Early Life History
Dr. Samuel H. Gruber
Why do we do the PIT tagging? 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. Here's what we did: We set up a big pen in the lagoon and fished large sections at a time. We knew from John Morrissey's comprehensive tracking study and more recently Bryan Frank's that baby lemon sharks have a VERY restricted activity space. We thus set 180-meter long gill nets at three stations at sundown and every 15 min. over the 12-hour summer night we walked/swam those nets. When we came upon a lemon shark we removed it brought it to a skiff and ran it out to the tagging crew at the big pen. The tagging crew weighed, measured, sexed and tagged sharks as they arrived. We also took a fin sample for DNA analysis.
After years of searching we came upon a tag with virtually no shed rate and absolutely no deleterious effects on the little sharks. It is called a passive integrated transponder (PIT) tag and is tiny--rice grain sized with the diameter of a #2 pencil lead. It is a glass encapsulated electronic transponder that needs no power source. Excite the circuit with microwaves and it spits out the number, which comes up on the reader device. We actually inject the tags into the base of the dorsal fin where we hope it stays for life.
Anyway, 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. But by the end of the three nights of fishing we are exhausted and have to take a day or two off. During the rest days we send out a boat to feed our little charges then eventually release them en-masse. We know from telemetry studies that they go back to their activity spaces, which are only several hundred meters away in any case.
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.
One other thing: We work at Bimini, an island complex surrounding a mangrove fringed shallow lagoon. The islands are only 48 miles east of Miami in the Bahamas. As far as I know, Bimini has the only viable nursery grounds within miles, so the baby lemon sharks cannot migrate--they are stuck for at least 3-years in the lagoon. If they venture out they get eaten--quickly.
Well, our first attempt suggested that there were only about 80 baby lemon sharks born into the nursery at the North Sound, a part of the lagoon. We did this study before we began the catch-them-all technique. We used a sampling technique and estimated abundance. But starting in May 1995 we have caught around 100 in the North Sound and a further 100+ in a nearby place we call Sharkland. As I write we have data on over 2000 lemon sharks at Bimini each with a PIT tag. Probably 70% are dead from natural causes-juveniles have high mortality of 40-60% in the first years. Lots of these are bigger sharks with dart tags but after many years of trying this and that we have settled on the PIT method I outlined above.
The thing is that only because of the species of choice, the lemon shark, and the unique conditions at Bimini --resembling a lake more than an open, infinite marine environment can I do these comprehensive and detailed studies. The usual situation is far more daunting: If you get a 3% recovery rate from your tagged sharks, you are doing very well indeed!! Eventually at Bimini we will get a 99% recapture rate since all the sharks will carry a PIT tag-and we have often gotten a 94% rate. Thus we have been able to learn a lot about the early life history of my model species--the lemon shark.
Even in the bad old days (the 1980s) our recapture rate approached 45% and we often tagged and released the same shark 5 or more times over the course of several years!! But that's Bimini and that's the lemon shark!!
Well you shark lovers asked for it--and I got carried away. But I thought you shark folks would like to hear a little about the nitty gritty of it. One other thing: We don't have a monster Federal grant. Our work is carried out with volunteers. If any of you out there in cybershark land are interested in helping, drop me a line.
For more information on our annual PIT Tagging Program, see Eric Cheng's PIT Journal from 2003.
Also, to see coverage of the 2004 PIT Program, click here.
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