Shepard Point in Prince William Sound. Photo by Dahr Jamal courtesy of the Eyak Preservation Council

Hello Scientist,  

Can you tell me the life history of the North Pacific Spiny Dogfish, Squalus suckleyi, including at what age females mature and why the males and females travel in different schools? 

-Anonymous 

The Answer: 

Great question, spiny dogfish are an interesting species. I reached out to Cindy Tribuzio for help answering this question. Cindy is an Alaska Fisheries Science Center Research Biologist at Auke Bay Laboratories.  

Life History:  

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Spiny dogfish can be aged, using their dorsal fin spines. Studies using that method have estimated that they can live at least 80 years; likely, they can live more than 100. Spiny dogfish are one of the slower growing shark species that have been studied. Females reach maturity at about 36 years of age, males at about 19 years of age.  

Spiny dogfish are live bearers, meaning that the female carries the young until birth. However, similar to egg laying, the pups are fed by a yolk sac, not by maternal connections — as seen in mammal placentas. Gestation can last about 22 months. Often, the females are developing ova — eggs within the ovaries — for the next pregnancy while they are carrying the pups of the current pregnancy. This means that they start another pregnancy shortly after pupping. It takes about four years from when the ova starts to develop in the ovaries until a pup is born. They have an average of eight pups per litter, but this can range from one to 22 pups. Larger females tend to have larger pups and possibly higher survival of their pups. Some females “take breaks” between pregnancies as well. Instead of having pups every two years, they have pups every three or more years. This species has a very slow growth rate: they mature later in life and produce pups at a very slow rate.  

Schooling:  

Spiny dogfish tend to school by size when they are immature, not by sex. As immature and smaller dogfish, they likely school together for safety. A lot more things can eat you when you are small.  

As they mature, they start to segregate by sex, but females can also segregate by pregnancy stage. Cindy says that nothing is a hard rule with spiny dogfish. She believes it is possible that adults, especially the larger ones, may venture out on their own. They all can also congregate on an abundant food source. As adults, females and males may segregate by size, but also to protect the pups.  

A number of studies have shown that female sharks will utilize specific areas to give birth for the protection of the pups, so the males don’t eat them. Spiny dogfish are not picky eaters. Males do not get nearly as big as females and can be more susceptible to being eaten by others. Cindy expects to see adult males more schooled up than large females. Catch data suggests that, but again, there are no hard rules.  

In summary, Cindy says “If I had to wrap up the schooling question in one sentence, I’d say they do it for protection: either safety in numbers or to protect pups. And sometimes they just do their own thing.”  

Thank you so much, Cindy Tribuzio, for helping us answer this question. We are currently seeking more Ask a Scientist questions. If you have a question about something interesting that you observed, submit it to [email protected]. 

Cristina Reo is an education specialist at the Prince William Sound Science Center. 

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