Overberg Angling

SHARKS — Do We Really Need Them? For the longest time after the 1975 blockbuster "JAWS" gave us a spine-tingling ride, there was an often used saying that "the only good shark is a dead shark". This man- against-beast thriller and its many progeny shark horror flicks still pervade the public's psyche, anointing all sharks as human-eaters and keeping many beach-goers out of the ocean.

The public and media's morbid fascination with sharks as killing "machines" continues today. There is a steady stream of media coverage when fishers catch and drag back a large shark for photo-ops. In some minds, catching and killing a large shark is almost heroic and fashionable, and a testament to man's superiority in the "battle" against the beast.

Meanwhile, the enormous toll taken on shark numbers worldwide due to indiscriminate fisheries continues unabated. All this shark killing causes some to wring their hands in anguish about longer-term ecological impacts. Others say "what's the big deal if sharks are killed?"

Who's right? Should we care if many of the oceans large sharks are exterminated? Is there really enough of an impact on the marine environment to worry about?

New studies show that sharks also influence the behavior of their prey.

It seems intuitively reasonable that sharks, as top-level predators, play an important role in maintaining stability in the ocean's food chain. Most people objectively or "in their gut" understand that life on earth is a series of complex interactions, with connections through food webs. Simply put, species at the top of the food chain eat species in the middle of the food chain which in turn eat species on the bottom of the food chain. And therefore, changes in the abundance of one community segment will affect the other segments. In fact, recent studies have indeed documented that overfishing of large sharks (the apex predators) has resulted in numerical increases in populations of their normal prey such as smaller sharks and rays (known as mid-level predators or mesopredators) in a phenomenon called "predator release". In turn, the mesopredators are overeating their own smaller prey such as bay scallops and bony fishes even lower on the food chain. Scientists call such effects that ripple down the food chain "trophic cascades".

Still, will it really matter all that much if we overfish sharks? Won't some other large predatory species, such as billfish and barracuda, take over for sharks at the top of the food chain and keep the food webs functioning normally?

If only the interconnections of life were that simple…….

New studies in Shark Bay, Australia by Dr. Mike Heithaus and his team at Florida International University are showing that in addition to playing important roles in the food web by direct predation (or lethal) effects, including keeping prey population sizes in check, sharks also play a large role in maintaining the normal functioning of marine ecosystems by— get this— influencing the behavior of their prey!

How does this prey behavior to ecosystem function connection work?

Let's take the seagrass ecosystem as an example. Recreational fishers and patrons of the marine outdoors know that seagrass beds are critically important nursery areas for juveniles and sometimes even adults of all types of fishes and invertebrates. The health of seagrass ecosystems is woven into an intricate balance with larger animals such as sea-cows, sea turtles and birds that obtain their sustenance in seagrass beds, either by directly grazing on the seagrass or eating smaller creatures living on the seagrass or in the surrounding sediment. These large animals (mesopredators) are in turn, prey for tiger sharks.

Let's connect the ecosystem dots: Dr. Heithaus and colleagues have documented that sea-cows, sea turtles and birds avoid hanging out in seagrass beds when tiger sharks are seasonally present in Shark Bay, and jump right back in to devour their favorite foods after the sharks leave in winter. Makes survival sense doesn't it? What this shows is that the presence of tiger sharks causes the mesopredators to change their habitat-use behavior to avoid the risk of being eaten. And this risk-avoidance behavior keeps the seagrass beds and their inhabitants from being over-consumed.

The take home message is that sharks keep the marine ecosystem in balance not just by directly eating their prey — the role that gets the most attention, but also indirectly by altering the behavior of their prey. The importance of this indirect ecosystem role of sharks is just beginning to be recognized.

We hope that you will keep in mind the delicate balance required to keep our oceans healthy. Please enjoy our marine environment with respect for all of its remarkable life forms. If you catch a shark, enjoy its magnificence, keep its important ecosystem role in mind – and let it swim away.

by Mahmood Shivji at Marlin Magazine