The Rundown - Results from the

PIT Census 2007

Getting up close and personal with a juvenile lemon

Another PIT Census is in the books and the 2007 BBFS team has performed well under pressure, successfully adding yet another chapter to their extensive body of research into lemon shark biology. In both the North Sound and Sharkland lagoon areas we were able to capture and census the second highest number of total sharks in the last six years. In twelve intense nights of fishing the team netted a total of 233 juvenile lemon sharks.

As you can see in the table, the two lagoons totaled 143 and 90 sharks for Sharkland and the North Sound, respectively. The pattern of catching more sharks in Sharkland than the North Sound is typical of the patterns we have seen in past years (with 2005 being an anomoly). Males outnumbered females in both areas of the lagoon and for both newborn and recaptured sharks. There is some overlap in home range for sharks between the two lagoon areas, and this was witnessed by 15 sharks being sampled in both periods of fishing (ie both lagoon fishing locations). A look at the following figures shows the total catch for each sampling site in the lagoon over the last six years.

In Sharkland, we can see a slightly increasing trend in total sharks over the six year period. We have caught over 100 juveile sharks in this part of the lagoon in each of the last five years. This year was marked by the highest total of newborn sharks caught in this area of the lagoon since the project's inception. The number of recaps stays relatively consistent over the six year period (with 2005 being a high year). In every year but 2005, the number of newborn sharks exceeds the number of recaptured sharks. The last two years have seen a surge in newborn sharks, most likely due to the recruitment of new mother sharks into the Bimini lagoon. The Sharkland nursery seems to be highly productive but also somewhat volatile. This area is further down in the lagoon and more open to larger sharks to come in and predate on the juveniles. This is probably the main reason we don't see a consistent pattern forming and we eagerly wait each year to see how the numbers add up.

The North Sound shows a very stable pattern up until 2005. This was the only year we have caught more than 100 sharks in this part of the lagoon. As the graph shows this is due to an unprecedented amount of newborn sharks in 2005, on the order of at least twice as many as any other year. In 2006 and this year, the number of newborn sharks returned to a similar level to 2002-2004. The total number of sharks the past two years has been somewhat greater than 2002-2004, as is due to the higher number of recaptured sharks in the past two years. The recapture rates are probably influenced by the large number of newborns in 2005. If mortality is not population density-dependent in the lagoon we can assume that out of the higher number of newborns, an appropriately high number will survive and be recaptured i the subsequent years. The data seems to bear this hypothesis out, as 2006and 2007 are the only years in which the recaptured shark numbers exceeded the newborn shark numbers. In the following years the number of sharks captured in the North Sound may return to their relatively consistent state if no new spikes in newborns are recorded.

Looking at the graphs of the total length distributions, you can easily see two simple patterns emerge. The first thing to notice is that the scales on the x and y axes for these two graphs are not the same. The newborn sharks are distributed from approximately 52-68cm, while the recaptured sharks range from 60-112cm total length. The frequencies are also much smaller for the recaptured sharks. The distribution for the sizes of the newborns displays a very typical "Bell Curve" shape indicating a normal distribution. This is what we would expect to find in a sampling of newborns - a diversity of total lengths with most individuals falling within median values and a few outliers. This simply means that most of the baby sharks will be a similar size, though there will be a couple "runts" of smaller size, and a coupe "giants". The recaptures graph shows another expected result. The curve for this graph shows a dramatically shifted distribution with a long right tail. This means that the majority of the recaptured sharks are of small to medium size, with the number of individuals significantly dropping as the size increases. What we can interpret to be happening with the recaptures has numerous indications. Primarily we are probably seeing the effects of mortality, as over time more shark die and the numbers that survive to older age (and larger size) become few. The graph may also indicate the effects of sampling bias, as the larger sharks may be strong enough to rip through the monofilament and therefore avoid capture. Finally, the graph may indicate some of the effects of emigration from the inner lagoon as sharks attain lengths of over 1 meter. The majority of the recaptured sharks have a total length of 60-75 cm. This shows significant over lap with the newborn sharks' length distribution, indicating that the majority of recaptured sharks are from the previous year. These sharks have survived their first year and are still a size that is easily caught. There appear to be two more significant size ranges to this graph. there appear to be a lot less sharks in the 80-100cm region. Mortality probably plays a significant role in the lower number of this size class. Finally, there are only a couple sharks exceeding 1meter total length. Sharks in this size class may elude capture by having moved to the lower lagoon or by simply breaking through the nets. In summary, for both size distribution graphs the data bears out a very definitive and classic pattern that can easily be explained by shark biology.

This important data was collected by a diverse team of staff and volunteers who came together for a well-coordinated team effort. It always amazes me how a group of 20+ individuals can meet and become good friends in a few short weeks. The intensity of the PIT activities, the long nights on the boats, and close living quarters allow you (or practically force you) to drop many barriers in a short amount of time. The unity of purpose and shared interests usually allow complete strangers to be laughing like old friends within the first few days. The interactions with fellow lab mates and experiencing of new cultures, music, and lifestyles adds a real human dimension to the lab experience. The world has a lot to teach every one of us, and at BBFS not all the lessons have to do with shark biology. I have learned a lot about people and how to deal with them in many diferent situations in my time at the lab. Every year the PIT project brings together a different group and every year PIT volunteers bring different things to the table. It is always a new experience and always proves valuable to learning something about life. We had a great group for PIT (even with the great overabundance of Brits), and I think we managed to experience the science, the sharks, and each other in a very profitable and memorable way.

Just your average group of crazy shark lovers...

Author's Note

I like to reserve this small endnote to make a few last personal comments on this year's PIT experience for me. I realize that all the text is from my perspective, but I try to be as objective and descriptive as I can. So please humor me as I try to bring this whole project together in some sort of summary and distribute platitudes as they are deserved. I have been making trips out to the lab for almost a decade now, and I have a large store of experiences, friendships, and emotions that are inseparable from the Sharklab. Somehow, every year when June rolls around, I find my way back to the lab. In some ways, it is a lot like going home - the lab is a place where I truly feel I belong and I can let my inner shark geek completely free amongst like minded individuals. The Lab encompases many things and Doc's continued work has created several types of legacies. The scientific reputation of the lab and caliber of work that is accomplished here is cemented in place. But even more than that is the personell over the years that have put in their sweat, laughs, tears, and sometimes even blood (after all - there is the periodic shark bite...) who have all benefitted from our unique shared interests and communal work towards common goals. This intimacy of living space and daily work regime forges amazing relationships. I would like to say that Doc's legacy will not only be his impressive body of work, but additionally the large surrogate family of BBFS alumni he has acquired over the years. This is self-evident every AES annual meeting, when we have a banquet for lab alumni which in past years has hovered around 50 individuals. I have personally traveled to visit friends garnered through the lab, even gone to weddings and other events. I think it is amazing that Doc has done so much to open so many minds to the ways of field science, to train us, and to have us interact on such a level that we are a complete social and professional network. I hope I speak for many when I extend my sincere thanks to Doc for being a role model and wonderful positive influence in my life and career. Doc officially retired from the faculty of RSMAS earlier this year, but his dedication to the lab remains his driving force. As long as he continues to be dedicated to BBFS and all it stands for, I hope to be by his side aiding him.

I would also like to comment specifically on this year's PIT census project. The lab was in a transitional period and the new lab managers were somewhat "thrown into the fire". Not enough can be said about the level of effort Kat put in to this project as she was usually the first one up and the last one in bed. Without her constant overachievement and seemingly constant state of multitasking we would have been lost. Christian has been amazing in stepping up his role to learn and maintain all the house's systems, the truck, and the boat engines . While we had our usual level of small glitches, we had no catastrophic equipment failures and I think that is a token to his diligence and ingenuity. Tristan, Steve, and Joey did their usual stellar jobs of maintaining calm and cool in their command in the field. Emily showed up to lend an extra hand just when it was needed most. Last, but definitely not least, was the team of dedicated volunteers. I would like to thank everyone personally for their efforts in the intense schedule that comprises PIT. We caught an exceptional amount of sharks with only one mortality and the team worked like clockwork with an efficiency I was duly impressed with. Along the way we managed to have lots of laughs and make the work environment very enjoyable. Great job everybody and I hope to see many of you again in 2008!

- Matthew D Potenski, 2007

MDP with his weapon of choice (Photo courtesy J. Rajnohova)