New England Fisheries

Are We Headed Toward the Era of Two-Headed Sharks?

In New England, Atlantic cod has been overfished and subject to overfishing for decades. Image via NOAA.

The 2016 presidential election will likely go down in history books as one of the most polarizing of our time. For the last year and a half, we’ve been watching two talking heads bare their teeth at one another, taking the occasional bite, as each attempted to move the same body in two different directions. A figurative two-headed shark, you might say. But is a real two-headed shark even scarier?

Call it a coincidence but researchers at a university in Spain recently encountered the first-documented two-headed shark embryo in an oviparous, or egg-laying, shark species. The embryo was an Atlantic sawtail catshark (Galeaus atlanticus), whose populations are found only in a small section of the Northeast Atlantic and the Mediterranean Sea. The International Union for Conservation of Nature lists the species as “near threatened.”

This isn’t the first occurrence of a two-headed shark ­– and it certainly won’t be the last. In fact, two-headed sharks seem to be more common of late. As described in a recent National Geographic article, a two-headed bull shark fetus was found in Florida a few years ago, and in 2008, a two-headed blue shark embryo (the species with the most recorded two-headed embryos) was discovered in the Indian Ocean.

Researchers believe that a genetic abnormality may be the most likely cause of the two-headed Atlantic sawtail catshark. And although this particular specimen was found in a lab, in the wild, overfishing is a major driver of such genetic abnormalities, leading one researcher to believe this may be why we have been seeing an increase in two-headed sharks worldwide.

Linking Overfishing to Gene Pool Problems

As we’ve noted in previous posts, chronic overfishing can produce what geneticists call a “population bottleneck,” reducing the diversity of the remaining gene pool. It becomes an even greater issue when a specific age or sex of fish is targeted for fishing.  For example, here in New England, the larger, more fecund Atlantic cod females were often the most sought after – but removal of so many of these fish greatly reduced the productivity of the stock and contributed to its collapse.

A smaller gene pool not only hurts productivity, but it leaves fish populations more vulnerable to disease and defects (like perhaps a two-headed shark). It also leaves them less resilient to the stresses of environmental changes. This is a major issue given increasing ocean temperatures, particularly in New England where the Gulf of Maine is warming faster than almost any other part of the ocean worldwide.

So, our vote on Election Day is not the only important decision we need to make: We also need to prevent overfishing to ensure strong and healthy fisheries. Our fisheries must be managed on an ecosystem-based level, forage species must be protected, and the scale of management must mirror the range of the managed population.

Marine ecosystem health also requires protecting essential fish habitat and creating refuge areas, especially where there are spawning and nursery grounds. The best available science must be used to inform our management decisions, and managers must use a precautionary approach given uncertainty in that science, particularly in an era that will be defined by ocean changes associated with excessive greenhouse gas emissions.

Even as we fight to maintain and strengthen our nation for future generations on Election Day, we must commit to working harder over the long term to maintain and strengthen our conservation-minded national fishery laws and policies.


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