Keep Jaws in the Potomac

Fishermen trying to catch cownose rays in the Potomac River yesterday netted something larger: an 8-foot-long shark.  “I was quite surprised. It was a bull shark, the most aggressive shark there is, and the first one I’ve ever caught,” fisherman David Ridgell, told Bay Daily. He said he and his co-workers hauled in the shark (pictured above) in Cornfield Harbor, near Point Lookout, at the far eastern end in the Potomac.

NBC News dubbed the incident:  “Jaws in the Potomac” and played up the danger angle.  But here’s the irony. It’s large sharks -– uncommon in the Chesapeake Bay -– that are much more in danger than the people who might come in contact with them. While shark attacks on humans are exceedingly rare, sharks are victims of severe overfishing up and down the Atlantic Coast.

Bull shark populations declined more than 99 percent between 1970 and 2005 because of rising demand for shark fins (for soup) and meat, according to a report in the journal Science. The "declines in great sharks...imply their likely functional elimination," wrote biologist Ransom A. Myers and colleagues.  And as shark populations have plummeted, numbers of cow-nose rays -– a favorite food of the sharks -– have skyrocketed, triggering a chain reaction in the Chesapeake Bay’s chain of life.  The theory is that not enough sharks means too many rays, and this translates to not enough oysters –- because rays love to gobble up oysters and disrupt oyster planting projects.

So, for the sequel, let’s rewrite the plot and keep Jaws in the Potomac.

By Tom Pelton

Chesapeake Bay Foundation

(Photo courtesy of Buzz's Marina)

 

Comments

[Wild Bill]

Thanks Tom for speaking up for the shark.

Killing creatures just because we can is often not the best thing to do. I did not know much about bull sharks, but somehow killing it just did not seem right.

[Chuck]

And, to add further irony, many folks are now involved in a new campaign to create markets Chesapeake Bay rays. They even have a catchy name. Alas, rays possess many of the exact same characteristics as sharks (mature late, low birth rates, etc), so they too are highly vulnerable to overfishing. In a few years, it is highly likely that we'll be asking the question, "where have all the rays gone?"

[Tom Pelton]

I'm with you on that, Chuck. I'm a little suspicious of this whole "hey, lets kill and eat the rays!" campaign. That doesn't sound like a wise way to solve an ecosystem problem started by overfishing (of sharks). Better to find a way to cut down on the harvest of sharks.

[Chuck]

Commercial fishermen used to say that we didn't have enough crabs because the now-rebounded rockfish were eating them. They recommended additional striped bass harvests as a remedy.

Oysters? They used to say we weren't working the bottom enough, so we had to allow power dredging of more oysters to clean off the sediment.

Now they say the rays are eating the clams and oysters and that the solution is catching more rays.

Notice a pattern? I'm not sure "catching more" is a good conservation strategy. It hasn't really worked in the past 200 years anyway.

[Tom Pelton]

Words of wisdom

[R. Dean Grubbs]

Hello Mr. Pelton,

Virginia is near the northern extent of
the summer migration of bull sharks. Some enter the Bay every year
but bull sharks are never common in the region. During the years I
ran the VIMS shark longline survey, we would occasionally get calls
about them being caught and we would occasionally catch them, but
primarily on the Eastern Shore and in nearshore coastal waters. As
far as their population status, there has been no formal assessment
for this individual species due to a lack of adequate data. However,
most data that are available suggest the population is stable. They
do not have an offshore component of their migration like sandbar
sharks and dusky sharks, and therefore weren't subject to as much
fishing mortality in the 1980's which caused declines in these other
large coastal species.

There are two papers (from the same lab) that suggest very large
declines in bull sharks. I will describe them briefly below and I am
happy to send these to you, but neither is considered credible by many
biologists that study sharks and shark populations. O'Connel et al.
(2007) claimed bull sharks declined by 98.6% in a lake in Louisiana
based on gillnet catches, but that decline was based on a single year
(1978) when a total of 21 sharks were caught. Using beach seine data,
they claimed a decline of 99.9% based on 10 animals in the 1950's, 4
in the 1970's and only 1 since. This is not a robust analysis. Myers
et al. (2007) suggested bull sharks along the East Coast declined by
99% based on a total of 23 sharks caught over a 35-year period at two
sites off North Carolina. Again, far too few data to make such
assertions.

By the way, Myers et al. (2007) is the same paper that made the claim
that overfishing of sharks led to a trophic cascade resulting in
dramatic increases in smaller sharks and rays, including cownose rays.
I can't stress strongly enough that this paper and this theory has
no credibility among the scientists that study sharks and shark
populations. The populations of large sharks along the East Coast
declined dramatically in the 1980's reaching a minimum in the early
1990's. The magnitude of declines was between 70% and 90% depending
on species (based on many data sets), not 99% as alleged by the Myers
et al. (2007) based on one inappropriate data set. Coastal sharks
have been managed since 1993 and as a result some species have fully
recovered (e.g. blacktip sharks), some began to recover but recovery
has been stymied (e.g. sandbar sharks), and some are still in trouble
(e.g. dusky sharks), but none suffered the magnitude of declines put
forth in this paper and others published by the same group.

Likewise, while there is no doubt that the cownose ray population is
very large, there is very little evidence that the population is
overpopulated or that it has increased dramatically. Given that they
produce a single pup following a one-year gestation, a tenfold
increase would require 50-70 years, not ten. Even if the
sensationalized declines in sharks and increases in cownose rays were
true, there is no evidence that cownose rays make up a large enough
component of any shark diets to cause such a cascade. In fact,
juvenile cownose rays are a larger component of the diets of some
bony fishes (e.g. cobia) than they are of any shark species. The life
history model of most batoids is such that they grow extremely rapidly
during their first year. By the time cownose rays born in Chesapeake
Bay emigrate during fall, they have already outgrown much of their
predation risk, except by very large sharks.

This is probably more information that you wanted. I am happy to
provide more detail or discuss this in more fully if you would like.

Best wishes,

R. Dean Grubbs, Ph.D.
Assistant Scholar Scientist
Florida State University Coastal and Marine Laboratory
St. Teresa, FL 32358

[Charles H. Peterson]

Yes, a 99% decline in bull sharks over the 35 years in the North
Carolina fishery-independent shark survey.

Charles H. Peterson, Ph.D.
University of North Carolina
Institute of Marine Sciences

[RW]

Marine biologist throwdown!

[Bob Jones]

This shark is a great icon. We should not kill them but let the sharks swim free. That is what I think we should do.

[Bob Jones]

IF you think a a shark will stop me, then you are sorely mistaking, sorely mistaking.