Not just Shark
Pictures: Elasmodiver contains photos of sharks, skates, rays, and
chimaera's from around the world. Elasmodiver began as a simple web
based shark field guide to help divers find the best places to
encounter the different species of sharks and rays that live in
shallow water but it has slowly evolved into a much larger project
containing information on all aspects of shark diving and shark
There are now
more than 5000 shark pictures and sections on shark evolution,
biology, and conservation. There is a large library of reviewed
shark books, a constantly updated shark taxonomy page, a monster
list of shark links, and deeper in the site there are numerous
articles and stories about shark encounters. Elasmodiver is now so
difficult to check for updates, that new information and pictures
are listed on an Elasmodiver Updates Page that can be accessed here:
At first glance the Spiny Dogfish is an unimpressive little
shark. It is barely five feet in length when fully grown and more commonly
tops out at a modest three feet. However, dig a little deeper and you find out
that there is more to this pint sized predator than meets the eye.
It belongs to the order Squalidae which contains more than
seventy species collectively referred to as dogfish sharks. Labeled
by science, it has been given many common names in the past including: Blue
dog, Common spinyfish, Darwen salmon, Grayfish, Pacific dogfish, Piked
dogfish, Rock salmon, Spiky dog, Spotted dogfish, Spring dogfish, Spur dog,
Victorian spotted dogfish, White-spotted dogfish, and Mud shark, and that’s
just the English names!
Although Spiny dogfish are not seen by divers in the tropics,
they are exceptionally common along sub-arctic and temperate coastlines around
the world. In fact, Squalus acanthias is the most
common shark in the sea – at least for now. Not confined to the surface
layers, it has been recorded at depths down to 900m and it may hunt even
Spiny dogfish are by nature social animals that school by the
thousands. Doug Biffard of Parks Canada put it well when he said: “Back in the
early 80’s, there were so many of them that when they appeared ‘en masse’ they
would block out the sun”. It must have been awe inspiring to look up and see a
completely impenetrable wall of sharks swimming your way.
Sadly, their very abundance has led to their own demise. Since
the 60’s fishing efforts have been concentrating way too heavily on Spiny
dogfish in both the Atlantic and Pacific Oceans. They are utilized for all
manner of products including England’s famous ‘fish and chips’, shark liver
oil, laboratory specimens for thousands of budding marine biologists, and
perhaps even sadder; hundreds of tons of fertilizer.
In the Western Atlantic between 1989 and 1996, annual catches
grew from an already precarious 2,500 metric tons to an unfathomable 21,000
metric tons. By this time the species as a whole was teetering on the brink of
collapse and sure enough, the following year’s yields were significantly down.
Part of the reason for the Spiny dogfish’s inability to respond to over
fishing lies in its incredible physiology. One fact you may want to commit to
memory for your next game of Trivial Pursuit, is that our lowly Spiny dogfish
has the longest gestation period (pregnancy) of any vertebrate in the world –
up to two years.
Spiny dogfish can have twenty pups but usually brood around
six. Like other squaloids, they are ovoviviparous, meaning that their young
develop inside thin egg cases within the mother’s uterus. Once the embryonic
sharks are fully developed, the egg cases are discarded and shortly afterwards
the foot long dogfish are released into the ocean.
The long gestation period alone makes dogfish vulnerable to
over-fishing, but the real obstacle that they face is their late maturity.
Depending on the region, females mature at anywhere from 11 to 26 years of
age. This means that they have to run the gauntlet each fishing season simply
to survive long enough to begin the process of procreation. It is not
surprising that they are struggling.
Yet another problem is that some populations of Spiny dogfish
seasonally migrate over vast distances. Individuals tagged in British
Columbia, have shown up off the coast of Japan. An epic journey of some
7000km. This brings these populations in contact with multiple fisheries,
increasing their chances of ending up on a dinner plate.
It’s not just humans that eat dogfish. Their small size makes
them easy targets for larger shark species, killer whales, and seals. Even
Giant pacific octopuses will bag the occasional Spiny when they can get one.
Fortunately, Spiny dogfish are hardy little sharks. Tagging
programs and examination of the growth rings on their dorsal spines (a bit
like counting tree rings) indicate that they can live up to 70 years or more.
Some marine biologists believe that on rare occasions they may even reach the
ripe old age of 95.
Due to the diligent efforts of marine conservation groups, in
the last couple of years catch quotas have finally dropped to within what many
scientists consider sustainable levels. Only time will tell. It will take many
generations to return the stocks to their former glory and the over fishing of
smaller species that they like to eat will not help in their recovery.
It’s a gloomy picture but all is not lost. Spiny Dogfish like
many other species are versatile and adaptable. There is even evidence that in
response to declining numbers, females may be starting to mature earlier.
Given time and protection we may yet see the species rebound and perhaps in
our lifetime, we will again be able to look up on a dive at an impenetrable
wall of Spiny dogfish blocking out the sun.