Sunday, March 18, 2012

Sharks and MPAs!

Our Whitetip Reef Sharks - clearly resident and doing great. Stellar pic by Lill.

Nice to see our intuition confirmed!
It may sound trivial, but recent research finally confirms that Sharks thrive within Marine Protected Areas. There may be more, but these are the two new papers I know about.

Evaluating marine protected areas for the conservation of tropical coastal sharks.
Danielle M. Knipa, Michelle R. Heupel, Colin A. Simpfendorfer


Global declines in shark populations have created uncertainty in the future status of many species and conservation efforts are urgently needed.
Marine protected areas (MPAs) are used increasingly as conservation tools around the world, but how they benefit mobile and wide ranging species like sharks remains unclear. To evaluate the degree of protection MPAs may provide for sharks, we used an array of acoustic receivers to examine the movements and spatial use of two tropical coastal species within two MPAs in the Great Barrier Reef Marine Park, Australia. Juvenile pigeye (Carcharhinus amboinensis) and adult spottail (Carcharhinus sorrah) sharks were fitted with acoustic transmitters from 2009 to 2010. Both species displayed long-term use of MPAs, with some individuals detected for longer than 600 days. The mean percentage of time C. amboinensis and C. sorrah spent inside MPAs was 22% and 32%, respectively. MPA use varied seasonally, with C. amboinensis spending a higher percentage of time inside MPAs in summer (mean = 28%) and C. sorrah spending a higher percentage of time inside MPAs in winter (mean = 40%). Although sharks used large areas inside MPAs, most individuals tended to use only half of the available protected space. In addition, all sharks made excursions from MPAs and individuals exited and re-entered at consistent locations along the MPA boundaries.
These results demonstrate that MPAs have conservation benefits for shark populations by providing protection across different species and life stages, and tracking studies can be used to help tailor MPA design to maximize effectiveness.


Reef Sharks Exhibit Site-Fidelity and Higher Relative Abundance in Marine Reserves on the Mesoamerican Barrier Reef
Mark E. Bond, Elizabeth A. Babcock, Ellen K. Pikitch, Debra L. Abercrombie, Norlan F. Lamb, Demian D. Chapman


Carcharhinid sharks can make up a large fraction of the top predators inhabiting tropical marine ecosystems and have declined in many regions due to intense fishing pressure.
There is some support for the hypothesis that carcharhinid species that complete their life-cycle within coral reef ecosystems, hereafter referred to as “reef sharks”, are more abundant inside no-take marine reserves due to a reduction in fishing pressure (i.e., they benefit from marine reserves). Key predictions of this hypothesis are that (a) individual reef sharks exhibit high site-fidelity to these protected areas and (b) their relative abundance will generally be higher in these areas compared to fished reefs. To test this hypothesis for the first time in Caribbean coral reef ecosystems we combined acoustic monitoring and baited remote underwater video (BRUV) surveys to measure reef shark site-fidelity and relative abundance, respectively. We focused on the Caribbean reef shark (Carcharhinus perezi), the most common reef shark in the Western Atlantic, at Glover's Reef Marine Reserve (GRMR), Belize. Acoustically tagged sharks (N = 34) were detected throughout the year at this location and exhibited strong site-fidelity. Shark presence or absence on 200 BRUVs deployed at GRMR and three other sites (another reserve site and two fished reefs) showed that the factor “marine reserve” had a significant positive effect on reef shark presence. We rejected environmental factors or site-environment interactions as predominant drivers of this pattern.
These results are consistent with the hypothesis that marine reserves can benefit reef shark populations and we suggest new hypotheses to determine the underlying mechanism(s) involved: reduced fishing mortality or enhanced prey availability.

Very cool!
The latter paper is thankfully open access so you can read it in its entirety. There are also excellent synopses, one by Juliet right here and others here, here, here and here. What is particularly interesting here is the deployment of totally non-invasive bait cams and the correlation to the lack of fishing pressure (and thus, increased fish density) within the MPA.
Helen tells me that she has observed that Fish appear to know and take refuge within the confines of her Waitabu MPA, and from my days as roving underwater image hunter, I've learned to recognize whether a particular reef is being visited by spear fishermen by observing the behavior of the Fishes, particularly the Groupers - so it comes as no surprise that the Sharks who would be following their prey would also aggregate within the protected areas where they would be equally sheltered against any fishing pressure, find plenty of prey and thus thrive.

And what about the Shark Reef Marine Reserve?
Well we did set it up in the belief that in order to conserve a species one needs to preserve its habitat - but as always, it's complicated.
The Blacktip Reefs and Whitetips live, mate and give birth to plenty of babies right there so the conclusions of the papers are being fully confirmed.
Maybe even when it comes to the Grey Reefs but even there I'm less confident. From all I know, your typical resident aggregation of Greys consists of mature females plus juveniles and sub-adults of both sexes, whereas I thought that the adult males were non-resident and always roving, thus assuring gene flow - but on Shark Reef, most of the adults are males and then, like right now, we see a lot of juveniles. And in May/June, everybody leaves for a month or so, something we believe is correlated to mating opportunities somewhere else. But in reality, we don't quite know what's really going on there, so the whole scenario certainly warrants more investigation.
But when they are here they sure look perfectly happy!

The Silvertips, Nurses and Lemons?
As I stated somewhere else, they are increasingly being displaced by the assertive and ever more numerous Bulls, likely due to competitive exclusion - so although I'm sure that much like they've done in the past, they would love to turn up much more frequently, I equally fear that the deterrent is currently much too big for us to ever witness an improvement in numbers, MPA or no MPA.

And the Bulls and Tigers?
Mike's hypothesis stipulates much larger home ranges that have been confirmed by Juerg's telemetric studies - but they sure love to come visit for a juicy snack and maybe, even for some good company! :)

Anyway, all very interesting!

H/T: Demian and Rick!

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