Elasmobranch “Outbreak” Caused by Migration?


One of the most contentious topics on modern fisheries management and elasmobranch ecology has been the supposed “explosion” of skates and dogfish in Georges Bank.  The fact that this coincided with the crash in commercially important groundfish populations (cod, haddock, flounder, and other delicious bottom-dwellers) in the 80s and 90s has riled up fishermen, managers, and biologists ever since.  Add to this this much lower market value for these species, the fact that some of them are potential predators of the species we’d rather be eating, and that some of them have shown declines themselves (resulting in protections for species that many fishermen consider pests) and you have a recipe for repeated disastrous interactions between fishermen and regulators.  However, what is often lost in these arguments is what initially caused this sudden uptick in dogfish and skates.  This debate is often mired in fishermen vs. environmentalist politics, which is why the prevailing theory is still that fishing pressure caused the whole Georges Bank ecosystem to go out of whack, either in favor of or to the detriment of small elasmobranchs.  However, the sheer speed at which these squalids and rajids increased in numbers simply can’t be explained by their own life history characteristics: simply put, their slow growth rates and low fecundity (especially compared to the millions of eggs released by any one Atlantic cod) mean these increases don’t make a whole lot of sense.  Enter an alternative theory.

The importance of the Georges Bank fishery can’t be overstated.  Historically, these were among the first American waters fished and were one of the driving economic forces behind colonizing this continent in the first place.  Georges Bank cod were being heavily fished before the United States was anything more than a series of English trading ports along the Atlantic coast.  For this reason Georges Bank is virtually synonymous with American fisheries, and a highly disproportionate amount of work goes into finding ways to keep it productive (including some ideas that may stray a little too far outside the box).

Because Georges Bank has so much history behind it, you also have generations of fishermen and fishing communities that have grown up reviling spiny dogfish and skates and their ability to ruin a good day’s fishing with sheer numbers.  This historical ecological knowledge may be why the majority of New England fishermen found it impossible to believe that spiny dogfish stocks had crashed in the late 90s/early 2000s, and the dogfish certainly didn’t help their case by apparently rebounding dramatically after only about half a decade of strict management (though their population dynamics and distribution may remain affected).  But wait, aren’t dogfish incredibly long-lived animals that don’t mature until their teens and take two years to crank out maybe a dozen pups max?  The debate rages about whether the previous work on dogfish life history characteristics was anywhere close to accurate, but there is another distinct possibility: the dogfish (and their skate cousins) swam in from somewhere else.

I liked Mike Frisk’s talk at the AES conference, and was pleasantly surprised to find some of his work from 2008 (that’s two research blogging posts on papers from ’08 now.  Must have been a good year for fisheries research).  Frisk et al. (2008) actually deal with winter skate populations, but make sure to mention that their findings are potentially applicable to spiny dogfish and other skate populations as well.

Using a massive data set from NMFS and various state trawl surveys stretching back to the 1970s, Frisk et al. (2008) found evidence that the life history characteristics of winter skates alone don’t explain the sudden surge in their numbers, which was initially blamed on competitive release from overfishing of cod and flounder stocks.  What the authors did find was that Georges Bank and southern New England skates may not be all that isolated from the winter skate stocks in Canadian waters.  The prevailing conventional wisdom is that skate populations on Georges Bank are distinct from those on the Scotian Shelf, and are kept that way by the Northeast Channel between the two areas.  However, trawl survey data and good old deductive reasoning show that there’s no reason why winter skates couldn’t just swim across the channel.  Rather than a sudden upswing in winter skates popping out offspring, what we may be seeing is mass immigration of (shudder) Canadians.

Frisk et al. (2008) are refreshingly honest about their findings being fairly preliminary, but they are probably on to something.  What needs to be done in response is corroborating abundance data from the Scotian Shelf with Georges Bank data from the same time series.  It’s also possible that the Scotian Shelf is not the only place winter skates are coming from.

The ocean is paradoxical in that it is simultaneously a gigantic open system while still supporting highly localized species.  What ends up determining a species’ range more often than not is that species’ own environmental tolerances.  Dogfish and skates are more than capable of living in areas accessible from Georges Bank and we know this because Canadian and even northern European waters have stocks of these species.  What needs to be determined is just how interconnected these areas are and how common travel between them is.  Do dogfish and skates naturally run between these areas or are they doing so as a result of human influence?  European nations are actually worried about the rarity of dogfish and skates in the North Sea.  Could spiny dogfish have vacated those waters in search of a lower-stress environment?  The North Sea is one of the few places on earth that could probably be considered more overfished than Georges Bank.

Though-provoking stuff, and a subject that could definitely be explored using methods that are already very familiar to fisheries management.  I’d love to see work done on this on an international scale.

Frisk, M., Miller, T., Martell, S., & Sosebee, K. (2008). NEW HYPOTHESIS HELPS EXPLAIN ELASMOBRANCH “OUTBURST” ON GEORGES BANK IN THE 1980s Ecological Applications, 18 (1), 234-245 DOI: 10.1890/06-1392.1


  1. WhySharksMatter · July 18, 2010

    So, what you’re saying is that large numbers of undocumented migrants from a neighboring country are taking resources that affect the jobs of hard-working Americans? Should we, like, build a fence or something?

    • Chuck · July 18, 2010

      Unfortunately it seems all the sea creatures drew up their boundaries without consulting us. Guess we’ll have to declare war on ’em.

      • WhySharksMatter · July 18, 2010

        “Guess we’ll have to declare war on ‘em.”

        I suppose you could argue that we’ve already done that.

      • JohnnieCanuck · July 18, 2010

        In all too many cases, a genocidal war, or actually a succession of genocidal wars.

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  3. Monado, FCD · July 18, 2010

    If a cod releases a million eggs, doesn’t that mean that it takes about a million eggs to make two cod? I think that with part of the population being fished off, the herrings and others are getting ALL the eggs instead of just MOST of them.

    I know zilch about the distribution of dogfish: are there southern populations that could be moving north?

    • Chuck · July 18, 2010

      It’s actually the comb jellies that people are really worried about in terms of consuming fish eggs and larvae, though I’m sure herring and other planktivores take their share. What I was getting at there is that cod and other groundfish are much more r-selected than the area’s elasmobranchs, which does come with much higher juvenile mortality.

      Spiny dogs in New England waters do show a north-south migration (the subject of my thesis is actually what they’re eating while they’re off the Carolinas during the winter) but there’s some variation there. Some populations stay north all year long, and we just had a big school of dogfish stick around North Carolina until almost June. They have a general pattern they tend to follow but like all fish they basically end up doing whatever they want.

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