT tag and if none is detected, a tag is injected into the muscle just below the first dorsal fin. At this stage further samples such as blood or stomach contents can be obtained or acoustic and accelerometer tags implanted or attached. On completion sharks recover in semi-captive holding pens (Research Techniques - Captive Experiments) usually overnight before being released, or used in behavioural trials. Our nets are designed specifically for the capture of juvenile lemon sharks so bycatch is minimal; however if other animals are captured they are removed and released immediately.

Rod and Reel Fishing

Photo Credit: CJ Crooks

 

Rod and reel fishing is used by BBFS staff and volunteers as an effective non-lethal method to target particular shark species around the Bimini Islands.  Field crews will use heavy duty trolling rods with a minimum of 40 pound test with either 1/16” gangion wire or 80 lb. fishing wire rigs. Hook sizes vary from 5/o to 15/o circle hooks. Baits are typically suspended in the water column using floats.  This method is preferred when targeting juvenile lemon sharks and blacktip sharks as they normally feed mid water.  The fishing field crews create a slick, by either deploying chum blocks in mesh bags or by scraping small pieces of baitfish in the water in order to draw sharks to the baits floating down current from the boat or dock.  Once hooked, the field crews reel the shark next to the boat where they will either net the shark and place in a portable shark tub or tie the shark to the side of the boat. The size of the shark will determine the work-up method. Smaller sharks can be brought on board and easily worked-up placing them in a circular tub.  

Rod and reel fishing 'sharklab style' is fast and efficient and allows for a quick capture to work-up time.  Rod and Reel shark fishing crews normally set up in the shallow flats around Bimini to draw in sharks.  Many juvenile lemon sharks have also been caught from our dock at the Sharklab.  In addition, rod and reel fishing allows our scientists to target particular individuals identified by color coded tags or within a desired size range.

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