A Workup

Most of the work at the Shark Lab has historically focused on the lemon shark, and over the years, standard procedures for taking data when one is caught have been developed. Some of the ways the sharks are caught include gill netting, long-lining and old-fashioned rod and reel. What follows is a description of how juveniles are "worked up".

Let's say, for example, a few juveniles are needed for observation. First we might set a gillnet perpendicular to the shore by tying one end to the mangroves and the other to a rebar stake. When a shark is caught and we intend to temporarily keep it in a pen behind the lab, it is placed in a "transport box" and driven over to the pen. We can then go back to the pen at any time to start a workup.

For a workup, we drive one of the Carolina Skiffs out to the pen (which is about chest-deep at high tide) and anchor up. On the boat we have dip nets, a scale, a measuring trough, PIT tags and PIT tag reader, and other equipment necessary for the workup. First, a dipnet is used to scoop up the shark in the pen. Someone will then reach into the net and hold the shark behind its head, in front of the first dorsal fin. The shark is held just under the surface of the water and walked over to the boat, where a measuring trough filled with water is waiting.

The trough, a PVC pipe sliced lengthwise in half, has a ruler along the bottom. Three measurements are recorded: precaudal lenth (PCL), fork length (FL) and total length (TL). PCL is measured from the tip of the nose to the notch just before the shark's caudal (tail) fin; FL is measured from the tip of the nose to the fork in its tail; and TL from the tip of the nose to the tip of its tail.

A PIT tag reader is waved in the vicinity of the first dorsal fin, where PIT tags are always implanted. If the shark has been caught and tagged before, the reader beeps and displays a number. This number is recorded, and all of the data will be entered into its already-existing section of a master database. If there is no beep, a new PIT tag is implanted. To do this, a tiny incision is made with a scalpel just below the base of the first dorsal fin. A syringe is loaded with a PIT tag, and it is injected into the incision. The number of the tag is recorded, and the shark will be added to the database.

It may be necessary to take a DNA sample from the shark. There are several ways to take a sample, and the most common ways are by clipping the corner off of a fin with scissors or by using a small hole-punch in its tail fin.

The status of the umbilical scar is noted by turning the shark over and seeing if it is open (a newborn), just closed (a little older) or closed. The sex of the shark is also recorded (males have two claspers).

The sharks are weighed using a hand-held scale. One
person holds the scale while another lifts the shark out of the measuring trough and places it in the scale trough.

Finally, observations are made about the condition of the shark: stress (there are signs such as white gills), fin rot (very common - tips of fins look discolored and/or tattered), any scars or other distinguishing characteristics, etc.

Once all measurements have been made, the shark is either walked back to the pen or released, depending on what needs are. All of the data is recorded in a standard format in log books and later added to the master database at the lab. It's through careful measurements like these that things like growth rates have been able to be calculated. If you catch a shark one day and measure it, then you catch the same shark again a year later, you know exactly how much it has grown in one year. Multiply this by the incredible number of sharks in the database, and you have a great snapshot of the overall population.

Bimini: Round 2

I couldn't stay away. The two months I spent at the Shark Lab in September and October of last year were so incredible that I have to go back. Luckily, they'll have me. For the month of June, I'll be back at the lab in bimini for PIT.

PIT (Passive Integrated Transponder) is the annual tagging program the lab runs, where for 3 weeks, juvenile lemon sharks are caught, tagged and released. (For more info on the program, click here.) Gillnets are set overnight for 12 hour periods (a full tidal cycle), and we'll sit out in the lagoon all night catching the sharks and working them up (tagging, measuring, weighing, etc.). Why? Good question. Here's an abridged version of the answer on the Shark Lab's website...

What it came down to for us was to assume that we had no immigration or emigration in the lagoon/nursery where we were studying young lemon sharks. We also assumed no fishing--only natural mortality. Then we chose a sure sampling procedure: We caught and tagged every last lemon shark in the nursery.

We fish the same stations for six nights: The first night we get maybe 65 sharks; the next perhaps 30 and then after a few days maybe 1 or 2 or none. Why? We have caught them all and they are in our big pen. It’s quite a sight to see 100 little lemon sharks all schooling together in the pen. Once we are satisfied that we have all the sharks in that particular nursery area we go on to the next one.

Then we do it all over again in an adjacent location. We find only little overlap between the two nursery grounds as the situation repeats itself: 60 the first night; 30 the second and 2 or 3 the third and so on. Perhaps only 1 or 2 are recaptured from the nights before.

This summer, I will be heading back to Bimini for PIT. There will be mosquitoes, sandflies, extreme heat, sleep deprivation, and hours spent hunched over in the blazing sun fixing gillnets, but then there will also be opportunities for pictures like this:


Last time, I wasn't really able to keep up the blog while I was there because of getting only 30 minutes of internet time a week on one computer on a dial-up connection. This time it will be even worse because there just won't physically be time. So instead, what I'm going to do is post pictures and stories about when I was there before, and I'll continue that up until the point I leave for Bimini at the end of May. When I get back, I'll let you know how it went. Stay tuned...