Chief Science Correspondent
Discovery Channel wrapped up its annual Shark Week programming today and, in the spirit of the occasion, I chose to highlight a recent study on the predicted fate of our friends from the deep blue.
In an article published in eLife earlier this year, a team of more than 300 international scientists reported that chondrichthyan fishes (sharks, rays and chimaeras) face an alarmingly high risk of extinction - much higher than that of other vertebrates. Drawing from survets around the world, the group estimates that one-fourth of these species are threatened worldwide, with the lowest percentage value of "least concern" individuals worldwide. They note that the depletion of these marine species is a result of overfishing (both targeted and incidental), although they note that habitat destruction, pollution and climate change have contributed to the losses as well.
The decline of the chondrichthyans is actually an indirect result of overfishing as a whole, as these species becoming more commercially valuable as teleost fishes, such as tuna, have become increasingly depleted. The authors note that shark fins are particularly valuable in Asian countries such as China, where they are considered a delicacy.
Sharks are particularly vulnerable to extinction simply by their nature. They are large in body size and tend to reside in relatively shallow waters. Although scientists do not know exactly why, both of these factors tend to lead to a greater risk of population loss. The greater body size likely relates to greater energy requirements, as well as fewer offspring.
Sharks also tend to have very narrow fundamental niches, meaning that they are restricted by the areas they can inhabit. As pollution and human activities result in habitat fragmentation, the liveable ranges for these species becomes even further restricted.
The problem is that the fishing of sharks and other chondrichthyans is largely unregulated, meaning that there are relatively few restrictions on the practice, but also that the population decline has been difficult to track. With the current system in place, scientists may realize how threatened the populations have become until it is too late.
Despite their bad reputations, sharks are a very important part of our world. As keystone species, they stand at the top of food chains and help keep other fish species in balance. For example, if sharks did become extinct, their prey would greatly increase in size, meaning that these fish would have a higher demand for their prey. Then, these fish would overprey on the smaller fish, causing a second extinction. Then, the first group of fish would not have sufficient resources to survive, potentially causing a third extinction.
It is time to revisit the regulation of chondrichthyans fishing, and determine ways to save these species. If we don't, Shark Week may have to start airing on the History Channel.