The BBFS Annual Lemon Shark Census

PIT 2009 Rundown

Having some fun with cylumes...

DATA SUMMARY

The entire point of this crazy adventure called PIT 2009 is to get data on the life history of lemon sharks. Data collected this year will go into a huge database spanning the last 15 years. Lemon sharks take approximately 14 to 15 years to reach sexual maturity. This means that this yearly census project has spanned one entire generation, and the fact that we can begin to look at data on this scale is highly important. This year we managed to catch a grand total of 255 lemon sharks, the second highest total in the last seven years (save 2005). This includes 91 sharks in the North Sound and 164 sharks in Sharkland. Below is a table with a breakdown of this year's shark catch.

As per usual, we captured more sharks in the lower area of the lagoon (Sharkland) than in the North Sound. Another trend that continued was the higher capture rates of newborns versus recaptured sharks. The first-year mortality for even these well adapted predators can exceed 50%, so it is no surprise that smaller numbers survive to be recaptured in comparison to the new cohort of newborns. The final trend that has held is the total numbers for females exceeding the males in both parts of the lagoon (with the exception being recaps in the North Sound). The sex-ratio this year was 1:1.16 M to F, which is very close to unity and quite promising. The sex-ratio between the recaps was a perfect 48:48 and the newborns were not far off. The biggest disparity in sex-ratio was displayed in the newborns in the North Sound, where there were 20 more females than males captured. The North sound will continue to be monitored to see if the females survive better or worse than the males and in which direction the ratio moves. The disparity could be a simple aberration or could be indicative of some environmental influence.

The 2009 PIT Census ranks high on the all-time list for the prodigious catch we had this year. The total of 255 sharks was the second best total in the last seven years. Below are graphs showing catch data for both the Sharkland and North Sound, respectively, for the last seven years.

Shark catches for Sharkland, 2003-09

Shark catches for North Sound, 2003-09

Looking at the graph for Sharkland, it is easy to see that this was a "bumper crop" year. We had very high numbers of both neonates and recaptures, whch bolstered a very high total catch. This is very comforting as we had two years of diminishing catches (2007,2008) since a high point in 2006. Sharkland has been consistently productive over the last seven years registering more than 100 individuals throughout this entire period. The high total this year may result in a higher number of recaptures during next year's census - something we will keep our eyes out for.

Looking at the catch in the North Sound we can see a much different pattern. Last year the catch in the North sound was quite poor, but this year the population bounced back with a slightly above-average catch. The number of recaptures was somewhat low, but this can be attributed to the small catch last year. We captured a large number of neonates this year as well and that bolstered the total catch in the North Sound. Looking to next year, we will have to investigate if the high number of neonates translates into a higher recapture rate in 2010.

While it is interesting to see the year to year differences in each individual area of the lagoon, we also need to consider the big picture. These sampling areas are arbitrarily defined by us, and together form a continuous habitat. The juvenile lemon sharks tend to set up overlapping home ranges in which they patrol. Their movements may take them from the North Sound to Sharkland, or vice versa. This is evidenced by captures of sharks tagged in the North Sound the next week during Sharkland fishing. Looking at both lagoonal areas we try and see patterns in the pupping by mother lemon sharks. The large total this year is evidence that we potentially had one or more mother sharks just come to maturity and start birthing in the lagoon. Another option, particularly after the weak North Sound catch from last year means that one or more mothers may have skipped a year. The normal pattern is for a mother to pup every second year, but this is not always the case, and sometimes they can go three years between litters. The genetic samples we secured this year may give us answers to this question. We will be able to determine how many mother sharks came to pup into the lagoon, and identtify exactly who these mothers are. This marks the 15th year of the census, and maybe a mother shark that used to patrol these very lagoons has given birth for the 1st time. The circle becomes complete, and data taken during this census will serve to illustrate the life history of the lemon shark by connecting its initial and adult stages.

The BBFS PIT 2009 Crew has characters...er, character

Author's Endnote

I like to end the PIT blog with a little self-indulgence in the form of this endnote. Every PIT is different, and this year was simply a pleasure. This is one of the closest teams I think we have had in a long time. The entire project went smoothly, with a bare minimum of the usual mechanical failures, weather setbacks and swarms of biting bugs. The catch was impressive, and the entire team from newbies to grizzled vets performed well. Last year's PIT was very emotional, as it was the last one with two Principal Investigators before they left the lab. This year definitely spoke of a new regime and new leadership. I wish to commend and congratulate our new PI, Kristine Stump, for her excellent job at her first year with the reins of this Census Project. As usual, the staff of Em, Jim, Kat and Sean dealt with the sleep deprivation of the last few weeks to make sure everything ran smoothly. While I am not going to go down the entire list by name, I want to send a special thanks to the volunteers. The experienced vols realy showed up this year and lent a big hand. Several of the newbies were "thrown into the fire" (or literally thrown to the sharks, as it were) and met the challenge with fervor. I know I speak for not only myself when I say thank you to the volunteers for all the work they put in (especially long hours in the hot sun fixing gillnets). I thank everyone who contributed to this blog - it was refreshing to have many veiwpoints this year instead of just my own. I have enjoyed coming to Bimini and the SharkLab for a decade now. The friendships and camaraderie we forge through this crazy business we all love so much prove to be robust. I love coming to the lab and becoming temporary family with all the other waterlogged, nomadic, uber-geeky shark pirates who share my passion for adventure and ability to live life simply. Our backgrounds range across geography, time, and experience but we all come together to get the job done, enjoying ourselves along the way. We all appreciate the simple aesthetic beauty inherent in nature and embrace the battles necessary for their preservation. SharkLab has instilled many things in me over the years, none of which is greater than this. I strive to express this aesthetic through my photography and bring my experiences to others. To everyone on the PIT 2009 Crew - I hope you have derived as great a value as I have from my times here, and furthermore I hope this blog can be a reminder in text and pictures of your experiences here. To our readers - I thank you for following the journey and hope that we have entertained, educated, and empassioned you with all that we do. Finally, with everyone doing such a great job this year, I only hope I have performed to task as well. Bigtime thanks to all - Doc, the staff, and the volunteers for having me back and dealing with blinding paparazzi flash while trying to pull bitey sharks out of the nets. I see life through my lens and my images speak to express the natural beauty in the world. I implore you and thank you for taking the time to see the world as I do...

“Eventually there will be a time when natural beauty is more valuable than gold.”   S. ZWINGER

Cheers!

-Matthew D Potenski

June 2009

MDP & his camera in action Photo by S. "SugarTs" Heck