Shark Savers - Would the world be better off without sharks?

Would the world be better off without sharks?

The world would be even more precarious without sharks. For 450 million years, sharks have usually acted as the apex predator in the oceans.

The apex predator is the lead actor in any habitat’s story. The apex predator governs the balance of all species below it. They keep other species of fish in check, ensuring that certain species do not grow uncontrollably. They also play a critical role in the evolution of other species.

The loss of an apex predator can cause unpredictable havoc in an ecosystem that has come to depend on it. Some species benefit and others may get wiped out by those beneficiaries.

When sharks go, they alter the marine ecosystem as a whole, well beyond the specific species they feed on. Several studies are already identifying the problems.

The Integrative Ecology Group in Sevilla, Spain and the Scripps Institution of Oceanography at the University of California, San Diego showed that when sharks in the Caribbean were overfished, the fish they feed on increased significantly in number. Those fish in turn prey on parrotfish, which graze on the algae on coral reefs. With less parrotfish, the reefs become unhealthy and overgrown with algae. Coral reefs are often referred to as the “nurseries of the ocean.” Without a healthy reef system we risk losing thousands of species of fish.

Enric Sala, deputy director of the Center for Marine Biodiversity and Conservation at Scripps stated, "It appears that ecosystems such as Caribbean coral reefs need sharks to ensure the stability of the entire system."

Photo: Rob Stewart

In another example, reported in Science by Ransom Myers and colleagues at Dalhousie University in Halifax, Canada, the elimination of sharks resulted in the loss of mollusks. One result is that the 100 year-old bay scallop industry in North Carolina came to an end.

In the Myers study, an analysis of 17 fishery surveys on the east coast of the United States between 1970 and 2005, with the addition of computer modeling, indicated that the populations of scalloped hammerhead sharks, tiger sharks, bull sharks, dusky sharks and smooth hammerhead sharks may have declined by as much as 97-99%. The decline was largely driven by the dramatic increase in demand for shark fins, as described elsewhere on this site.

Without the sharks, former prey have flourished, including rays, skates and small sharks. The cownose ray was especially fruitful, growing tenfold in population. "There may now be over 40 million rays in the population," the researchers say, “which is remarkable for a species with such a low reproductive rate”. And, they love mollusks. This population now consumes 840,000 tons of mollusks each year, several times that of the entire fishery for that part of the mid-Atlantic states.

There is increasing alarm around how the loss of sharks may affect the oceans as the great lungs of the earth. Phytoplankton are the micro-plants of the ocean that collectively represent the biggest quantity of vegetation on the planet. Phytoplankton consume more carbon dioxide than all the trees in our forests and produce most of the oxygen we breathe. And, they are food for lots of different species of fish. And many of those fish are, in turn, shark food. Will phytoplankton go the way of North Carolina’s scallops?

Sources:

Research Shows Overfishing of Sharks Key Factor in Coral Reef Decline
Scripps News April 11, 2005

Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean
Ransom A. Myers, Julia K. Baum, Travis D. Shepherd, Sean P. Powers, and Charles H. Peterson Science 30 March 2007 315: 1846-1850 [DOI: 10.1126/science.1138657]

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