Despite their small size, Pacific spiny dogfish sharks play a critical role in the Puget Sound food web, shaping life-and-death competition over food supplies.
Some experts call dogfish a “keystone species,” because their presence shapes the entire ecosystem. Although nobody disputes the significant role of these small sharks, some argue that their numbers are too plentiful to fit under the definition of “keystone” — a species that has a “disproportionately large effect relative to its abundance,” according to the original definition by zoologist Robert Paine (background in Scientific American).
In Central Puget Sound, dogfish are the most abundant of the bottom-dwelling creatures that prey on fish, according to a study led by Chris Harvey of NOAA’s Northwest Fisheries Science Center. They make up about five percent of the total fish biomass and 87 percent of the biomass of top-level predators, according to Harvey’s report in the journal Estuaries and Coasts.
Females don’t normally give birth until around age 35, and some live to be 80—one of the longest lives among shark species.
Beyond their abundance, however, dogfish (with the scientific name Squalus suckleyi) can play different roles at different times, expanding their influence in the ecosystem. Acting as apex predators, they can go after over-abundant predators, such as fish, jellyfish and squid that consume forage fish needed by other species. As scavengers, they can maintain an energetic system by eating the dead remains of other animals. While they often eat smaller fish, they can also eat shrimp and crabs, providing food-web links to creatures at other depths. Dogfish themselves can become prey for larger fish, including other sharks, as well as marine mammals. It’s all a matter of balance. Dogfish, like several other shark species, are considered generalists in their food choices.
“What they eat depends on where they are,” said Ethan Personius of Oregon State University, who has studied dogfish along with sevengills and soupfins. “In Hammersley Inlet, they were eating a lot of crab. They also eat baitfish like smelt.”
In his research in Hammersley Inlet, Personius learned that dogfish were eaten by sevengill sharks, which are also generalists.
While fishermen may consider spiny dogfish a nuisance, many experts believe this important population could be vulnerable because of particular weaknesses in their life history. For example, females don’t normally give birth until around age 35, and some live to be 80—one of the longest lives among shark species, according to several studies, although other estimates can be found.
The gestation period for dogfish lasts about two years—one of the longest prenatal periods in the animal kingdom. Asian elephants, believed to have the longest pregnancy among mammals, support their unborn calves about the same amount of time, 21–25 months. While most fish produce hundreds or thousands of eggs at a time, the litters of female dogfish typically contain between five and 12 baby sharks. These factors result in a low level of productivity to support the dogfish population.
Dogfish in Puget Sound are considered part of a single coastwide stock, which has experienced steep declines during periods of heavy fishing activity, according to a 2021 stock status report (pdf) by NOAA’s Northwest Fisheries Science Center. A major population decline occurred during the 1940s, when dogfish were sought as a source of vitamin A. Fishing was minimal from about 1950 to 1974, but in the mid-70s, a decline in Atlantic dogfish in Europe (now considered a different species) created a market partially filled by West Coast fishermen in the U.S.
Commercial fishers seeking more valuable fish along the coast frequently catch and discard dogfish as unwanted bycatch. Since 2015, harvest levels have been stabilized with more careful management, and the fishing industry has implemented practices to reduce incidental take.
In Puget Sound, most dogfish are caught — and generally released — by anglers trying to catch salmon or bottom fish. A few sport fishermen will target these small sharks and take them home to eat. Current regulations allow for a daily limit of 15 dogfish — the same as for other unnamed “groundfish” in the regulations.
The small size of dogfish litters, uncertainty about their reproductive cycles, and ongoing changes in habitat and climate have raised concerns that the population could become rapidly depleted if fishing or other forces remove a relatively small number of individuals. And yet, while some studies suggest that the Puget Sound dogfish population remains on a long-term decline, these sharks are not yet considered at risk of extinction.
Relationships between dogfish in Puget Sound and those in waters off the West Coast are the subject of current studies, but it has been shown that dogfish swimming in Puget Sound during the summer generally head out to the ocean in the fall, where they stay until spring.
Whether or not dogfish are a “keystone species,” these migrations can have a significant effect on the Puget Sound food web, an issue that must be considered in food-web models that predict how changes in one marine species can reverberate through the entire food web.
“Incorporating movement data into the bioenergetics model resulted in a 70 percent decrease in the predatory impact of dogfish in the winter and a 30 percent decrease in the summer, compared to a year-round resident Puget Sound population,” states a 2013 research paper by Kelly Andrews and Chris Harvey for NOAA’s Northwest Fisheries Science Center.
The study involved the placement of acoustic tags in 17 dogfish, although three apparently died soon after. After tagging in 2008, the remaining 14 dogfish all migrated through the Strait of Juan de Fuca and out to the Pacific Ocean, primarily in November and December — although one individual waited until February to leave.
In 2009, nine of the 14 dogfish moved back through the Strait beginning in June, with seven of the nine reaching the main part of Puget Sound. For those fish, the pattern was repeated that fall with migration occurring in November and December, all except the one that left in February. The pattern was again repeated in 2010 and 2011, with fewer dogfish returning each year.
Within Puget Sound, the movements of the dogfish varied, but individuals stayed fairly consistent from year to year. Some took direct routes to South Puget Sound, where they stayed until it was time to leave. Others stayed in the north, making quick forays into Central Puget Sound where they were originally tagged.
Based on this study and catch records by recreational fishermen, it appears that the vast majority of dogfish migrate back and forth from the ocean each year, but a few remain in Puget Sound year-round. This differs from sixgill sharks, which seem to stay in Puget Sound for several years before they leave for the outer coast.
“It is unclear what incentive immature dogfish might have to migrate out of Puget Sound waters for the winter,” the paper states. “By contrast, the largest tagged female (likely mature) also had the most distinct migration pattern. It exited Puget Sound the first year at the same time as the rest of the individuals. But, unlike the others, it never returned, instead inhabiting coastal California waters in the summers of 2009 and 2010.”
Pacific spiny dogfish sharks get their name from the sharp spines in front of each of their two dorsal fins. Their range extends from Alaska to Baja California in the eastern Pacific as well as from Japan to the Sea of Okhotsk, Russia in the western Pacific. Males are typically about 2.5 feet long with females about three feet long. Maximum sizes in the Pacific Ocean have been reported as large as five feet.
Up next: More sharks of Puget Sound