Glow-in-the-Dark Sharks at Risk?

July 17, 2007 — One of the first ever detailed studies on deep water lantern sharks, so named for their ability to glow in the dark, has found they are in danger of extinction.

Recent studies have linked declines in shark populations to the collapse of entire marine ecosystems. The loss of lantern sharks could devastate other ocean life globally, as many lantern sharks have wide ranges.

Researchers focused on one species in particular, the smooth lantern shark, commonly caught as by-catch in Portuguese trawling and longline fisheries. Since the bioluminescent shark has little commercial value, the fishermen usually just discard it.

"What we now know is that this species has a vulnerable life cycle characterized by slow growth rates, low fecundity and late maturity," lead author Rui Coelho told Discovery News.

"When fisheries mortality increases, the populations start to decline and cannot compensate for this," added Coelho, a shark researcher at the University of Algarve, Portugal.

He and colleague Karim Erzini analyzed 614 by-catch sharks over a two-year period. Their measurements showed the species grows anywhere from 5 to 19 inches long.

The sharks have special light-producing organs, called photophores, mostly found on their sides.

"These photophores may be used to allow individuals to escape from predators, approach prey without the sharks being detected or for species recognition, such as during the mating season," explained Coelho.

The scientists developed a unique way to determine the age of the sharks: The inner portions of their spines grow continuously. By counting growth bands in this spinal area, similar to counting tree rings, the researchers could estimate each shark's age.

Their findings have been accepted for publication in the journal Fisheries Research.

The oldest male they found was 13 years old, while the oldest females were 17 years old.

Eggs found within females numbered, on average, 10.44. Since this shark's reproductive cycle may last as long as three years, the birthrate is extremely low when compared to most other animals and fishes. The researchers also noticed that many females miscarried — likely due to stress — when they became by-catch.

Prior studies conducted by Coelho and Erzini suggest that a number of strategies can reduce by-catch and help prevent lantern shark populations from declining further.

Since the sharks often swim in very deep water, the researchers suggest that fishermen remove hooks from their gear at these levels. For deep water trawling, rigid grids may help to keep fish out of desired crustacean catches.

Laws, such as one that now prohibits trawling in the Mediterranean at depths deeper than 3280 feet, might also afford some level of protection to the sharks and other deepwater species.

Shark expert Maurice Clarke of the Marine Institute in Ireland told Discovery News that the new study is "very comprehensive" and "is the first paper to validate age estimation for a deepwater shark."

Of the smooth lantern shark, Clarke said, "It is clear that it is a slow-growing species that matures late in its life. It is likely to be vulnerable to exploitation."

Clarke added, "The next step is to quantify the amount of this species being removed from the sea. This information, along with the data from Rui and Karim's study can form the basis of an analysis of the vulnerability of this species being caught and discarded in commercial fisheries."
Jennifer Viegas, Discovery News
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QUESTIONS FOR DISCUSSION:
Based on the quote at the end of the article by Maurice Clarke, why is there a connection between the shark being a slow-growing species and being vulnerable to exploitation?

Do you agree with laws that limit fishing in certain depths?

Some context:
US Fish and Wildlife: http://www.fws.gov/

U.N. Fishing Conventions (this is a long pdf file) http://untreaty.un.org/ilc/texts/instruments/english/conventions/8_1_195...