Considering that this blog started with a post on jellyfish, it’s been a while since we explored marine biology. Oceans cover 71% of the surface of the Earth, but remain terra incognita to many (or would that be mare incognita?). Ocean inhabitants range from the familiar (clams, tuna) to the almost unimaginably strange - a future post, I promise, will feature the ecosystems surrounding deep sea vents, tube worms and all.
I’m sure you’re all very excited.
Today we’re going to look at something more familiar, better known, and yet far more misunderstood. A creature of vital importance to the oceans, and to humans, and yet widely feared, vilified and hated. Meet your friend, the shark.
Yes, it’s big (sometimes), scary (occasionally) and has rather a lot of teeth, but sharks play an important role in ocean ecosystems. While there are a diversity of sharks out there, many of them, unsurprisingly, occupy the role of higher-level predator in a food web. This is an important position - from here, sharks effectively control the entire food web. Sharks consume smaller marine animals, which in turn consume smaller marine animals (down several levels, if necessary), until we arrive at the animals which consume plankton/seaweed/other so-called primary organisms (whatever is synthesizing its food energy rather than consuming it, via photosynthesis, or several other processes). Remove the shark from the web, and things rapidly get out of kilter. Say you have an ecosystem in which sharks prey on sea lions, who prey on fish, who eat plankton. No sharks, and your sea lion population explodes, and rapidly depletes the population of smaller fish. No smaller fish, and plankton growth takes off - possibly to the detriment of other, competing species. Rarer species may be pushed out of an area all together, and if any of the species experiencing a rapid population increase are potentially harmful to species outside the food web (stinging jellyfish, the organisms that cause red tide, etc.), the effects spread even further. Eventually the food web will stabilize, but only after disturbances which contain the potential for extinctions.
This gets to really be a concern for humans when the species further down the food chain are commercially important fishery species. In the waters off North Carolina, sharks were all but eliminated, leading to a population explosion in the cownose ray. More cownose rays meant more intense predation on their food sources, including the bay scallop. The once-important bay scallop fishery of North Carolina is now gone. Scientists suspect a similar dynamic is responsible for the lagging recoveries of a number of shellfish fisheries on the US Atlantic coast, despite recent improvements in water quality. Disturbances to the shark population are important for anyone who works as a fisherman, lives in a fishing community, or just likes to eat fish.
So, since sharks are so important, let’s look at them as something more than big sets of jaws that go around regulating ocean food webs. For starters, those jaws are attached to cartilaginous bodies. Sharks are fish (they have gills, can’t live outside the ocean, etc.), and more specifically, belong to an order of fish characterized by a lack of bones. Sharks have skeletons, but those skeletons are made of the same substance as human ears and noses - it makes for a more flexible body, albeit one that could only be particularly mobile in an aquatic environment. Sharks are an old form of life on Earth, first appearing on the scene about 400 million years ago in a time period sometimes called the Age of Fishes. Most of the action on Earth was confined to the seas back then, although land plants were continuing to evolve. For reference, the basic form of the shark has been around for roughly as long as the basic form of the vascular plant (not algae or moss). Over the millenia, sharks have survived mass extinctions that wiped out as much as 95% of all life on Earth. It’s sad to say that the 400 or so species of sharks in existance today are facing the biggest threat they have ever encountered - humans.
All sharks, from the unintimidating (I would go so far as to say cute) spiny dogfish to the definitely intimidating great white are under threat.
Sharks are being hit by a combination of habitat loss in coastal regions, being caught up in fishing nets (bycatch), and the rising demand for a traditional foodstuff made from shark fins. Shark fin is an incredibly value commodity these days, and for fishermen, especially desperately poor fishermen in the developing world, the temptation is too strong to catch as many sharks as possible. Like any other higher-level predator, shark simply don’t breed or grow to adulthood quickly. While fish like sardines or anchovies can be caught in relatively large quantities without endangering the survival of the species, it doesn’t take much fishing to endanger a local shark population.
The image problem sharks face doesn’t exactly help. Contrast the cases of eagle and wolf conservation. The bald eagle is the symbol of the United States of America. In the U.S., the eagle is a species with tremendous symbolic value - freedom, independence, truth, justice, the American way, etc. So, restoring the bald eagle was, as these things go, a relatively angst-free process. The wolf, on the other hand, while an important predator, has the reputation of a vicious animal that will attack anything in its sight. This isn’t really true, of course - while wolves do periodically devour valuable livestock (given a choice between a fat cow and a stringy elk, I know what I’d go for), they tend to be afraid of humans. Wolves exert a similar influence over food webs as sharks - in the absence of wolves, deer and elk become overpopulated, and consume enough vegetation to stunt tree growth, encourage erosion and impair water quality. In spite of this, the process of wolf re-introduction has been anything but smooth or simple, largely because of the negative associations many people have with wolves. Sharks face a similar problem. As long as sharks are more associated with being mindless killers than key components of ocean food webs, shark conservation is going to remain an uphill battle. This being in spite of the fact that since 1978, fewer Americans have been killed by shark attack than have been killed in vending-machine related incidents. Until perceptions change, all sharks, from the angelshark to the zebra bullhead shark, are going to be in trouble. And that’s really a shame.
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