In Defense of the Spiny Dogfish

"Voracious beyond belief, dogfish deserve their bad reputation among many fishermen."  - Bigelow and Schroeder's Fishes of the Gulf of Maine

As anyone with any kind of familiarity with Squalus acanthias will tell you, these are deeply unpopular animals.  Fishermen have cursed the name "dogfish" for almost as long as commercial fishing has existed, and recent efforts at dogfish management have not made them any more sympathetic.  Despite evidence of overfishing, these small sharks are still capable of turning up in massive numbers, swamping and damaging fishing gear, and devouring the catch.  They lack the nobility of many of their shark cousins; you're not likely to see them making majestic leaps like great whites and makos, and they haven't earned the respect of sport fishermen like porbeagles or thresher sharks.  For these reasons conservation of the spiny dogfish has been contentious at best. 

So why would anyone in their right mind want to study these fish except to try and find ways of eradicating them?  I make no secret that I fall on the side of conservation when it comes to this issue (though I want to see management of these sharks done right, but that's for another post).  Hopefully this post will illustrate some of the reasons I think spiny dogfish deserve at least a little respect.


Face it, these are pretty impressive animals.  They don't do tricks, they don't use tools, they're a pain in the butt to get out of a gillnet, but spiny dogfish exhibit a lot of interesting behaviors that are well worth study and, in my opinion, make them fairly charismatic.  For one thing, those huge aggregations that can ruin a perfectly good day of fishing are fairly unprecedented in most species of sharks.  Here is a shark that is highly social and has been virtually untapped in terms of animal behavior studies (most likely due to the difficulty in observing a fast-moving highly migratory school of fish in any detail).  They segregate by size and sex, but why?  The answers to this are still unclear.  These sharks appear worldwide in all kinds of environments.  How are they getting there?  How far are they migrating?  How is it that a fish that takes 12 years to mature and has a two-year gestation period is able to appear in large enough numbers to saturate fishing gear?


What has been studied in some detail is their feeding behavior, which, as the quote from Bigelow and Schroeder suggests, can be pretty hard to hyperbolize.  These little sharks are highly adaptable, appearing en masse around sources of food that may only exist for a matter of days.  A study by Beamish et al. (1992) showed that spiny dogfish in the Strait of Georgia would appear within the week that salmon hatcheries would release their smolts into the rivers.  These inexperienced young salmon would venture out into salt water and immediately be pounced upon by up to 1.4 million hungry dogfish, just waiting for prey that had never seen a predator before.  One year this veritable sea of sharks devoured nearly every smolt released by the Big Qualicum River hatchery, a release that represented 7.7 million smolts.  Horrifying if you run a salmon hatchery, but downright awe-inspiring for a shark fan.  Add to that evidence of pack-hunting (!) and you have a fish that has become a powerful and highly influential predator despite its relative lack of size or showiness.  Basically, you have a shark that acts like a piranha, and there is no way that is not impressive.

Another interesting thing about spiny dogfish is that they be an example of evolution in action in a very long-lived, slow-growing species.  Atlantic dogfish are a fairly k-selected species by any standard, with females only maturing after 12 years and a total lifespan of 40-50 years.  However, in the Pacific the same species has a life history nearly identical to that of a human (except for that nasty two-year pregnancy), living up to 80-100 years!  There is no doubt that the Atlantic and Pacific populations are reproductively isolated, since they have the small matter of the entire continent of North America between them.  Is speciation occurring in spiny dogfish, or has it already happened?  

Dogfish act as models for other sharks.  Much of what we currently know about shark behavior, physiology, and hydrodynamics comes from studies on spiny dogfish.  Thanks to their relatively small size, abundance, and ease of handling, this species is about as close as any shark gets to being a lab rat.  Dogfish have been poked, prodded, dissected, run through flow tanks, and even had transmitters implanted in their brains so the Navy can use them as sea-going spies.   Research on spiny dogfish has lead to developments in such disparate fields as submarine design and swimsuit materials.

One further way that spiny dogfish can act as models for their larger, more charismatic kin is in fisheries management.  Much of the controversy and conjecture in spiny dogfish management comes form the fact that sharks are unlike any other fish that is managed.  They are invariably slow-growing, late-maturing, and produce few offspring, meaning that once a shark stock is overfished it may take decades to recover, and may never recover at all.  Most historical attempts at establishing a commercial shark fishery have resulted in a "boom and bust" progression as the population of the target species suddenly collapses.  With dogfish we may or may not have hit the "bust" period (I'll leave the flak-catching for making that call to others more informed than I) but there may be an opportunity to manage the species before it sees the kind of massive population declines that have afflicted larger sharks.  Dogfish management has had more than its share of growing pains, but if scientists, fishermen, and managers can get it right, then it may be possible to responsibly manage other shark species and figure out a way to recover those species that have already been hit hard by overfishing.  But maybe that's just my idealistic take on the subject.

Dogfish just may end up being important.  Marine biology and fisheries management are fraught with cautionary tales.  Overfishing and irresponsible management have lead to all manner of unintended consequences.  The increased abundance of spiny dogfish and other "mesopredator" elasmobranchs could be the result of two fishing-related problems.  The first is competitive release due to the removal of the large predatory fish that used to compete with them for food, such as the case with Atlantic cod on Georges Bank.  The other is the removal of the larger sharks that used to prey upon them.  While the latter has yet to be positively proven with regards to spiny dogfish, it has been cited as the cause for the rapid increase in cow-nosed rays in the estuaries of North Carolina.  Without the big hammerheads and tiger sharks to keep them in check, these rays now run rampant on their shellfish prey and may be responsible for the crash of the North Carolina bay scallop fishery.

The point here is that today's pest species may turn out to be much more important tomorrow.  It was once considered that the only good shark was a dead shark, but now that there aren't as many policing the ocean it has become clear just how important a healthy stock of these "man-eaters" is to putting seafood on the plate.  While it is extremely unlikely that roving packs of dogfish will ever menace bathers, these sharks also have quite an image problem.  A species doesn't exert as much influence over the marine food web as the spiny dogfish without being important to the overall health of the ecosystem.  It's already known that dogfish prey upon ctenophores (better known as comb jellies), which can be voracious predators of fish larvae.  Would a crash in dogfish populations lead to lower reproductive success for other fish?  Sound management of a species requires thorough knowledge of how that species fits into the big puzzle that is the marine ecosystem.

So that's my turn on the soapbox.  I don't expect anyone who reads this to turn into a raging dogfish conservationist, but I do hope that it at least encourages deeper thought about that little shark that just stole your bait.

Sources cited and further reading.

Beamish, R.J., B.L. Thomson, and G.A. McFarlane.  1992.  Spiny dogfish predation on chinook and coho salmon and the potential effects on hatchery-produced salmon.  Transactions of the American Fisheries Society 121:444-455.

Camhi, M., S. Fowler, J. Musick, A. Brautigam, and S. Fordham.  1998.  Sharks and their relatives: ecology and conservation.  Occasional Paper of the IUCN Species Survival Commission 20.

Fogarty, F.J. and S.A. Murawski.  1998.  Large-scale disturbance and the structure of marine systems: fishery impacts on Georges Bank.  Ecological Applications 6:8-22.

Wallace, S.S., G.A. McFarlane, S.E. Campana, and J.R. King.  2009.  Status of spiny dogfish in Atlantic and Pacific Canada.  Pages 313-334 in V.F. Gallucci, G.A. McFarlane, and G.G. Bargman, editors.  Biology and management of dogfish sharks.  American Fisheries Society, Bethesda, Maryland.