I want to try something a little different this week,
because it seems to work so well for the Discovery Channel. Millions of people
tune in to Discovery Channel’s week long bonanza of programming related to our
cartilaginous friends of the ocean. And because it is fundraising week here on
WERU, we could use that kind of excitement and exposure. So, today on the World
Around Us, I bring you Shark Week.
To be fair, and a bit more accurate, Shark Week shouldn’t
just be about sharks. Sharks are in the phylum Chordata, like us, meaning they
have something called a notochord as part of they embryonic development. The
notochord is the primary axis of the developing embryo, and is found where, in
most of us, our spinal columns end up. The notochord actually develops into the
discs that end up between each of our vertebrae. If we go a bit further out on
the branch of this phylogenetic tree, we see that Sharks are members of the
class Chondrichthyes, the cartilaginous fish. Chondrichthyes are a group of
animals that are best known for their cartilaginous skeletons. Its actually
calcified cartilage, so it is structurally quite strong and can support an
animal of a very large size, its just very different from the bone that makes
up our skeletons. This group includes not only sharks but skates and rays,
chimaeras (also known as rat fish) and the super bizarre saw fish. They are mostly marine and mostly predatory.
That being said there are a few that live partially or entirely in fresh water,
and still others that are scavengers or even filter feeders. There’s always an
exception to the rule, in biology.
Chondrichthyans date back to the Devonian period of Earth
history, about 400 million years ago. Those cartilaginous skeletons don’t
fossilize very well though there have been shark fossils found with the bones
of fossilized prey still in their stomachs.
Sharks’ teeth are very fossilizable, and they are undisputedly in the
Devonian fossil record. They are thought to have evolved quite quickly and
became rather “advanced” for the time. Many of today’s sharks strongly resemble
fossil specimens. This group struck on a body plan and life history that was,
and still is, highly effective.
That makes what I have to talk about next, all the more
disturbing. A recent study showed that fully one quarter of chondrichthyan fish
are threatened, using “Red List” criteria developed by the International Union
for Conservation of Nature. The primary reason for this threatened status is
over fishing, both targeted and incidental or bycatch, meaning fish that are
caught by mistake, or aren’t the primary target species. About a third of
threatened sharks are part of a targeted fishery, much has been made in the media
in the past few years about the practice of hunting sharks for their fins, a
culinary status symbol in parts of Asia. A related threat is the direct
persecution of this group, particularly the sharks, simply because we are
afraid of them. We kill them because they are sharks. However most of the
threatened Chondrichthyans are simply bycatch, and five of the seven most
threatened groups are actually rays.
The life history of Chondrichthyans makes them more vulnerable
to overexploitation than your average species. They tend to be slow growing and
slow to mature. They have long gestation periods, and reproduce slowly. These
factors all add up to a slow population growth rate, in the literature they use
the phrase “intrinsically more sensitive” and I think that sums it up. The
study futher identified the particular risk factors that make certain species
especially vulnerable. If you are large, live near shore in shallow water and
have a narrow range of depths at which you function, you are much more likely to
be a threatened species. You are just that much easier to catch, as most
fishing is still quite near shore and in relatively shallow water.
These are not good odds. There are just over 1000 species of
Chondrichthyans, and fully a quarter are threatened, some critically. The study
found that for many species, there is insufficient data to accurately assess
the threat level and estimated that that as many as half of the Chondrichthyans
could actually be threatened. I’m just going to come out and say it. This is
not OK. That we still have fishing practices that result in substantial bycatch
is ridiculous. Is this not the 21rst century? We can do better, and those of us
with the technology and resources to do better need to help those who lack such
access. I don’t want to point the finger at any particular nationality or
ethnicity, but seriously, soup made of sharkfins? Just to show off your wealth,
and throw one more endangered species at your erectile dysfunction? Lastly,
killing them just to kill them because they are scary and have big teeth and
sometimes mistake us for their normal prey. I think that history has shown,
over and over again, that this kind of problem solving never works.
So that is a little different perhaps, than what you would
find on the Discovery Channel’s Shark Week, but it is an important part of the
conversation. And that is what WERU does best, it gives voice to any one of us
who wants to stand up and say our piece. If you support this kind of
programming, that kind of conversation, please call 1 800 643 6273 or pledge
your support on line at www.weru.org Thanks
for listening, and as always join us next week for another look at the world
around us.
References:
Discovery Channel Shark Week website: http://www.discovery.com/tv-shows/shark-week
The original study and article: http://elife.elifesciences.org/content/3/e00590
Nicholas Dulvy from Simon Fraser
College, and others. Here’s the abstract: “The rapid expansion
of human activities threatens ocean-wide biodiversity. Numerous marine animal
populations have declined, yet it remains unclear whether these trends are
symptomatic of a chronic accumulation of global marine extinction risk. We
present the first systematic analysis of threat for a globally distributed
lineage of 1,041 chondrichthyan fishes—sharks, rays, and chimaeras. We estimate
that one-quarter are threatened according to IUCN Red List criteria due to
overfishing (targeted and incidental). Large-bodied, shallow-water species are
at greatest risk and five out of the seven most threatened families are rays.
Overall chondrichthyan extinction risk is substantially higher than for most
other vertebrates, and only one-third of species are considered safe.
Population depletion has occurred throughout the world’s ice-free waters, but
is particularly prevalent in the Indo-Pacific Biodiversity Triangle and
Mediterranean Sea. Improved management of fisheries and trade is urgently
needed to avoid extinctions and promote population recovery.”
The Science Daily digest version: http://www.sciencedaily.com/releases/2014/01/140122202304.htm
The sponsoring organization, the International Union for
Conservation of Nature: http://www.iucn.org/
On Shark Week ratings: http://www.businessweek.com/articles/2013-08-07/shark-week-is-discoverys-great-white-ratings-machine
Lots of pretty pictures and videos of various
chondricthyans: http://www.bbc.co.uk/nature/life/Chondrichthyes
Tree of Life: http://eol.org/info/442
Not a bad overview of the group: http://en.wikipedia.org/wiki/Chondrichthyes
Lots of interesting stuff particularly about the fossil
evidence and other strange details: http://petrifiedwoodmuseum.org/SOChondrichthyans.htm
