Friday, April 27, 2012

Reef Sharks - Going, going, ...... gone. Decline of Pacific Reef Sharks.

Black tip reef sharks cruising at HIMB - photo M Heckman

A new study led by Hawaii based research institutes JIMAR and CRED points to declines in coastal shark populations by 90 - 97% Pacific wide, or declines of 93 - 97% for the Main Hawaiian Islands. Try and think what a reduction of 93 - 97% of a population might mean. If it was your paycheck for instance, and you had a 97% reduction, a 50K salary would be worth 3% of that or $1,500 - for the whole year. You could still pay for your cell phone, but you would not get to eat. Maybe you could take pictures of food with your phone and somehow enjoy that.  Or how about if the coral cover on Oahu dropped by 97% - just Hanauma Bay and a couple of other bits of reef left, the rest of the island stripped of fish and corals. Things would be grim. So these numbers are rather dramatic and disturbing.

We all know that shark stocks are depleted, but the how much and why are the tricky parts. The Conservation Biology paper, Re-Creating Missing Population Baselines for Pacific Reef Sharkstakes a look at what reef shark populations should be across the Pacific, as opposed to what they are. They used data from over 1,600 towed diver surveys, checking for all of the usual suspects relative to influences on shark populations. They considered factors such as oceanic productivity, the amount of reef in an area and it's complexity. Temperature was included, since, as one of the study's authors Julia Baum noted, "They (sharks) like it warm and productive." 

After comparing remote and pristine reefs, to slightly remote reefs, to not at all remote reefs, one factor became clear - even a few humans nearby led to declines in shark populations. We might expect this in areas with high human concentrations; we tend to alter the environment around us. There would be direct interactions, such as recreational fishing or commercial fishing of sharks, incidental fishing (caught a shark when going for something else), declines of reef fish due to human fishing or coastal impacts (no food left for the sharks) and so on. But the authors found that even the presence of  less than 100 people in an area was correlated with distinctly lower shark populations. We do not seem to be good neighbors.

Why should we care?  We reported before on a study that found a shark in Palau might be worth 1.9 million in tourist dollars over its lifetime, while a second more recent study notes that shark diving in Fiji is worth over 42 million dollars a year  (from the Australian Institute of Marine Science (AIMS) and the University of Western Australia). Certainly there is possible evidence of cascading effects on other animals when top predators are removed (for a nice technical discussion, see "Patterns and ecosystem consequences of shark declines in the ocean"), but really there is much we do not know.

So how many sharks should be here? The authors estimate that there should be 1.1 to 2.4 sharks per hectare in the Main Hawaiian Islands. Since I do not like the idea of a partial shark and I have trouble with words like hectare, lets say that there should be 1-2 reef sharks per every  group of eight 50-meter Olympic swimming pools (100 meters x 100 meters) or 2 football fields if you prefer. The sharks would probably not be evenly distributed and we are talking about a number of species here (including white tip reef sharks resting in the caves during the day, small black tip reef sharks patrolling the drops and shallows and so on). Generally speaking, these are not sharks I am very concerned about being predated upon by, so it would be very cool to have more around (for the last article we did on shark attacks, see "Jaws - the Shark Attack Files").

Lets hope this new paper leads to humans doing a better job of coexisting with sharks.

For some entertaining reading and other references see:

HIMB's Christie Wilcox's excellent Scientific American Blog on this same breaking research: 
Mounting Evidence Suggests Sharks Are In Serious Trouble

Carl Myers pages on HIMB Shark and Reef Fish Research

The NOAA Coral Reef Ecosystem Division (CRED) page

Info on towed diver survey methods - general description and use in the NWHI and taking shark photos for the study via towboard.



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