Behavioral thermoregulation after mating by female nurse sharks
For thousands of years, adult nurse sharks ( Ginglymostoma cirratum) have most likely been returning to the Dry Tortugas shallows to court and mate in June and July. Using kayaks and simple nets, our research team has tagged, released and studied over 100 female nurse sharks during the past 21 years.
Sharks chose specific habitat for various reasons at different times of their lives. The relevance of this project is to understand the requirements for reproductive success in shark populations by elucidating the need of pregnant females for specific temperature regimes.
Temperature is one of the most critical factors determining the distribution and success of animal populations. Female nurse sharks mate in shallow lagoons in June and July. Some of these females return to the same lagoons in autumn to bask in the warm shallow waters. It is our hypothesis that they seek out these waters to warm their bodies, much as reptiles do, to facilitate gestation and an early parturition.
With the help of Hai-Stiftung, we purchased 14 CEFAS G5 temperature/depth recording tags to place on adult female nurse sharks. As our sharks return to the Dry Tortugas mating site bi- and triennially, it was decided to deploy half the tags in 2011 and the other half in 2012.
Activities in 2012
In 2012, 13 adult females and seven adult males were captured or recaptured. Eight of these females were net captured and tagged or retagged, three were observed photographically and two electronically. The seven males were 'recaptured' electronically or visually/photographically. Eight of the females were fitted with CEFAS G5 temperature/depth loggers.
This year our research team spent 28 days on-site. We conducted around 140 hours of observation. 137 shark courtship and mating events which is comparable to other years.
Results until June 2012
To date, 145 adult sharks have been tagged. Forty-seven of these adults have been recaptured a total of 117 times (range 1 to 6 recaptures) in the breeding and refuging grounds where they were tagged. As noted, our sharks only mate once a year. They have not been available to capture and tag since our last reporting period in June of 2011.
In 2013 we will make our 23rd annual trip to the Dry Tortugas shark mating site to deploy another group of CEFAS G5 tags and to download the fixed loggers. The group we tagged in 2012 will be expected to return in 2014.