Give Thanks to the Spiny Dogfish, Squalus acanthias. By Don Orth

We should be grateful for our many fishy blessings on Thanksgiving. The Spiny Dogfish Squalus acanthias plays many roles in our world. It may be a bloody nuisance for commercial fishermen, but Spiny Dogfish once provided abundant fish oil and vitamin A.  Today they are a substitute fish for fish n chips. Why the dogfish even has its name on a brewery.  Through history, the Spiny Dogfish has posed many challenges. To the Spiny Dogfish and those who helped to overcome the challenges, we give thanks. 

Spiny Dogfish is the representative of the cartilaginous fishes (Chondrichthyes) that is dissected in anatomy labs throughout the world. Each year biology majors and pre-med students are challenged in comparative vertebrate anatomy lab before advanced study in human anatomy.  Here students dissect the dogfish shark and begin to compare organ systems among the chordates. Students name structures and consider their functional adaptations.   At night they review and cram from 3-pound dissection guides, such as Wischnitzer (2006) or Fishbeck and Sebastiani  (2015).   

Dissection of Spiny Dogfish in process in Ichthyology Lab. Photo: DJ Orth

The body of the Spiny Dogfish is slim with a narrow pointed snout and asymmetrical caudal fin lobes. Spiny Dogfish are slate gray on the dorsal surface and sides, and have characteristic white spots and a white belly.  The irregular white spots are typical of younger fish and may be lacking on older individuals.  It has two widely separated dorsal fins and two sets of paired fins.  It lacks an anal fin – how do you explain that? Maybe God meant for Ichthyology Professors to have a little fun with student lab practicals?  "Label the anal fin on this specimen."

Squalus acanthias (female 678 mm TL) from Ebert et al. (2010)

Commercial fishers are challenged to harvest high-valued fishes without catching unpopular dogfish. Spiny Dogfish grow to 3.3 (males) to 4 feet (females) and adults school near the bottom where they overlap with many more valuable food fishes, such as cod, halibut, flounder, and sole. When these valuable groundfish were overfished in the late 1980’s (Buchsbaum et al. 2005), commercial fishermen targeted the larger female Spiny Dogfish, leading to overharvest.  Spiny Dogfish are not the first choice of fishermen, and are an impediment to harvesting groundfish, such as the Silver Hake (aka, whiting) Merluccius bilinearis.  Trawlers targeting whiting use a small mesh trawl with a raised footrope to avoid catching some unwanted bottom fishes. However, the Spiny Dogfish is still captured with this gear.  An excluder grate installed on the trawl net reduces the catch of Spiny Dogfish and subsequent handing time and harvests more of the target fish. 

Dogfish excluder grate on a hake trawl.   Photo from Chosid et al. (2012)

Surprisingly, the naming was another challenge. Carolus Linnaeus first named the Spiny Dogfish in 1758.  Since that time 24 other species of Squalus have been discovered and named. For example, in 1854, Charles Girard named the North Pacific Dogfish, Squalus suckleyi.  Like many other complex taxonomic issues, reasonable people may disagree until careful measurements of meristic and morphometric measurements and genetic data from representative samples are analyzed.  In 1960, the American Fisheries Society committee on names of fishes, grouped the disconnected North Pacific and Atlantic populations into a single species Squalus acanthias, ignoring the work of Girard. It took 50 years before Ebert et al. (2010) examined meristic, morphological, and molecular data and resurrected the endemic North Pacific Squalus suckleyi.  Consequently, there is some confusion around names used in previous literature.  Further, there are still many other species of Squalus that need to be evaluated in order to understand the phylogeny of the the dogfish genus, Squalus.

The common name, dogfish, has always been a challenge.  It just doesn't sound like a chef’s delicacy. The name, dogfish, was been used since the 15^th century as fishermen observed them hunting in packs.   Dokefyche is a middle English word that combines “dog” and “fish.”   Cabinet makers in the 19^th century used the skins of dogfish to polish hardwoods.  Livers were also used by tanneries. Fisheries still considered the dogfish a trash species in the mid 20^th century. Harvest was used for livestock and pet feed or fish meal.  However, the liver is high in natural vitamin A and fisheries once harvested the dogfish and used the liver oil to derive vitamin A. 

Many years ago it was marketed as Flake, Grayfish, Cape Shark, Japanese Halibut, or Rock Salmon.   Like all marine elasmobranchs, the Dogfish maintains osmotic balance with its external environment with a mixture of urea and trimethyl amine oxide (TMAO) in its blood stream.  Otherwise they would lose freshwater to the marine environment and be unable to function. So that urea thing may keep you from ever trying a Dogfish entrée. But do try it. Just remember Dogfish must be bled, gutted, and iced immediately after harvest to avoid the urea settling in the tissues.  Watch the cleaning video. Soon after death, bacteria can quickly convert the urea in their blood and tissue into ammonia.  Yuk!  The taste of ammonia is not appealing, in fact, it's a sign of kidney disease!  So prepare the dogfish correctly and even consider marinating dogfish in something slightly acidic.  The result will be another delectable dish, thanks to the Spiny Dogfish.

Spiny Dogfish, although once overfished in the Northwest Atlantic, are now the largest shark fishery in US. The Spiny Dogfish is often maligned by commercial and recreational fishers and its role is misunderstood.  Also, the leading edge of their dorsal spine is a big, white, needle-sharp spine, a formidable weapon capable of inflicting agonizing pain. Bigelow and Schroeder (1948) in Fishes of the Western Atlantic, wrote “from a practical aspect the spiny dog in the Western Atlantic is chiefly important because it is undoubtedly more destructive to gear and interferes more with fishing operations than does any other fish – shark or teleost.”    

Spiny Dogfish are opportunistic feeders, preying on what is locally abundant.  They feed primarily on crustaceans when young, and move on to comb jellies, jellyfish, squid, and fish as they get larger.  Consequently, they are blamed for competing with valuable groundfish for food. They also eat commercially valuable fishes, such as mackerel, herring, and squid.   Ctenophores (comb jellies)  have become more prevalent in diets of Spiny Dogfish between 1981 and 2000; this increase likely reflects increased abundance of ctenophores (Link and Ford 2006; Ford and Link 2014).  Spiny Dogfish are slow growing and there are many predators on the Spiny Dogfish, including the Cod, Red Hake, Goosefish, larger sharks, seals, and orcas.   Many fisheries management decisions today are still made without quantifying many of these key ecosystem interactions (Murawski 2000).

Although Spiny Dogfish are not the most desirable fish in the sea, there is high demand in the United Kingdom for Fish n Chips and in Germany for a Beer Garden Snack, called shillerlocken.   Shillerlocken should not to be confused with the pastry named after the same German poet. Schaumrollen, or Schillerlocken, is something my grandfather Orth made in his bakery each Christmas.  It’s ironic that Spiny Dogfish are now protected under EU fishing regulations to stop it being caught and sold with chips, but it is legal to sell it in the UK provided it has been caught outside the EU and imported. 

 

Spiny Dogfish becomes the fish in "Fish n Chips"  Photo by Alamy (Source)

Changing demands on the Spiny Dogfish led to historical fluctuations in abundance. It is clear now from their life history, that the Spiny Dogfish is vulnerable to rapid overharvest without controls on harvest. Spiny Dogfish are difficult to age.  Validation of methods for examining annual marks on the spines or vertebrae were only recently validated via radiocarbon dating (Campana et al. 2006).  

Annual rings on the spines of Spiny Dogfish (from Campana et al. 2006, left) and on sectioned and stained vertebra (from Bubley et al. 2012,  right)

Growth and maturity studies indicate slow growth, late maturity, and low fertility.  Spiny Dogfish are viviparous and the embryos, called pups, feed off their yolk sac until parturition. The gestation period is long and fertility is low; gestation is 18 to 22 months.  Therefore, females give birth every other years to between 2 and 15 pups (average: 6).  The maximum reported size is 117 cm (=46 inches) total length and maximum reported age is 40 years. 

Growth (left, Bubley et al. 2012) and maturity, as measured by the female gonadosomatic index, of Spiny Dogfish (right, Bubley et al. 2013)

Fisheries scientists estimate population size and the removals from targeted fisheries and discards from bycatch. Large trawls are used to estimate catch per swept area and population size.  However, the Spiny Dogfish school and often school just ahead of the trawls.  Therefore, discard mortality and abundance, derived from trawl surveys, are uncertain estimates  (Rago et al. 1998; Rago and Sosobee 2009).  Female fish dominate the harvest of Spiny Dogfish. Harvesting results in truncated size and age distributions, which further reduces population productivity. Consequently, catch limits were imposed on harvests in US waters starting in 2000. These regulations allowed the Northwest Atlantic stock of the Spiny Dogfish to rebuild (Rago and Sosobee 2011).  However, commercial interests lobby for increases in catch limits and stock assessments are updated periodically to inform future management actions.   Meanwhile, Spiny Dogfish stocks in the eastern Atlantic are dwindling, which puts demands on Spiny Dogfish populations from US fisheries.  So challenges will continue. 

Remember to give thanks to the following on this Thanksgiving Day:  the Spiny Dogfish in your comparative anatomy lab, Carolus Linnaeus for naming the Spiny Dogfish, modern taxonomists for getting the names right, inventors for the dogfish excluder grate, fisheries biologists for revealing the life history of Spiny Dogfish, population analysts for trying to get the population numbers right, and fisheries managers for acting to limit harvests and sustain fisheries.  

References

Bubley, W.J., J. Kneebone, J.A. Sulikowski, and P.C.W. Tsang. 2012. Reassessment of spiny dogfish Squalus acanthias age and growth using vertebrae and dorsal-fin spines.  Journal of Fish Biology 80:1300-1319.

Bubley, W.J., J.A. Sulikowski, D.M. Koester, and P.C.W. Tsang. 2013.  Using a multi-parameter approach to reassess maturity of spiny dogfish, Squalus acanthias, following increased fishing pressure in the western North Atlantic.  Fisheries Research 147:202-212.

Buchsbaum, R., J. Pederson, and W. E. Robinson, editors. 2005. The Decline of Fisheries Resources in New England: Evaluating the Impact of Overfishing, Contamination, and Habitat Degradation. MIT Sea Grant College Program Publication No. 05-5.  

Campana, S. E., C. Jones, G.A. McFarlane, and S. Myklevoll.   2006. Bomb dating and age validation using the spines of spiny dogfish (Squalus acanthias). Environmental Biology of Fishes 77, 327–336.

Chosid, D.M., M. Pol, M. Szymanski, F. Mirarchi, and A. Mirarchi. 2012.  Development and observations of a spiny dogfish Squalus acanthias reduction device in a raised footrope silver hake Merluccius bilinearis trawl.  Fisheries Research 114:66-75.

Dell'Apa, A., C.W. Bangley, and R.A. Rulifson. 2015. Who let the dogfish out? A review of management and socioeconomic aspects of spiny dogfish fisheries.  Reviews in Fish Biology and Fisheries  DOI: 10.1007/s11160-014-9379-1

Ebert, D.A., W.T. White, K.J. Goldman, L.J.V. Compagno, T.S. Daly-Engel, and R.D. Ward. 2010. Resurrection and redescription of Squalus suckleyi (Girard, 1854) from the North Pacific, with comments on the Squalus acanthias subgroup (Squaliformes: Squalidae). Zootaxa 2612, 22–40.  

Fishbeck, D.W., and A. Sebastiani. 2015. Comparative Anatomy: Manual of Vertebrate Dissection 3rd Edition. Morton Publishing Company, 576 pp. 

Ford, M.D., and J.S. Link. 2014. Bounds on biomass estimates and energetic consequences of Ctenophora in the Northeast U.S. Shelf ecosystem. International Journal of Oceanography http://dx.doi.org/10.1155/2014/851809

Link, J. S., and M. D. Ford. 2006.  Widespread and persistent increase of Ctenophora in the continental shelf ecosystem off NE USA. Marine Ecology Progress Series 320:153–159

Murawski, S. A. 2000.  Definitions of overfishing from an ecosystem perspective.  ICES Journal of Marine Science 57:649-658.

Rago, P.J., K.A. Sosebee, J.K.T. Brodziak, S.A. Murawski, and E.D. Anderson. 1998. Implications of recent increases in catches on the dynamics of Northwest Atlantic spiny dogfish (Squalus acanthias). Fisheries Research 39, 165–181.   

Rago, P. J. and K. A. Sosebee. 2009 The agony of recovery: Scientific challenges of Spiny Dogfish recovery programs. pp 343-372. In V. F. Gallucci, G. A. McFarlane and G. G. Bargman eds. Biology and Management of Dogfish Sharks. American Fisheries Society, Bethesda Maryland.

Rago, P. and K. Sosebee. 2011.  Update on the status of Spiny Dogfish in 2011and initial evaluation of alternative harvest strategies. Mid Atlantic Fishery Management Council Science and Statistical Committee 

Wischnitzer, S. 2006. Atlas and Dissection Guide for Comparative Anatomy 6th Edition. W.H. Freeman, 368 pp.