The BBFS Annual Lemon Shark Census
PIT 2008 Rundown
We came, we saw, we caught some lemons...
June 26, 2008
A BBFS "Sharkdive" with snorkellers lined up near the Twin Vee
PIT is officially complete, but there is one additional activity we decided to participate as a sort of reward for our hard work. We chopped up some bait and piled into the Twin Vee to head out to Triangle Rocks. The whole team came out to see some large Caribbean reef sharks on a lab "Shark dive". A series of three anchor lines kept the Twin Vee suspended next to a sandy bowl. The water was murky, but a few curious sharks started to circle. Once everyone was on the line, the bait started to fly. The splashes of the bait hitting the water excited the reef sharks and they came jetting in to swallow the food. There was a large female with an old casey tag completely grown over with algae. This girl is a familiar sight on our shark dives, but she was especially frisky today, even going so far as to nose her way through our line of snorkellers. She swam in tight circles and chased fish and other sharks off to claim the bait. Sharks work on a simple social system - the largest shark is dominant and this female knew where she stood. She definitely got the lion's share of the bait, but kept coming back for more. In all, it was an exciting experience for the crew and a nice way to cap-off our shark experiences for PIT.
A reef shark glides in to say hello
Another PIT Census has been completed and we now can take a look at the data we worked so hard to gain.
You will find below a table that shows a breakdown of the shark catches by location, sex, and number of newborns and recaptured sharks.
Looking at the data we can see that we caught more sharks (much more actually) in the Sharkland area of the lagoon than the North South. This is the common pattern from past years with only one exception (2005). All thirty-eight sharks in the North Sound were either newborns or recaptured previously tagged sharks. However, in Sharkland we managed to catch twenty-seven sharks that were untagged and not newborns. This means they could have been "hiding" in the mangrove channels off of the lower lagoon and we missed catching them before. In both lagoon areas we managed to snare more recaptured sharks than newborns. The sex ratios for both lagoon and classes of sharks showed good balance, and although females slightly outnumbered the males it is very close to a 1:1 sex ratio. It is interesting to note that in the North Sound the males actually outnumbered the females, despite the aggregate trend towards more females.
The following graphs depict the shark catches for each lagoon location over the last seven years of the project. Since the PIT Census project has been conducted for the last fourteen years, this graph shows half of the existing catch totals we have archived.
Looking at the first graph we can see a fairly consistent pattern in the number of shark catches in Sharkland. We have caught over 100 individuals in this area of the lagoon for the past six years running. Last year we caught the highest number of newborns in any PIT census, and while this year we caught less newborns, we are are easily in the range of the years 2002-2005. We caught more recaptures than last year, and a few less than the peak year of 2006. In all the numbers and distribution of the shark catches in Sharkland fall within observed levels and indicate another year of normal recruitment to the lemon shark population of the lagoon.
While all is relatively status quo in Sharkland, this year we witnessed a large deviation in shark catches in the North Sound. This year we had the lowest catch for the North Sound in the last seven years. The main cause of this low catch can be attributed to a very low number of newborns being recruited into this area of the lagoon. With the exception of the hug spike in 2005, the catch rate of newborns is fairly consistent over the last six years. This year's total of newborns is about half of the historical catch rate. Recaptured shark catches is also down from the past few years, but is not too far off from 2004 levels. While several factors could have contributed to the low catches, the lack of more newborn sharks is evidence that there were less females who used this area of the lagoon for parturition. Exactly what caused less females to drop their pups in the North Sound can only be guessed at for now. Since the Sharkland catches fell within their normal range, it can be assumed that none of the mother sharks who usually use the North Sound had instead chosen to pup in Sharkland. This simply means that there are some mother sharks that did not make it to Bimini to pup. Mortality - natural or human-related is a possibility. In this case, we did not see a corresponding recruitment of new mother sharks to replace the old guard. Many of the mothers reproduce every other year, so maybe a few sharks were thrown off their cycle and may come in to pup next year. In this case, we can expect a bonanza year of catches for PIT 2009 due to the extra mother sharks. We will have to wait a year to see evidence for this hypothesis. A last theory could be that there is some sort of environmental degradation in this lagoonal area that may be altering the mother shark's desire to leave their pups there. As with most problems in biology, there is probably a complex of reasons for the pattern we are seeing, and teasing out the precise causes can prove difficult. The data definitely shows that something interesting is occurring in the lagoon this year and we will look into it further to see if we can decipher the story the lemon sharks are trying to tell us.
This marks the third straight year I have documented the PIT experience and the seventh PIT Census I have been a participant in. I made two trips to Tiger Beach during the course of the fishing and missed out on some significant time of the project. That made completing the updates a little challenging. I would like to thank everyone on the PIT team and especially those individuals who helped pick up the slack for me and gave me photo or written contributions for this year's coverage. I have to admit that this eased the creative burden on me as well, as after 3 years I am running out of ways to depict sewing gillnets. But seriously, it was another wonderful lab experience for me surrounded by old friends and making new ones. I admit that this year is tinged with a little sadness, as the current group who I have had the honor of knowing and working with these last few years will soon be going their separate ways. Joey, Steve, and Tristan will all end their tenure as principal investigators at the lab this year and proceed to complete their PhDs. I know we have all had some great times together both in the field and socially and I will miss the comfortable and productive feeling I get around all of you. I wish you the best of luck in your future endeavors and I look forward to our lives meeting in other sharky corners of this great big rock we all reside on. I wish to commend the staff - Kat, Sean, Emily, Duncan, and Jim for all their hard work in this crazy time of year. On top of PIT we had Tiger Beach, film crews, and Geo magazine. Somehow you figured out a way to organize and get it all done. Special thanks go out to Bryan and Joy. Bryan has assumed the mantle of lab manager, and has passed his first test (which is no big surprise) with flying colors. With Doc taking a step back from the Lab, I look forward to you taking the reins of the non-profit organization and leading the Lab into the future. Hardly overlooked, I especially thank all the volunteers who worked so hard to make PIT happen. Your long hours at night on the nets and braving the weather, hours baking in the sun endlessly repairing gillnets, and miscellaneous cleaning, organizing, preparing, and working allowed us to get the job done. It was a pleasure getting to know everyone of you and I wish you the best in the near future. Last but not least, I thank Doc for having me back on staff and especially for including me in what was simply an amazing trip to Tiger Beach. I think the most pleasurable moment of the entire month was after we came up from the first dive there - the look of sheer boyish joy on Doc's face was a type of reward that cannot be measured. This year Doc spent very little time on the island, but the lessons and methods he has taught us continue to hold us in good stead. In many ways we are looking at the parent bird letting its young try out its wings. Doc will forever be involved with the Lab, it is his creation, his legacy. Although his hand in the Lab will get lighter and lighter, we will never lose his little touches that truly make the whole Sharklab experience. The Lab and all the people there have given me another great experience for my life, and I hope I have given something worthwhile back. These web pages document our research and our journey together for the PIT 2008 Lemon Shark Census Project. Again, I thank all of you on the crew and wish you the best of luck in future endeavors. Who knows maybe some of us will find ourselves back in Bimini for PIT 2009. Until then - great job to everyone!
-Matthew D Potenski
July 1, 2008
The author and his underwater camera rig