Sharks, Seals and Climate Change

July 11, 2012

Wednesday morning, NPR’s Morning Edition aired a story by Richard Harris connecting the recent heat wave to climate change. Then, about fifteen minutes later, they aired a story by WCAI’s own Brian Morris about the increase over the past few years in great white shark sightings near outer Cape beaches.


The natural next question is whether or not the shark sightings are connected to climate change. And that is exactly what WGBH’s morning host Bob Seay asked me. We talked for about five minutes, and I’ll get the audio up as soon as I can. In the meantime, here’s my answer, in a nutshell:

• Climate change is absolutely real, it affects the ocean, and it can change where animals show up. In fact, the ocean has absorbed the vast majority of heat trapped by greenhouse gases. The result is logical; water temperatures around New England have risen 2-4F in the past several decades. That has a lot of marine species (at least half of all commercially exploited species monitored by the National Marine Fisheries Service) moving around to stay within their comfort zones.

• Even though global warming is a relatively slow process (in the eyes of most, but certainly not geologists or evolutionary biologists who tend to think in terms of millennia and eons rather than years or decades), it can manifest as rapid shifts. The collapse of the southern New England lobster population is a good example. When summer water temperatures in Buzzards Bay started to regularly exceed 68F – a threshold temperature above which lobsters don’t do well – juveniles and adult females very quickly declined, followed by the population at large.

At this point, it’s looking like the answer might be “yes.” Most sharks are lovers of warm-water, so they could be moving north as waters warm. And the fact that they suddenly started to frequent near-shore waters three years ago might be attributable to a threshold temperature effect, like the lobsters. It all seems reasonable. But - and this is a really big “but” – that’s not the story Dr. Greg Skomal, Massachusetts’ state shark biologist, told me when I spoke to him last summer.

• First of all, it’s not clear whether there are more sharks in the waters around Cape Cod or whether they’re just coming closer to shore. There have always been great whites around Cape Cod, they just have tended to stay further offshore. So sightings have tended to come from fishermen, not sunbathers.

• In either case, the primary reason there are great whites hanging around outer Cape beaches is the abundance of a favorite food: seals. And, before you ask whether climate change is what brought the seals, on to the next point.

• The resurgent seal population is most likely a consequence of the cessation of hunting brought about by the Marine Mammal Protection Act of 1972. It’s possible that climate change could be altering the food chain that supports the seals. In fact, it probably is. But whether that has anything to do with the recent increase in seal populations is more a matter of conjecture than science, at this point.

All in all, Greg said he would consider the current situation to be a return to something more closely approximating normal. That is, a return to what might have been seen 150-200 years ago, before fishing pressure and seal hunting dramatically intensified. In that regard, it’s a sign that some of the environmental regulations passed in the 1970’s are doing what they were intended to do.
That doesn’t mean climate change isn’t real (see the first bullet), or that it isn’t affecting sharks and won’t continue to do so in the future. It just means that it probably isn’t responsible for shark sightings off Chatham beaches.

But don’t take my word for it. Ask the shark himself. He’s on Twitter, you know. Check out@ChathamShark.

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