|The Science of the Brazil Shark Mission--
Dr. Samuel Gruber's shark research in Brazil, and associated research missions in Bimini, Bahamas and the Marquesas (in the Florida Keys) are parts of a long-term study funded by the National Science Foundation. The lemon shark (Negaprion brevirostris) is one of 39 shark species protected by the United States government (NOAA/ NMFS) and results from this study will have both practical and theoretical implications. Data gathered throughout the study will provide the clearest picture to date of the breeding system, recruitment patterns (how and where the young take up residence), and species-wide population structure of a large coastal shark species. This information in turn will allow more informed conservation and management decisions to be made for sharks species with similar ecologies and life histories, and will clarify the population dynamics of a top predator in the coastal environment.
Dr. Gruber's research team is investigating the mating habits and population structure of the lemon shark. "Nothing at all is known about the breeding strategy of requiem sharks like the lemon shark, " said Gruber in a recent interview, "...and they are the most evolutionarily successful kinds of sharks. They are apparently the most modern sharks as well. And they are the most heavily targeted group--fished and overfished for fin and flesh. We don't want to see them go extinct before we even know how they go about reproducing."
No animal, shark, or fish will be purposefully sacrificed during the course of the study. It is important that the research not be a source of shark mortality since the population numbers are so small that even one death could adversely affect our estimate of natural mortality in particular and population dynamics in general.
Requiem sharks are the most abundant of the sharks, named for their tendancy to school near the surface. Reqiem sharks are in the family Carcharhinidae which includes the tiger shark, the bull shark, and the lemon shark.
Dr. Gruber lists as his primary research objectives--
1. genetically characterize, for the first time, the mating system of a cartilaginous fish;
2. clarify the use of a tropical nursery ground by juvenile sharks and females in parturition (birthing); and
3. examine population structure at both a local and extensive geographic scale.
The missions in Brazil, Bimini, and the Marquesas will test several hypotheses about lemon shark mating habits. Among these are Dr. Gruber's hunch that lemon sharks, like salmon, tend to return to their birthplace to mate and give birth. If the sharks are indeed returning home year after year to mate among their siblings and cousins, they will display evidence of a high degree of inbreeding. This evidence will be found in their DNA...if the hundreds of sharks sampled for their genetic material all proved to be close relatives, it will indicate that Gruber's hunch is a good one.
DNA samples also will be used to determine whether lemon sharks are on a two-year birth cycle--with mature sharks getting pregnant one year and resting the next. If this is the case, then every spring at the breeding grounds there will be two distinct types of female sharks swimming around--those who are about to have pups, and those who are just mating. Since one group of female sharks would continue to have pups on even years, and another group would give birth on odd years, the shark populations would tend to display distinct genetic patterns. Namely, sharks born on even years should be more closely related to each other (since they share the same set of mothers) than they are to sharks born on the odd years.
How DNA Samples Are Collected and Analyzed--
To collect DNA, shark researchers corral young sharks (called "pups") in a net at night and bring them to a tagging boat. The sharks are weighed, measured, and sex determined. Then, leather hole punches are used to collected a 2 mm diameter piece of each pup's fin for DNA analysis. The sampled pups are collected in a separate pen so they won't be sampled twice. When every coralled pup has been tagged, all are released to go about their normal lives.
After the missions, DNA is extracted from Gruber's shark skin samples by scientist Mary Ashley and her student, Kevin Feldheim in Chicago. Using a technique called PCR, Mary and Kevin make millions of copies of a special portion the DNA genome, called 'microsattellites.' These microsattelites show the individual variation between sharks, using the same photographic methods used during the O.J. Simpson trial.
Activities at Atol das Rocas--
1. Catch, measure, tag, and collect fin samples from the
approximately 64 lemon sharks in the lagoon; and put color coded tags
on a subset of group. We hope to collect stomach samples from the
sharks as well.
2. Observe home ranges and activity spaces of the lagoon-bound lemon
sharks by ultrasonic telemetry-tracking and direct observation of
select lemon sharks. Tracking of at least 5 lagoon-bound lemon sharks is planned.
3. Catch larger lemon sharks outside the lagoon using bottom-set long
line gear as well as rod and reel fishing. Larger sharks will be
tagged and measured. Fin clips will be taken.
4. Attempt to track least two of the larger lemon sharks
outside the lagoon. Do extensive visual survey
using the entire scientific field party to completely encircle the
atoll in an attempt to locate and observe all the sharks residing
outside but near the island. Catalogue fishes using a
digital video and a laser measuring device.
5. Set long lines twice for a 24 h period, once at the start
and once at the end of the stay at Rocas to do a Peterson estimate of
the local population of lemon sharks outside the lagoon
6. Measure hydrographic parameters both within and without
the lagoon. We will place data logging monitors (Sealab) at locations
within the lagoon to measure physical changes associated with tidal
fluctuations through the diel cycle. Data will be automatically
collected over the entire stay.
7. Using the ship as a platform, measure oceanographic
factors with CTD (Conductivity/Temperature/Depth), ADCP (Acoustic Doppler Current Profiler), Plankton tows and water sampling.
Activities at Fernando de Noronha--
1. Long line for lemon sharks to continue sampling the population.
2. Hire local fishers to catch lemon sharks.