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Commercial Fisheries News
Volume 35 Number 9
May 2008
Dogfish in the Gulf of Maine eat cod, herring
BIDDEFORD, ME – Fishermen are painfully aware that dogfish are insatiable predators that will eat just about anything.
But new information based on stomach content analyses from 401 dogfish caught by commercial fishermen in the Gulf of Maine is giving researchers deeper insight into exactly what dogfish in this particular area are devouring.
At a March 29 dogfish forum here, James Sulikowski of the University of New England said his research team really wanted to zero-in on what dogfish were eating on a regional basis.
“Spiny dogfish are everywhere, so obviously their diet is going to vary by where they are,” he said. “We wanted to look at the Gulf of Maine.”
Sulikowski said the team also wanted to get at the root of some key questions.
“We wanted to know, ‘Are they competing with groundfish by eating groundfish or are they competing with them because dogfish eat what groundfish eat?’” he said.
What they found
From September 2006 through December 2007, fishermen from Maine and Massachusetts collected dogfish for the study largely from the Stellwagen Bank, Jeffreys Ledge, and Massachusetts Bay areas – prime fishing territory for commercial and recreational fishermen.
Of the 401 fish sampled, 361 were females with an average total length of 85.93 centimeters (cm) or 34.4", and 40 were males with an average total length of 70.67 cm or 28.3".
Sulikowski received considerable help with the work from university students, who spent roughly one hour per fish tediously sorting through stomach contents.
They found that 36% of the dogfish didn’t have anything at all in their stomachs.
But in the remaining 340 fish, they found – with a few notable exceptions – just about everything: herring, sand lance, flounder, hake, cod, haddock, a few crabs, anemones, worms, and even a couple of rocks, which probably were a byproduct of groveling for crabs.
All told, 87% of the stomach contents from these particular Gulf of Maine-caught dogfish consisted of bony fish – with cod, herring, and sand lance being the top three species.
Diets may vary
All of the fish sampled were adults, so Sulikowski and others in the room warned that juvenile dogfish in the Gulf of Maine might be feeding on entirely different species.
And, inevitably, dogfish collected elsewhere along the coast – say off North Carolina – likely would have a different diet as well.
“They’re very opportunistic eaters,” said Sulikowski. “That’s why you really need to see what’s happening in each individual area.”
The two most significant species that were missing from the stomachs of the dogfish caught in the Gulf of Maine were jellyfish and lobsters.
The lack of jellyfish was particularly noteworthy since several other stomach content studies have shown that dogfish eat a lot of jellyfish.
As for lobsters, Sulikowski said he had not yet sampled dogfish from the more inshore areas of Midcoast and Downeast Maine, so he had yet to determine whether dogfish feeding in those areas have a lobster component to their diets.
That lack of information might change soon, however. Harpswell fisherman Don Sproul immediately volunteered to provide additional dogfish samples in April from the Midcoast.
How much do they eat?
The big question on people’s minds was: So how much do dogfish eat in total each year?
Using the most recent total dogfish biomass estimate of 478,000 metric tons (mt) and assuming dogfish consume roughly 1.5% of their body weight per day, Sulikowski estimated that dogfish need somewhere on the order of 2.4 million mt of prey to support the population at its current size.
Forum co-organizer Phil Grondin, a tuna fisherman, said, “These dogfish are out-eating commercial fishermen three-to-one.”
Even if the prey estimate was deemed to be on the high end and dogfish were consuming only one-half-to-one-third of that tonnage, Sulikowski concluded, “The impacts could be great.”
Janice M. Plante
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